Structure evolution and optical properties of hierarchical ZnO micro/nanorods fabricated by a...

ISSN 1753-3562

August 2012 Volume 53 Number 4

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Physics and Chemistry of GlassesEuropean Journal of Glass Science and Technology B

Volume 53 Number 4 August 2012

Cover: Figure 1. Optical micrographs (1,2) and SEM (3-8) images of as-prepared glasses 21-02 (1,3), 21-07 (5) and 21-21 (2,7) and their surfaces after heat treatment and etching (4,6,8, respectively) of the paper: Structural features of high-Fe2O3 and high-Al2O3/Fe2O3 SRS HLW glasses by Sergey V. Stefanovsky et al (Phys. Chem. Glasses: Eur. J. Glass Sci. Technol. B, August 2012, 53 (4), 158–166)

CONTENTS143 Viscous flow and the viscosity of melts and glasses

Michael I. Ojovan151 Effect of alkali cations on the compressibility of MAlSi3O8 glasses (M=Na, K,

Rb, Cs) in the pressure range up to 6·0 GPaR. G. Kuryaeva

158 Structural features of high-Fe2O3 and high-Al2O3/Fe2O3 SRS HLW glassesSergey V. Stefanovsky, Kevin M. Fox, James C. Marra, Andrey A. Shiryaev & Yan V. Zubavichus

167 Structure evolution and optical properties of Co-doped zinc aluminosilicate glass-ceramicsI. Alekseeva, O. Dymshits, V. Ermakov, V. Golubkov, A. Malyarevich, M. Tsenter, A. Zhilin & K. Yumashev

A35 Abstracts

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Physics and Chemistry of Glasses: European Journal of Glass Science and Technology Part B Volume 53 Number 4 August 2012 167

Phys. Chem. Glasses: Eur. J. Glass Sci. Technol. B, August 2012, 53 (4), 167–180

IntroductionСobalt is a widely used colouring agent in the pro-duction of glasses, glass-ceramics, paints, cements, porcelains, crystals and plastics.(1–11) Common blue ceramic pigments are based on Co-containing com-pounds, i.e. Co olivine (Co2SiO4), cobalt aluminate, magnesium-doped Co-Al spinel (Mg0.2Co0.8Al2O4), cobalt-doped ZnAl2O4 (gahnite),(5–8,9 and references therein) willemite,(8,10) and Ca2Zn1−xCoxSi2O7 (hardystonite).(11) Cobalt-doped crystals were also studied as materials that demonstrate laser operation and spectral hole burning.(12) Recently interest in Co-doped crystals was revived because Co2+-doped spinel single crystals and glass-ceramics were found to be the materials of choice for passive Q-switching of Er:glass lasers.(13–17) Tanaka et al(18) for the first time reported absorption and luminescence spectra of glass-ceramics contain-ing gahnite nanocrystals doped with tetrahedrally coordinated Co2+ ions in the spectral range 500–750 nm. Soon after, the first Co-doped gahnite glass-ceramic saturable absorbers were reported in the same system.(16) A study of the luminescence, linear

and nonlinear absorption of Co-doped gahnite glass-ceramics(17) demonstrated that in spite of the absence of crystalline phases other than gahnite, this mate-rial had unsaturable losses due to some additional absorbing centers.

The aim of the present study is to follow the evolution of structure of zinc aluminosilicate glass-ceramics doped with cobalt oxide and prepared at different heat treatment temperatures and to connect it with their optical properties using small angle x-ray scattering (SAXS), x-ray diffraction analysis (XRD), Raman and optical spectroscopy.

Experimental

Glass with the molar composition 25ZnO–25Al2O3–50SiO2 nucleated by 10 mol% TiO2 was chosen for the study. The glass was doped with 0·5–5·0 mol% CoO. Batches to produce 200 g of glass were melted in crucibles made of quartz ceramics in a laboratory electric furnace at 1560°C for 4 h with stirring. The glass melt was poured onto a metal plate. The glasses were annealed at 670°C and heat treated in isother-mal conditions in the temperature range from 680 to

Structure evolution and optical properties of Co-doped zinc aluminosilicate glass-ceramicsI. Alekseeva,a O. Dymshits,*,a V. Ermakov,b V. Golubkov,c A. Malyarevich,d M. Tsenter,a A. Zhilina & K. Yumashevd

a NITIOM S.I. Vavilov State Optical Institute,36/1 Babushkin Street, Saint-Petersburg 192171 Russiab National Research University of Information Technologies, Mechanics and Optics, Kronverkskiy pr., 49, 197101, Saint-Petersburg, Russiac I.V. Grebenschikov Institute of Silicate Chemistry, Russian Academy of Science, Odoevskogo str., Saint-Petersburg 199155, Russiad Institute for Optical Materials and Technologies, Belarus National Technical University, 65 Nezavisimosti Ave., 220013, Minsk, Belarus

Manuscript received 20 October 2011Revised version received 18 February 2012Accepted 28 February 2012

Temperature induced structure evolution of zinc aluminosilicate glasses nucleated by TiO2 and doped with cobalt oxide was studied by small angle x-ray scattering, x-ray diffraction analysis, Raman and optical spectroscopy. According to the SAXS data, the phase separation by spinodal decomposition that occurred during glass melt cooling and anneal-ing proceeded upon further heat-treatments. Cobalt oxide facilitated amorphous phase separation of the parent glass that resulted in formation of a cobalt-doped amorphous zinc aluminotitanate phase and gahnite, ZnAl2O4, crystals. Cobalt-doped gahnite nanometre sized crystals were formed at temperatures as low as 720°C and remained the only crystalline phase if heat treatment temperature did not exceed 950°C. Absorption and luminescence properties of these glass-ceramics were defined mainly by tetrahedrally coordinated Co2+ ions located in ZnAl2O4 gahnite nanocrystals. The luminescence decay assigned to the transition from the 4T1(4P) to the 4A2 energy level of tetrahedrally coordinated Co2+ ion and absorption saturation under excitation of the 4A2Æ4T1(4F) transition were shown to depend on the Co concentration in the parent glass and to be independent of heat treatment conditions. The sources of unsaturable losses even after high temperature heat treatments were Co2+ ions remaining in the amorphous zinc aluminotitanate phase and in octahedral sites of inversed gahnite spinel.

* Corresponding author. Email [email protected], [email protected]

168 Physics and Chemistry of Glasses: European Journal of Glass Science and Technology Part B Volume 53 Number 4 August 2012

1200°C. Zinc aluminosilicate glass-ceramic samples prepared according to procedure described above will be referred to as Co:ZAS; a number in front of this symbol designates the content of CoO (in mol%) in the parent glass.

Polished samples with thickness of about 0·2 mm were investigated by small angle x-ray scattering (SAXS). The SAXS measurements were carried out with a home made instrument over a range from 6 to 450 arc minutes which corresponds to scattering vec-tor Q range from 8×10–3 Å−1 to 0·5 Å−1 where Q=(4π/λ)sin(φ/2) and λ is the x-ray wavelength and φ is the scattering angle. Cu Кα radiation with a Ni filter and an “infinitely” high primary beam was exploited. The level of monochromatization achieved enabled the intensity of the primary beam to be measured in absolute units by a scintillometer after attenuation by an absorber with a known attenuation factor and the mean square difference of electron densities, |(Δρ)2|, to be obtained.

For a monomodal system of scattering regions the angular dependence of the scattered intensity at small angles and asymptotic dependence at large angles are respectively given by

I Q NnQ R

( )≅ −

2

2 2

3exp g QRg <<1 (1)

I Q SQ

( )≅ −( )2 1 22

4π ρ ρ QRg>>1 (2)

where N is the number of scattering regions, n is the number of scattering electrons in the scattering region, and Rg is the radius of gyration. In the case of spherical scattering regions of radius R, R2=⁵⁄₃Rg

2. ρ1 and ρ2 are the mean electron densities of the in-homogeneous regions and the surrounding matrix, and S is the interfacial surface between the phases.

In the case of two-phase materials,

|(Δρ)2|=(ρ1−ρ2)2w1w2 (3)

where w1 and w2 are the volumes of the inhomo-geneous regions and of the surrounding matrix respectively. In the case of a multiphase system, the |(Δρ)2| value characterizes the inhomogeneity of the whole structure of the material.

The position of the maximum on SAXS curves and its change with the heat treatment temperature were measured. One of the reasons for its appearance is the regularity in a distribution of scattering regions in the glass volume. Then Q corresponding to the maximum on the curve, which is denoted Qm, and the distance between centres of regularly distributed particle are connected by the simple correlation:

L≅(3·3–3·5)×103Qm−1 (4)

where L is the distance in Ångstroms. The position of the maximum of the Q depend-

ence of QI(Q) corresponds to the inverse radius of

the scattering regions while the shift in its position reflects changes in this radius. For instance, in the case of monomodal system of scattering particles, when the Guinier equation is valid, the dependence between the radius of scattering regions, R, and the Qm value corresponding to maximum on the curve of QI(Q) versus Q(19) is given by:

R Q= −52

1m (5)

If there are several phases with mean sizes that differ substantially (by two times or even more), the curve of the angular dependence of QI(Q) can exhibit several maxima. Then intensity of scattering by large and small regions of inhomogeneity can be separated and the mean size of each type of inhomogeneous region and the SAXS intensity scattered by each frac-tion can be obtained from their maxima.

X-ray diffraction (XRD) patterns of powdered samples were measured with a Shimadzu XRD-6000 diffractometer using Cu Kα radiation with a Ni filter. The mean crystal sizes were estimated from broaden-ing of x-ray peaks according to the Scherrer equation:

D K=( )λθ θ∆ 2 cos

(6)

where λ is the wavelength of x-ray radiation, θ is the diffraction angle, Δ(2θ) is the width of peak at half of its maximum, and K is a constant, which was assumed to be 1.(20) Relative amounts of gahnite crystallized during heat treatments were estimated by measur-ing the integrated intensity of the main diffraction peak of gahnite around 2θ=36·8°, which corresponds to the reflection from the (311) plane. The unit cell parameter a was estimated from the position of the (440) plane of the gahnite crystal.

Absorption spectra of the samples in the wave-length range 340–2500 nm were recorded on 1 mm thick polished samples using a Shimadzu model UV 3600 recording spectrophotometer.

A filtered Xe-lamp producing wavelengths in the range 500–600 nm was used to excite luminescence. A CCD camera used as a detector and recorder. For the luminescence decay measurement, luminescence was excited using 10 ns pulses of the second harmonic (532 nm) from a Nd:YAG laser. The collected light was recorded with a fast Si diode and a digital oscil-loscope. The absorption saturation measurements were performed with the aid of passively Q-switched 1·54 µm Er3+:glass laser delivering pulses with pulse widths τp=75 ns. The transmission of the Co:ZAS samples was measured with two photodetectors. The input pulse energy was varied with a set of filters.

Raman spectra were recorded in backscattering geometry using a Renishaw 1000 Micro-Raman spectrometer equipped with TE cooled CCD camera. Ar+ CW laser line of 457 nm with power of 10 µW (1 µW on the sample) was employed as an excitation

I. AlekseevA et al: structure And propertIes of co-doped zInc AlumInosIlIcAte glAss-cerAmIcs


source. A Leica 50× (NA=0·71) objective was used to illuminate the sample; the scattered light was col-lected by the same objective. A notch filter was placed before the spectrograph entrance slit (width 70 µm). A spatial resolution of 2 cm−1 was obtained using a 1800 grooves/mm diffraction grating. The acquisition time was 120 s. Each sample was carefully positioned at the focus of the excitation laser beam, which resulted in a good reproducibility of the spectra and allowed the intensities of Raman bands in spectra of different samples to be compared.

Results

SAXS findingsThe dependence of SAXS intensity on Q for parent glasses with various CoO contents is shown in Figure 1. All parent glasses are nano-inhomogeneous, with 0·5Co:ZAS glass being the most inhomogeneous. In glasses doped with 0·5 and 1·0 mol% CoO there are two types of inhomogeneous regions significantly different in size (Table 1). The SAXS curve of the 2Co:ZAS parent glass is very similar to that of the non-doped glass, the inhomogeneous regions formed in these glasses have sizes smaller than in the two other glasses (Table 1). The inhomogeneity exhibited by the parent glasses is the result of incipient phase separation during melt cooling and annealing.

The selected portion of the plot of angular depend-ence of I(Q) for Q ranging from 8×10–3 Å−1 to 10−1 Å−1 (which corresponds to scattering angles from 7 to 80 arc minutes) is shown in Figure 1(b). Angular depend-ences of I(Q) exhibit maxima. Their appearance could be connected with interparticle interference, which in turn suggests a certain regularity in the distribu-tion of scattering regions in the glass volume. The position of the maximum is inversely proportional to the mean distance between regularly distributed centres of the scattering regions (see Equation (4)). The mean distance between these centres decreases with an increase of the doping ion concentration, as

shown in Figure 1(b), which implies that addition of CoO influences the structure of the parent glass.

Angular dependencies of SAXS intensity of the parent and heat treated glasses are shown in Figure 2. For 0·5Co:ZAS glass (Figure 2(a)) SAXS intensity increases after heat treatment at 720°C for 6 h, how-ever the position of the maximum practically does not change. Thus the structural changes of 0·5Co:ZAS glass during low temperature heat treatment are the result of a continuation of the phase separation that started during glass preparation. Both types of inhomogeneity, with large and small sizes, are developed in the previously formed regions. Figure 3(a) shows that the position of the maximum on the curve of QI(Q) versus Q at small Q shifts slightly to smaller values justifying the size increase of the cor-responding regions from 15 nm in the parent glass to 18 nm resulting in growth of the volume of phase separated regions. However, the main contribution to this increased volume of inhomogeneous regions comes from the precipitation of small (5·6 nm in size) scattering regions; the maximum on the QI(Q) curve

Table 1. Structural characteristics of parent and heat treated Co:ZAS glasses obtained by means of SAXS and XRD dataCoO Heat XRD, SAXS, |(Δρ)2| Position of thecontent, treatment 2R of 2R ×103 maximum on(mol%) scedule: gahnite (Å) (e/Å3)2 the angular T (°C), τ (h) (Å) dependence of ISAXS, Qm ×103, Å−1

0 Parent glass - 58 3 - 720, 6 - 60 14 19 900, 6 75 66 78 320·5 Parent glass - 150; ~50 7 11 720, 6 41 180; 56 11 10 900, 6 70 120; 61 100 121·0 Parent glass - 106, ~50 7 13 720, 6 44 64 17 21 900, 6 78 72 80 182·0 Parent glass - 60 3 - 720, 6 47 58 34 32 900, 6 70 58 91 35

Figure 1. Dependences of SAXS intensity I(Q) on Q for parent glasses. (a) Plotted on a log–log scale. The dashed line indicates the scattering slope of 1/3. (b) A portion of the plot for Q values from 8×10−3 Å–1 to 1×10−1 Å−1. The dashed line marks the position of the maxima on the I(Q) curves

10 100

10

100

1000

Q ⋅103 (A-1)

0.5Co:ZAS 1Co:ZAS 2Co:ZAS 0Co:ZASSA

XS in

tens

ity (a

rb. u

nits

) a

20 40 60 80 100

500

1000

1500

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2500

Q ⋅103 (A-1)

0.5Co:ZAS 1Co:ZAS 2Co:ZAS 0Co:ZAS

SAXS

inte

nsity

(arb

. uni

ts) b



corresponding to Qm≈6×10–2 Å−1 is clearly seen (Figure 3(a), Table 1).

The angular dependences of the SAXS intensity of the parent glass 0·5Co:ZAS subjected to prolonged heat treatments at 720°C are shown in Figure 4(a) in a log–log scale. The bimodality of structure is clearly seen for the sample heat treated just for 6 h as the bend in the I(Q) versus Q curve, which confirms the

formation of large and small size inhomogeneous regions. The increase of the heat treatment duration results in increasing the scattering intensity by both large and small size regions, with the intensity of scat-tering by small size regions being predominant, while intensity at small Q values increases to a lesser extent.

Figure 4(b) presents the QI(Q) dependences on Q for the same samples. On the plot for the sample heated for 6 h, there are two peaks corresponding

0 10 20 30 40 50 60 70 80 90 100

0

2000

4000

6000

8000

10000SA

XS in

tens

ity (a

rb. u

nits

)

Q ⋅103 (A-1)

900oC 0.5Co:ZAS

glass

720oC

a

0 10 20 30 40 50 60 70 80 90 100

0

2000

4000

6000

Q ⋅103 (A-1)

SAXS

inte

nsity

(arb

. uni

ts)

900oC

720oC

glass

1Co:ZAS b

0 10 20 30 40 50 60 70 80 90 100

0

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3000

4000

5000

Q ⋅103 (A-1)

2Co:ZAS

900oC

720oC

glass

SAXS

inte

nsity

(arb

. uni

ts) c

Figure 2. Dependences of SAXS intensity on Q for parent and heat treated glasses with compositions (a) 0·5Co:ZAS; (b) 1Co:ZAS; (c) 2Co:ZAS. The values indicated in the figure denote the heat treatment temperature. The dura-tion of heat treatments is 6 h. The lower curves are for the parent glasses

0

5000

10000

15000

Q ⋅102 (A-1)

QI(Q

) (ar

b. u

nits

)

0 6 12 18 24 30 36 42 48

glass

720oC

900oC 0.5Co:ZASa

0

5000

10000

15000

20000

Q ⋅102 (A-1)

900oC

720oC

glass

1Co:ZAS

QI(Q

) (ar

b. u

nits

)

0 6 12 18 24 30 36 42 48

b

0

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10000

15000

20000

25000

Q ⋅102 (A-1)

2Co:ZAS

900oC

720oC

0 6 12 18 24 30 36 42 48

QI(Q

) (ar

b. u

nits

)

glass

c

Figure 3. Dependences of QI(Q) on Q for parent and heat treated glasses (a) 0·5Co:ZAS; (b) 1Co:ZAS; (c) 2Co:ZAS. The values indicated in the figure denote the heat treatment temperatures. The duration of heat treatments is 6 h. The lower curves correspond to the parent glasses



to the intensities of scattering by large and small regions. The dashed line corresponds to the impact of small size regions on the total dependence of QI(Q) versus Q. It is seen from Figure 4(b) that increasing the duration of heat treatment leads to a significant increase in the scattering by small sized regions while scattering due to large sized regions changes only a little with increasing heat treatment duration. Within the accuracy of the method, the diameter of the large regions is equal to 160 Å and does not change with heat treatment duration. The sizes of small inhomo-geneous regions slightly increase from 54 to 59 Å with increasing heat treatment duration from 6 to 48 h and do not change with further increases of the duration up to 120 h (Table 2, Figure 5). The slight discrepancy in the structural characteristics of the 0·5Co:ZAS glass-ceramics (compare data in Tables 1 and 2) prepared at 720°C for 6 h by two different heat treatments is the result of the accuracy of the temperature control in the heat treatment furnace.

After heat treatment of the 0·5Co:ZAS glass at 900°C for 6 h, SAXS intensity increases significantly

for Q ranging from 3×10−2 to 9×10−2 Å−1 (Figure 2(a)) which shows that, as in the case of low temperature heat treatment, it is mainly determined by scattering by the small inhomogeneous regions about 6 nm in size (Table 1, Figure 3(a)). SAX intensity at smaller Q increases partly due to impact of scattering by these small regions and partly because of growth of large size inhomogeneous regions.

Figure 2(b) and (c) shows the angular dependences of I(Q) for heat treated 1Co:ZAS and 2Co:ZAS glasses. As a result of heat treatment, intensity of scattering increases at all angles. For 1Co:ZAS sample heat treated at 720°C for 6 h, bimodality shows itself by asymmetric in the curve of I(Q) versus Q (Figure 2(b)).

10 100

1

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48 h24 h

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(arb

. uni

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a

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QI(Q

) (ar

b. u

nits

)

Q ⋅102 (A-1)0 6 12 18 24 30 36

48 h

24 h

12 h

6 h

b

Figure 4. (a) Dependence of SAXS intensity on Q for 0·5Co:ZAS glass heat treated at 720°C for 6, 12, 24 and 48 h; (b) Dependence of QI(Q) on Q for 0·5Co:ZAS glass heat treated at 720°C for 6, 12, 24 and 48 h. The values indicated in figures denote the heat treatment duration. The dashed line corresponds to the impact of small size regions on the total dependence of QI(Q) versus Q

0 20 40 60 80 100 1200

20

40

60

80

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0.5Co:ZAS720 oC

Time (h)

XRD

inte

nsity

(arb

. uni

ts)

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a∆ρ

2SAXS (arb. units)

1

2

0 20 40 60 80 100 1200

20

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b

Time (h)

Gah

nite

dia

met

er (A

)

0

20

40

60

80

0.5Co:ZAS720 oC

Diameter of sm

all inhomogeneity regions (A)

1

2

Figure 5. (a) Variation of relative amount of gahnite crys-tallized (1) and of (Dr)2

SAXS of small inhomogeneous regions (2) with heat treatment duration at 720°C. (b) Variation of diameter of gahnite (1) and of small inhomogeneous regions (2) with heat treatment duration at 720°C

Table 2. Structure characteristics of 0·5Co:ZAS glass heat treated at 720°C for 6–120 h and at 900°C for 6 h and obtained by means of SAXS and XRD dataHeat treatment |(Δρ)2|×103 (e/Å3)2 SAXS, XRD,conditions Integrated By large By small diameter of diameter ofTemp. Time particles particles small gahanite (°C) (h) particles (Å) (Å)720 6 19 8·2 11·8 54 31±1720 12 30·7 8·5 22·2 56 32±1720 24 40·8 8·6 32·2 58 33±1720 48 52·4 8·9 43·5 60 40·5±1720 120 60 8·5 51·5 60 42±1900 6 100 merged 72 61±1



For 2Co:ZAS parent glass heat treated at 720°C for 6 h bimodality manifests itself by decreasing the effective size of regions of inhomogeneity due to appearance of regions of inhomogeneity with smaller sizes (Figure 3(c) and Table 1): 2R=6·0 nm in the parent glass, and 2R=5·8 nm in the sample heat treated at 720°C for 6 h.

The primary structure formed upon melt cooling and annealing influences the structure of samples prepared by high temperature heat treatment (see Figure 6). The bimodal structure is clearly seen in the SAXS curve of the 0·5Co:ZAS glass-ceramics. 1Co:ZAS glass-ceramic also exhibits a bimodal structure, but in this sample the sizes of large in-homogeneous regions, formed upon melt cooling and developed during heat treatments, and sizes of small regions which appeared in the course of heat treatments, are close to each other. In 2Co:ZAS glass-ceramics the sizes of the two types of inhomogene-ous regions are probably very similar (Figure 3(c), Table 1) and the shape of the curve of I(Q) versus Q is typical for small size scattering regions regularly distributed in the volume of the material (Figure 2(c)). Figure 2(c) shows that for the 2Co:ZAS sample, the position of the maximum of the curve of I(Q) versus

Q almost does not change with an increase of heat treatment temperature from 720°C to 900°C, i.e. the liquid phase separation and crystallization takes place at a constant distribution of scattering regions in the glass volume, in other words the number of scattering centres remains unchanged.

Figure 6(b) shows dependences of SAXS intensity on Q on a log–log scale for glasses heat treated at 900°C for 6 h. When Q>0·1, log[I(Q)] linearly depends on logQ with a scattering slope of 1/3, which corre-sponds to the angular dependence of scattering by an interface in the case of an “infinitely high” primary beam. It is worth noting that such a scattering curve slope was not observed in Figure 1, for the parent glasses even at highest scattering angles, which means that scattering by parent glasses at large angles is due to the appearance of small sized regions, and the lack a scattering curve slope of 1/3 is a proof of the existence of small particles in the parent glasses.

XRD findings

According to the XRD analysis data, all parent glasses with the exception of 5Co:ZAS are x-ray amorphous. In the parent glass doped with 5·0 mol% CoO gahnite precipitates (Figure 7, inset). Parent glasses were subjected to heat treatments at various temperatures between 680 and 1200°C. XRD patterns of 1Co:ZAS and 0·5Co:ZAS samples are given in Figure 7(a) and (b) as an example. Glasses heat treated below 720°C are completely x-ray amorphous. Very broad peaks of low intensity attributed to gahnite appear after heat treatment at 720°C for 6 h (Figure 7(a) and (b)). Their intensity gradually increases as the heat treatment duration increases to 120 h (Figure 7(b)). Simultaneously the first diffraction maximum on XRD patterns (“an amorphous halo”) decreases and moves to smaller angles (to the position of the diffrac-tion maximum of quartz glass) because in the course of liquid phase separation and gahnite crystallization the residual glass composition becomes gradually depleted in Zn, Al, Ti and Co oxides.

Gahnite crystallizes within the whole studied tem-perature range of 720–1200°C. With the increase of heat treatment temperature in the range from 720°C to 900°C the peak intensity increases and almost comes to saturation after heat treatments at 900 to 950°C and rapidly increases after heat treatments at temperatures above 1000°C. The gahnite mean size increases with an increase in the heat treatment temperature in a similar way for all samples under study. It changes from about 3 nm to 10 nm as the heat treatment temperature increases from 720 to 1000°C and drastically increases by almost ten times, to about 60–80 nm as the temperature rises to 1200°C. Prolonged heat treatment at 720°C from 6 to 120 h re-sults in a slow increase of the gahnite crystal size from 3·1 to 4·2 nm (Table 2, Figure 5). The glass-ceramics

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Figure 6. Dependences of SAXS intensity on Q for glasses heat treated at 900°C for 6 h: (a) linear scale; (b) log–log scale. The straight line marks the scattering slope of 1/3. The values indicated in the figure denote the CoO content



prepared at temperatures ranging from 720 to 1000°C are blue-green coloured and transparent. The samples subjected to heat treatments at higher temperatures become opaque. According to the XRD data, glass-ceramics prepared in the temperature range of 720 to 950°C were single phase gahnite samples. Rutile, the stable modification of TiO2, crystallizes starting from heat treatment at 1000°C; the higher the CoO doping level, the larger its amount and the mean crystal size.

Cristobalite appeared only in glass-ceramics doped with 2 mol% CoO during heat treatment at 1200°C (Figure 7(a)).

The gahnite unit cell parameter a was calculated for glasses heat treated at 950°C and 1000°C. The unit cell parameter of gahnite increased with CoO-doping level from 8·104 Å for 0·5Co:ZAS glass-ceramics to 8·107 Å for 1Co:ZAS glass-ceramics and to 8·120 Å for 2Co:ZAS glass-ceramics obtained at 950°C. After heat treatment at 1000°C the unit cell parameter monotoni-cally increased with CoO from 8·099 Å for 0·5Co:ZAS glass-ceramics to 8·100 Å for 1Co:ZAS glass-ceramics and to 8·107 Å for 2Co:ZAS glass-ceramics.

Raman spectroscopy study

Figure 8 shows Raman spectra of parent and heat treated glasses doped with CoO. Changes in Raman spectra of 1Co:ZAS parent glass with heat treatment temperature are shown in Figure 8(a), while changes with duration of isothermal heat treatment at 720°C are shown in Figure 8(b) for 0·5Co:ZAS samples. Variation in Raman spectra with CoO doping level are shown in Figure 8(c) for glasses heat treated at low temperature of 720°C, the spectrum of a non-doped sample is shown for reference. It is seen from Figure 8(a) that the gahnite band at 657 cm−1 appears in the Raman spectrum of the sample heat treated at 750°C, the second band of gahnite at 417 cm−1 appears after heat treatment at 850°C. In Raman spectra of all the samples heat treated from 720°С to 850°С there is a weak band at ~280 cm−1 assigned to one of the two most intensive bands of an anosovite solid solution, xR2O3.yTiO2 (R=A13+, traces of Ti3+ and Fe3+, x<<y), which is often formed at the beginning of crystal-lization of titania-doped lithium aluminosilicate glasses.(21) After heat treatment at 1000°C, rutile bands at ~235, 440 and 604 cm−1 are clearly seen in the spectrum. These findings are in agreement with the XRD data within the accuracy of the slightly different sensitivities of both methods. Raman spectroscopy is a sensitive tool to investigate the behaviour of amor-phous phases formed upon liquid phase separation. Broad bands at ~920 and ~800 сm−1 are assigned to vibrations of [TiO4] tetrahedra incorporated into the glass matrix(22) and to vibrations of Ti–O bonds in an amorphous aluminotitanate phase,(21) respectively. It is seen from Figure 8(a) that the intensity of the band at ~920 cm−1 characteristic of vibrations of [TiO4] tetra-hedra in the structure of the parent glass diminishes while intensity of the band at ~800 cm−1 increases with heat treatment temperature indicating formation of an amorphous zinc aluminotitanate (ZAT) phase (the band due to Ti–O vibrations in amorphous phase is superimposed by a weak band assigned to Si–O vibrations in the parent glass(23)).

The Renishaw 1000 Micro-Raman spectrometer allowed us to obtain qualitative data on variation of

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Figure 7. (a) X-ray diffraction patterns of parent and heat treated glasses 1Co:ZAS and 2Co:ZAS. The values indicated in the figure denote the heat treatment tempera-ture. The heat treatment duration is 6 h. The lower curve corresponds to the parent glass. The rhombuses, circles, and triangles denote the diffraction lines attributed to gahnite, ZnAl2O4, rutile, TiO2 and cristobalite, SiO2, respectively. Inset. X-ray diffraction patterns of the par-ent glasses. The values indicated in the figure denote the CoO concentration (mol%). The rhombuses denote the diffraction lines attributed to gahnite. (b) X-ray diffraction patterns of 0·5Co:ZAS glass heat treated at 720°C for 6, 12, 48 and 120 h, and at 900°C for 6 h. The rhombuses denote the diffraction lines attributed to gahnite. The values indicated in the figure denote the heat treatment temperature and duration. The dashed line indicates the position of the amorphous halo maximum. The patterns are shifted for clarity



integrated intensity of Raman bands at ~ 800 and ~900 cm−1 with heat treatment temperature. Figure 8(d) shows the data for non-doped samples and samples doped by 2 mol% CoO (for the intermediate CoO doping level, the corresponding dependences are situated inbetween the dependences shown in Figure 8(d)). The intensity of the band at ~800 cm−1 increases with heat treatment temperature indicating formation of amorphous ZAT phase. Only after high tempera-ture heat treatments at 950–1000°C does the intensity decrease due to crystallization of rutile (Figure 8(a)). It should be noted that the band at about 900 cm−1 has a different nature in the Raman spectra of samples heat treated at 670–750°C and at higher temperatures. In the Raman spectra of parent glasses and samples heat treated at 720–750°C there is the band at 920 cm−1 assigned to [TiO4] tetrahedra still remaining in

the silicate network, while for samples heat treated at 800–1000°C there is a new band with a frequency of ~940 cm−1. The appearance of this new band is also accompanied by the appearance of the band at 1115 cm−1 (which is not shown in Figure 8(a)). The degree of depolarization, ρ, of the band at ~940 cm−1 is dif-ferent from the degree of depolarization of the band at 920 cm−1. The band at 920 cm−1, connected with [TiO4] tetrahedra incorporated in the glass network, is totally polarized (ρ=0),(22) while the band at 940 cm−1 is depolarized (ρ~0·40). The degree of depolariza-tion of the band at 1115 cm–1 is close to zero. Similar bands with similar degrees of depolarization were found in Raman spectra of glasses of the SiO2–TiO2 system and assigned to either antisymmetric vibra-tions of Ti–O–Si bridges(24) or to vibrations of [TiO4] tetrahedra incorporated into SiO2 (Refs 9 and 10 from

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Figure 8. (a) Raman spectra of parent and heat treated 1Co:ZAS glass. The values indicated in the figure denote the heat treatment temperature. The holding time is 6 h. The rhombuses and filled circles denote the bands due to gahnite and rutile, respectively. (b) Raman spectra of 0·5Co:ZAS glass heat treated at 720°C for 6–24 h. (c) Raman spectra of glasses heat treated at 720°C for 6 h. The values indicated in the figure denote the CoO concentration (mol%). (d) Variation of integral intensity of Raman bands at ~800 cm−1 (1, 2) and at ~900 cm−1 (1’, 2’) with heat treatment temperature. The holding time is 6 h. The values indicated in the figure denote the CoO concentration (mol%)



Ref. 25). Thus we suggested that the high frequency bands in the Raman spectra of heat treated glasses were connected with vibrations of [TiO4] groupings in the residual highly siliceous glass, which is in good agreement with XRD findings of cristobalite precipitation at 1200°C from only 2Co:ZAS glass, which according to our assumption contains the smallest amount of [TiO4] units (see Figure 7(a)).

Figure 8(b) shows the Raman spectra of the 0·5Co:ZAS glass heat treated at 720°C for 6–24 h. It shows that weakening of the Raman band at ~920 cm−1 connected with removal of titanium from the parent glass network correlates with growth of the band at 800 cm−1 due to formation of the amorphous ZAT phase. When heat treatment temperature increases from 6 to 24 h, intensity of the latter band increases by approximately a factor of two.

Data presented in Figure 8(c) and (d) show the influence of cobalt oxide addition on phase separa-tion of parent glasses at a heat treatment temperature of 720°С. According to the Raman spectra shown in Figure 8(c), the higher the CoO doping level, the lower the intensity of the band at ~920 cm−1 and the higher the intensity of the band at ~800 сm−1. The qualitative data on intensity of both bands at different heat treatment temperatures (Figure 8(d)) reflects the substantial growth of the Raman band at ~800 сm−1, assigned to the amorphous ZAT phase, and the simul-taneous decrease of intensity of the band at ~900 сm−1 reflecting titanium removal from the glass network, with the CoO doping level. Figure 8(c) shows that there is a simultaneous increase of the weak band assigned to vibrations of the glass network. In the spectrum of non-doped glass this band is located at ~460 cm−1. In spectra of samples doped with CoO it shifts to ~435 cm−1, a position close to that for quartz glass, while its intensity increases because of the decomposition of the glass network with formation of the amorphous ZAT phase and gahnite.

Optical absorption and luminescence study

Figure 9(a) shows optical absorption spectra of the 0·5Co:ZAS parent glass samples has broad and unresolved absorption bands at around 500 and 600 nm and a very broad band in the near IR at around 1000–500 nm. Cobalt ions are commonly found in glass as divalent Co2+,(1) and these bands are attrib-uted to six-coordinated and four-coordinated Co2+ ions, respectively.(1,2) With increasing heat treatment temperature, the absorption spectrum gradually change; both visible and IR absorption bands become narrower and more intense: the intensity of absorp-tion at 500 nm weakens, and a new band appears at about 550 nm while the intensity of the band with a maximum at 590 nm increases and the band at about 640 nm shifts slightly to lower wavelengths indicating that octahedrally coordinated ions in glass

move to tetrahedral sites during crystallization. The intensity of the band in the IR region increases, and five maxima at 1250, 1300, 1400, 1430 and 1500 nm can be distinguished in the broad absorption band. The intensity of absorption bands reaches maximum after heat treatment at 1000°C. The absorption spectrum of glass-ceramics bears a great resemblance to the absorption spectrum of single crystal gahnite doped with CoO.(26) We attributed the observed bands to the 4A2Æ4T1(4P) and 4A2Æ4T1(4F) transitions of tetrahe-drally coordinated Co2+ ions. The inset in Figure 8(a) shows a schematic energy level diagram for a Co2+ ion (electronic configuration d7) in a tetrahedral crystal field (Td). The structure of absorption bands observed near 590 nm and 1400 nm is the result of splitting of the 4T1(4P) and 4T1(4F) states in a lower symmetry crystal field that is characteristic of the Co:ZnAl2O4 single crystal.(26) The similarity of absorption spectra

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Figure 9. (a) Absorption spectra of parent and heat treated 0·5Co:ZAS glass. The values indicated in the figure de-note the heat treatment temperature. The heat treatment duration is 6 h. The lower curve corresponds to the parent glass. Inset shows the schematic energy level diagram of Co2+ ion in ZnAl2O4 crystal in Td symmetry. (b) Absorp-tion spectra of 0·5Co:ZAS parent glass and samples heat treated at 720°C. The values indicated in the figure denote the heat treatment duration. The lower curve corresponds to the parent glass



of Co:ZAS glass-ceramics and of Co:ZnAl2O4 single crystal implies that after heat treatments of the Co:ZAS glass Co2+ ions replace Zn2+ ions in the gahnite

nanocrystals and occupy tetrahedrally coordinated sites in them. Figure 9(b) shows the variation of the absorption spectrum of 0·5Co:ZAS parent glass with heat treatment duration at 720°C. The gradual change in the absorption spectrum is connected with the precipitation of Co-doped gahnite nanosized crystals (see Figure 9(b)).

Figure 10(a) shows evolution of the luminescence in the spectral range of 600–700 nm with heat treat-ment temperature for 1Co:ZAS glass-ceramics. The luminescence spectrum of the parent glass (not shown here) is different in shape and about 100 times less intense than the spectra of heat treated glasses. The luminescence properties of these glass-ceramics are defined mainly by tetrahedrally coordinated Co2+ ions located in ZnAl2O4 gahnite nanocrystals.(27) The increase of the luminescence intensity with heat treatment temperature can be assigned to increasing the cobalt-doped gahnite fraction crystallized in the glass-ceramics. Figure 10(b) shows the influence of Co2+ concentration on the luminescence spectra of the samples heat treated at 1000°C for 6 h. The spec-trum has an intense band at 650 and a less intense band centred at 880 nm. The fluorescence intensity increases with increasing Co2+ ion concentration. Interestingly Duan et al(28) in their study of optical properties of the Co2+:ZnAl2O4/SiO2 nanocomposite glass prepared by sol-gel reported the opposite dependence of the luminescence intensity on CoO concentration and attributed this phenomenon to concentration quenching.

Figure 10(c) shows the decay of the integral lumi-nescence signal for Co:ZAS samples with different CoO concentrations. For all the samples the decay is not mono-exponential. The characteristic times of luminescence decay as the time of decrease to the 1/e level τ1/e were estimated and values of 70, 40 and 25 ns were obtained for Co:ZAS samples with CoO contents in the parent glasses of 0·5, 1·0 and 2·0 mol%, respectively. These values are independent of heat treatment temperature and duration (Figure 10(c), inset). The shortening of the luminescence decay time can be connected with concentration quenching of the Co2+ ions in gahnite crystals because the fraction of gahnite crystallized at heat treatments temperatures above 720°C is practically independent of the CoO content while absorption of Co2+ ions in gahnite increases with CoO doping level, which implies that concentration of Co2+ ions in gahnite crystals increases with the CoO content.

Fergusson et al(26) demonstrated that luminescence from the Co:ZnAl2O4 single crystal in the visible region at liquid helium temperatures exhibited a sharp line structure and millisecond decay time. This luminescence band was attributed to the transition from the metastable 2E(2G) energy level to the 4A2 level of tetrahedrally coordinated Co2+ ions.(26) In our case the broad luminescence bands near 650

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Figure 10. (a) Luminescence spectra of 1Co:ZAS glass-ceramics under excitation by the 10 ns laser pulses at 532 nm. The values indicated in the figure denote the heat treatment temperature. The heat treatment duration is 6 h. (b) Luminescence spectra of Co:ZAS samples with various concentrations of Co2+ ions heat treated at 1000°C for 6 h under excitation by 10 ns laser pulses at 532 nm. The values indicated in the figure denote the CoO con-centration (mol%). (c) Decay of integrated luminescence signal for these glass-ceramics. The values indicated in the figure denote the CoO concentration (mol%). Inset shows variation of decay for 1Co:ZAS glass-ceramics with heat treatment temperature



and 880 nm with nanosecond decay were attributed to the electric dipole allowed transitions from the thermally populated 4T1(4P) level (Figure 9(a), inset). Similar emission features were observed for tetra-hedrally coordinated Co2+ ions in magnesium spinel single crystal(29) and magnesium aluminosilicate glass-ceramics.(30) Recently Ha et al(30) gave a similar interpretation for the origin of similar features in the emission spectra of ZnAl2O4:(Co2+) powders prepared by hydrothermal method.

For Co:ZAS glass-ceramics τ1/e was found to be independent of temperature and duration of heat treatment and this independence confirms the above suggestion that the concentration of Co2+ ions in tetrahedral coordination sites in gahnite nanocrystals does not change under increasing heat treatment temperature and duration.(27) So the increase in ab-sorption of tetrahedrally coordinated Co2+ ions that was observed in the optical absorption spectra (Figure 9) with heat treatment is caused only by the increase in the quantity of gahnite phase and the increase in the amount of Co2+ ions in it, but the concentration of Co2+ ions in gahnite nanocrystals remains constant.

Figure 11 shows absorption saturation under exci-tation of the 4A2Æ4T1(4F) transition of the 0·5Co:ZAS and 1·0Co:ZAS samples heat treated at different temperatures. As one can see, samples with the same CoO concentration have the same level of residual absorption, although they were heat treated at dif-ferent temperatures. The value of residual losses for samples is proportional to the content of cobalt ions in the parent glasses.

Discussion All parent glasses with the exception of the 5Co:ZAS glass were x-ray amorphous and inhomogeneous. The bimodal structure, i.e. the appearance of two kinds of inhomogeneous regions with different com-positions and sizes, develops in parent glasses upon cooling the melt and annealing of the glass as a result of phase separation. Depending on the cooling rate of the glass melt and the rate of phase decomposition, the volume and composition of the inhomogeneous regions formed in the parent glasses correspond to different stages of phase decomposition. At early stages of phase decomposition these regions can only be enriched in some components of the glass melt and their composition and volume are far away from the equilibrium values. However there is certain regular-ity in distribution of these inhomogeneous regions, which is proved by the appearance of maxima in the angular dependences of SAXS intensity of the parent glasses. The maximum on the SAXS curve means that phase separation develops by spinodal decomposition.(32) Variation of the Co-doping level influences the structure of the parent glass in such a manner that the glass with the lowest concentra-tion of 0·5 mol% CoO has the most inhomogeneous structure. In parent glasses Co2+ ions have octahedral and tetrahedral coordination, which is typical of alu-minosilicate glasses and was observed previously in glasses of the magnesium and lithium aluminosilicate systems.(30,33–35) After the low temperature heat treat-ment at 720°C the bimodal structure continues to evolve in the previously formed large and small size regions. The sizes of gahnite crystals precipitated during the heat treatment at 720°C are much smaller than the sizes of large inhomogeneous regions and similar to the sizes of the small inhomogeneous re-gions (Table 1, Figure 5(b)). The relative quantity of gahnite crystallized vary as shown by the values of (Δρ)2

SAXS of the small inhomogeneous regions when heat treatment duration increases (Figure 5(a)). This implies that gahnite precipitates from small sized re-gions that are enriched in ZnO and Al2O3. This result is in good agreement with the previously identified structural peculiarities of phase separation in ZAS glasses with no doping ions.(36) Then the large sized amorphous phase should be the ZAT phase. The re-sults of the Raman spectroscopic study also confirms the formation of two phases, crystalline phase in the form of gahnite (characteristic gahnite Raman bands are located at 420 and 660 cm−1) and an amorphous ZAT phase (the corresponding Raman band is at ~800 cm−1). For samples with different concentrations of CoO heat treated at 720°C for 6 h, the mean square difference of electron densities of inhomogeneous regions and surrounding glass matrix increases with CoO doping level (Table 1). These data are in agreement with XRD and Raman spectroscopic find-ings of an increased fraction of gahnite crystals and

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Figure 11. Absorption saturation for (a) 0·5Co:ZAS, (b) 1·0Co:ZAS samples heat treated at 750°C ( ), 850°C ( ) and 1000°C (◦) for 6 h. The values indicated in the figure denote the heat treatment temperature



of increased integral intensity of the Raman band at 800 cm−1 assigned to the amorphous ZAT phase with increasing CoO content in the parent glasses subjected to low temperature heat treatment at 720°C. The important role that the small inhomogeneous regions play in the total SAXS scattering is explained by the crystalline nature of these regions. As soon as gahnite crystals start to precipitate, the absorption spectra of the samples change, which testifies that Co2+ ions leave the aluminosilicate network and enter gahnite crystals. The higher the concentration of CoO, the more pronounced this change (Figure 12(a)). Thus at low temperature heat treatments liquid phase separation and gahnite crystallization is facilitated by increasing CoO doping level. Previously we prepared CoTiO3 polycrystals and measured their reflectance spectrum(35) in the visible spectral range. The spectrum contains a broad band in the range from 520 to 650 nm with maxima at 530, 580 and 650 nm and a band at 470 nm assigned to absorption of tetrahedrally coordinated Co2+ ions. We suggest that Co2+ ions have a similar absorption spectrum in the amorphous ZAT phase. A small contribution of this phase to the absorption spectrum can go unnoticed due to the amorphous nature of this phase and the overlap of its bands with Co:gahnite bands. Thus cobalt ions participate in formation of both phases, gahnite and the amorphous ZAT phase and facilitate the phase transformations during the early stages of heat treatment.

According to the SAXS data, continuous precipi-tation of gahnite nanosized crystals determines the structure of glass-ceramics in the main. Very slight increase of gahnite crystal sizes after heat treatment at 720°C for 6–24 h and the considerable increase of the amount of precipitated gahnite is evidence of the continuation of the gahnite nucleation at this temperature. An increase in the mean square differ-ence of electron densities |(Δρ)2| with increasing heat treatment time from 6 to 24 h is the result of increasing both the number of small size regions and of fraction of gahnite crystals within these regions. The fact that gahnite crystallization is accompanied by a gradual variation in optical absorption spectra due to Co2+ ions entering the gahnite crystals proves that gahnite crystals are not formed on centres enriched in cobalt ions, i.e. on CoAl2O4 nuclei, but that the concentration of Co2+ ions gahnite crystals does not change with heat treatment duration.

With further temperature increases the structural evolution continued: the large size regions of amor-phous ZAT phase were formed at all heat treatments up to 1000°C, the temperature at which rutile started to crystallize causing decomposition of the ZAT phase. Co-doped gahnite continuously crystallized in the whole temperature range of heat treatment (720–1200°C). Absorption and luminescence spectra of glass-ceramics were typical of Co2+ ions substi-

tuting for Zn2+ ions in gahnite crystals. However glass-ceramics demonstrated non-saturable losses. Their sources should be some additional absorbing centres other then tetrahedrally coordinated Co2+ ions in a crystalline phase. Previously we suggested that such centres can be formed in the amorphous ZAT phase and in the residual glass phase as a small por-tion of the octahedrally coordinated Co2+ ions could probably remain in the residual glass phase of glass-ceramics prepared at all heat treatment temperatures. This is because one cannot exclude the possibility that the band with a maximum at 520 nm is concealed in the region of superposition of the absorption edge and the left edge of the absorption band in the visible part of the spectrum (Figure 9).

Recently Duan et al(28) measured the variation of the lattice parameters of gahnite in Co2+:ZnAl2O4/SiO2 nanocomposite glass prepared by sol-gel with CoO doping level. The parameters were found to increase with increasing CoO content. Similar dependences were obtained in our study. A gradual expansion of the gahnite unit cell, unexpected on the basis of

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0.5% CoO

Figure 12. Absorption spectra of Co:ZAS glasses heat treated at 720°C (a) and at 950°C (b) for 6 h. The values indicated in the figure denote the CoO concentration (mol%). The dashed lines mark positions of the maxima of the absorption spectra of glasses with different concen-trations of CoO



Zn2+ and Co2+ ionic radii, can be explained either by a suggestion of a partly inversed structure in Co-doped gahnite crystals(37) or attributed to a change of the covalent character of M–O bonding.(10,11) However the last explanation seems not to fit our data as the position of absorption bands of Co-doped glass-ceramics does not change with CoO doping level (Figure 11(b)), which would be the case if the covalent character of Co–O bond were changed.(1) The structural formula of spinel can be expressed as IV[A1−δBδ]VI[B2−δAδ]X4, where IV[] and VI[] represent the tetrahedral and octahedral sites, respectively; δ is called the inversion parameter; it reflects the degree of disorder. There are two ordered configurations of cations in spinels; the normal spinel with δ=0; and the inverse spinel with δ=1. Gahnite possesses the normal spinel structure(38) while the structure of CoAl2O4 is partly inversed.(39,40) It was taken for granted that in gahnite crystals Co2+ ions substitute only for Zn2+ ions. However, recently Popović et al(37) proved that the increase of the unit cell parameters of gahnite with the CoO doping level is an indication of structure inversion in gahnite due to CoO doping, when a part of Co2+ ions substitute for Zn2+ ions on tetrahedral cation sites, and the remaining Co2+ ions substitute for Al3+ ions on octahedral cation sites in the gahnite lattice, causing an increase in the unit cell parameter. Interestingly as early as 1969, Ferguson et al(26) showed that an appreciable fraction of Co2+ must be present in the octahedral sites in gahnite single crystals, however, little attention was paid to this information. Absorption of octahedrally coordinated Co2+ ions cannot be observed in the absorption spec-trum of Co-doped gahnite glass-ceramics because the intensity of absorption for this ion would be expected to be at least one or two orders of magnitude less than for tetrahedral Co2+ ions.(1,26)

Conclusions

Temperature induced structure evolution of zinc aluminosilicate glasses nucleated by TiO2 and doped with cobalt oxide was studied by SAXS, XRD, Raman and optical spectroscopy methods.

According to the SAXS data, the phase separation by spinodal decomposition that occurred during the glass melt cooling and annealing proceeded upon further heat treatments. Bimodal structures developed in all parent glasses with heat treatment, large size inhomogeneous regions were assigned to amorphous ZAT phase while small size regions were ascribed to gradually crystallized gahnite, ZnAl2O4. Cobalt oxide was observed to facilitate amorphous phase separation of the parent glass and to enter both phases with the formation of Co-doped amorphous zinc aluminotitanate phase and Co-doped gahnite nanosized crystals.

Absorption and luminescence properties of these

glass-ceramics were defined mainly by tetrahedrally coordinated Co2+ ions located in ZnAl2O4 gahnite nanocrystals. The luminescence decay assigned to the transition from the 4T1(4P) to the 4A2 energy level of tetrahedrally coordinated Co2+ ions and absorption saturation under excitation of the 4A2Æ4T1(4F) transi-tion were shown to depend on the Co concentration in the parent glass and to be independent of heat treatment conditions.

The sources of unsaturable losses even after high temperature heat treatments were Co2+ ions that re-mained in the amorphous zinc aluminotitanate phase and in octahedral sites of the inverse gahnite spinel.

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