Extended X-ray Absorption Fine Structure (EXAFS) Analysis of Zirconium-Doped Lithium Silicate/Borate...

X-ray absorption spectroscopy of doped ZrO2 systems

S. Basu,1 Salil Varma,2 A. N. Shirsat,2 B. N. Wani,2 S. R. Bharadwaj,2 A. Chakrabarti,3

S. N. Jha,1 and D. Bhattacharyya1,a)1Applied Spectroscopy Division, Bhabha Atomic Research Centre, Mumbai – 400 094, India2Chemistry Division, Bhabha Atomic Research Centre, Mumbai – 400 094, India3Indus Synchrotron Utilisation Division, Raja Ramanna Centre for Advanced Technology,Indore- 452013, India

(Received 29 August 2011; accepted 9 February 2012; published online 14 March 2012)

ZrO2 samples with 11% Nd and La doping and with 7, 9, 11, and 13% Gd doping have been

prepared by co-precipitation route followed by sintering at 700 �C and 1100 �C, for potential

application as high conductivity electrolytes in solid oxide fuel cells. The samples have been

characterized by x-ray diffraction with laboratory x-ray source of Cu Ka radiation and extended

x-ray absorption fine structure (EXAFS) spectroscopy measurement at Zr K edge with synchrotron

radiation. The XRD spectra have been analyzed to determine the structure of the samples and the

EXAFS data have been analyzed to find out relevant local structure parameters of the Zr-O and

Zr-Zr shells, viz., bond distances, co-ordinations, and disorder parameters. The effect of change

in ionic radius as well as concentration of the dopants on the above parameters has been

thoroughly studied. The experimental results, in some cases, have also been corroborated by first

principle calculations of the energetics of the systems. VC 2012 American Institute of Physics.

[http://dx.doi.org/10.1063/1.3693470]

I. INTRODUCTION

Zirconia (ZrO2) has been studied extensively as a prom-

ising material due to its diverse applications, viz., as oxygen

sensor, refractory material, bioceramics, thermal barrier

coating, etc.1,2 Zirconia powders are also very important in

catalytic applications, either as supports or as electrolytes,

especially in solid oxide fuel cells (SOFCs).3,4 Pure ZrO2 is

monoclinic from room temperature to 1440 K, tetragonal

between 1440 and 2650 K and cubic up to its melting point

of 2950 K.5 It is well known that zirconia exhibits high ani-

onic conductivity when doped with aliovalent cations which

initiates the generation of oxygen ion vacancies for charge

compensation.6 Doping in this manner also stabilizes the

cubic fluorite phase of zirconia having relevant ionic conduc-

tivity, as a solid electrolyte, at the operating temperatures of

the solid oxide fuel cell. Yttria-stabilized zirconia (YSZ) typ-

ically with 8 mol% Y doped in ZrO2, is the most common

material used in SOFCs. The major limitation to the use of

YSZ arises from the need to operate at relatively high tem-

peratures of around 1100–1300 K to achieve adequate ionic

conduction. The high temperatures of operation lead to mate-

rial stability and compatibility issues. This has prompted the

search for alternate materials with equivalent ionic conduc-

tivity at lower temperature to enable operation of solid oxide

fuel cells at intermediate temperatures of 900–1000 K.

The YSZ system has been thoroughly studied in every

form, bulk, surface, films, and nanoparticles.7–11 On the con-

trary, reports on zirconia doped with ions, other than Y3þ,

are few and not very extensive.6,12–16 There have been few

studies on the x-ray absorption spectroscopy of doped ZrO2

systems also where efforts have been given to have insight

into the mechanism of creation of oxygen vacancies and on

their locations within the ZrO2 matrix, though there are con-

tradictory information in the literature regarding the same.

Sadykov et al.12 have observed for their Gd/La doped

CeO2-ZrO2 nanocrystalline systems that with increase in

Gd concentration, the oxygen coordination around Zr/Ce

increases while disorders at oxygen sites decrease. This

implies that with Gd doping oxygen vacancies are mainly

created near the dopant site. However, with La doping Zr-O

bonds get contracted and oxygen coordination decreases

around Zr atoms implying that for doping with over-sized

cations like La, oxygen vacancies lie near the Zr sites. Zacate

et al.6 by atomistic simulation have estimated the binding

energies of the oxygen vacancies with the cations in doped

ZrO2 systems and found that for smaller size dopants binding

energies of oxygen vacancy is more for a nearest neighbor

configuration of the dopant and for large size dopants the

binding energy is more for next nearest neighbor configura-

tion, which implies that for ZrO2 doped with cations having

smaller size than Zr, oxygen vacancies will reside near the

dopant and for cations having larger radius than Zr, vacan-

cies would reside near Zr atoms. Li and Chen13 have also

investigated the changes in Zr-O environment by EXAFS

measurements on their Gd, Y, Fe, and Ga doped systems and

concluded the same. However, Cole et al.14 have observed

for their cryochemically-prepared doped ZrO2 systems that

the Zr-Zr peak intensity increases and the disorder term

decreases as the size of the dopant cation changes from Er to

Gd to La which shows that the central Zr atoms has less

perturbation for doping with larger size atoms. They have

concluded that when the dopant size is close to that of Zr,

oxygen vacancies prefer to be located near Zr sites while for

larger size dopants the oxygen vacancies lie close to the

dopant site.a)Electronic mail: [email protected].

0021-8979/2012/111(5)/053532/9/$30.00 VC 2012 American Institute of Physics111, 053532-1

JOURNAL OF APPLIED PHYSICS 111, 053532 (2012)

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Thus, it can be seen from the above discussion that contra-

dictory reports are available in the literatures regarding the pos-

sible positions of oxygen vacancies created in ZrO2 matrix

when doped with different tri-valent ions. We have looked into

the above issue again mainly using the results of synchrotron

based EXAFS measurements corroborated by x-ray diffraction

data and also by theoretical simulations to some extent. In the

present study, two sets of samples have been investigated by

the EXAFS technique at the Zr K-edge, viz., (i) zirconia doped

with 11% mol of Gd3þ, Nd3þ, La3þ, where the dopant cations

are in the order of increasing ionic radii and (ii) zirconia doped

with 7, 9, 11, and 13 mol% of Gd. By analyzing the EXAFS

data, we seek the effect of size and concentrations of the dopant

on the local structure around the Zr sites.

II. EXPERIMENTAL SECTION

Wet chemical route provides simple method to synthe-

size different classes of compounds with preset properties

and structure.17 The rare earth substituted ZrO2 samples dis-

cussed in this study were prepared by co-precipitation route.

In the above process, a stoichiometric solution was first pre-

pared by mixing appropriate concentrations of ZrO(NO3)2and the nitrate of the dopant metal (viz., Gd(NO3)3 for Gd

doped samples). This solution was then added directly to am-

monia solution with constant stirring. The precipitate, which

consists of homogenous mixture of hydroxides of Zr and the

dopant metal, was allowed to settle down and was subse-

quently filtered and washed with distilled water. The precipi-

tate was dried at 100 �C and the samples were prepared by

calcining the dried powder at 700 �C, which is the minimum

temperature, required for obtaining cubic phase of ZrO2.

However, to test the stability of the samples at higher operat-

ing temperature of SOFC devices, another set of samples

were prepared by sintering at 1100 �C for 24 h. As has been

mentioned above, ZrO2 samples have been prepared with

11% doping of Nd and La and with 7, 9, 11, and 13% doping

of Gd (doping concentration is defined by the expected con-

centration of oxides of the respective dopants in the ZrO2

matrix). It should be mentioned here that during preparation

of the samples, precursors are added to excess ammonia so-

lution which ensures complete coprecipitation of all the

involved cations and allows us to assume that the chemical

composition of the powders follows the composition of the

initial aqueous solution.

X-ray diffraction (XRD) spectra of the samples were

recorded using a Philips X-ray Diffractometer (PW 1710)

with Ni filtered Cu Ka radiation. Identifications of the XRD

peaks have been carried out using the standard powder dif-

fraction files of x-ray diffraction data and the exact values of

the lattice parameters were generated by refining the spectra

using the POWD code.18

The extended x-ray absorption fine structure (EXAFS)

measurements on the above mentioned pellets were carried

out at the dispersive EXAFS beamline (BL-8) at the INDUS-

2 Synchrotron Source (2.5 GeV, 100 mA) at the Raja Ram-

anna Centre for Advanced Technology, Indore, India.19–22

The above beamline uses a 460 mm long Si (111) crystal

mounted on a mechanical crystal bender which can bend the

crystal to the shape of an ellipse. The crystal selects a partic-

ular band of energy from white synchrotron radiation

depending on the grazing angle of incidence of the synchro-

tron beam (Bragg angle) and disperses as well as focuses the

band on the sample. The radiation transmitted through the

sample is detected by a position sensitive CCD detector having

2048 pixels. The whole absorption spectrum can be recorded

simultaneously on the detector within fraction of a second. The

beamline has a resolution of 1 eV at the photon energies of 10

keV. The plot of absorption versus photon energy is obtained

by recording the intensities I0 and It on the CCD, without and

with the sample, respectively, and using the relation, It ¼I0e

ÿlt where l is the absorption coefficient and t is the thick-

ness of the absorber. For the present experiment the crystal has

been set at the proper Bragg angle so that a band of energy is

obtained around Zr K edge of�17 998 eV.

Samples of appropriate weights, estimated to obtain a

reasonable edge jump, have been taken in powder form and

have been mixed thoroughly with cellulose powder to obtain

total weight of approximately 150 mg and 2.5 mm thick ho-

mogenous pellets of 12.5 mm diam have been prepared using

an electrically operated hydraulic press. EXAFS spectrum of

a commercial YSZ sample has been used for calibration of

the CCD channels, where both the Zr and Y edges appeared

in the same spectrum at CCD channel nos. 253 and 460,

respectively, and assuming the reported values of Y K-edge

of 17 046 eV and Zr K edge of 18 014 eV in YSZ, the CCD

channels were calibrated in energy scale.23

III. RESULTS AND DISCUSSION

Figure 1 shows the x-ray diffraction spectra of ZrO2

samples with 11 mol% doping of La, Gd, Nd as synthesized

by coprecipitation route and heated for 24 h at 700 �C. XRD

peaks for all the samples are found to match with that of

cubic phase of yttria stabilized zirconia (JCPDS 30-1468),

implying that the cubic phase of zirconia has been stabilized

in these samples. Crystallite sizes in the samples have been

estimated from the broadening of the XRD peaks using the

Scherrer equation24 and are shown in Table I. The samples

prepared by co-precipitation route are found to have 22-25

nm particle size. XRD spectra of the samples sintered at

1100 �C have been shown in Fig. 2. It has been observed

that, while Nd and La substituted samples exhibit partial

transition to tetragonal and monoclinic phases, respectively,

the Gd doped sample retains the distorted cubic phase with

slight broadening of XRD peaks at �35� and 60� 2h value.

Splitting of these peaks is indicative of presence of tetrago-

nal phase. The overall data for the phases present in the three

types of samples after sintering at two different temperatures

are summarized in Table I. It should be noted that trace

amounts of monoclinic phases of ZrO2 are also present in the

Gd and Nd doped samples characterized by the presence of

small peaks at 2h � 28.2� and 31.5� in their XRD spectra

(Fig. 2). Thus, it can be seen from the above study that low

temperature synthesized rare earth substituted zirconia

showed cubic phase which on heating above 1100 �C get

converted to tetragonal and/or monoclinic phase. In case of

Gd doped sample, cubic phase could be stabilized to a

053532-2 Basu et al. J. Appl. Phys. 111, 053532 (2012)


greater extent as compared to other two substituted samples.

This could be due to identical ionic radii of Y3þ (1.02 A)

and Gd3þ (1.05 A), which also displays similar behavior.

The ionic size of La3þ (1.16 A) and Nd3þ (1.11 A)25 being

bigger, stabilization of cubic phase was not possible at sin-

tering temperatures in excess of 1100 �C. The appearance of

different other phases in samples sintered at 1100 �C has fur-

ther been investigated by EXAFS measurements.

XRD spectra of Gd doped ZrO2 samples with varying

concentration of gadolinium (7–13 at. %) sintered at

1100 �C, for longer period of 48 h, have been shown in Fig.

3. The sintering time has been increased to explore the gene-

sis for the peak broadening observed for above mentioned

Gd doped samples. All the samples exhibit phases similar to

distorted cubic phase (tetragonal) as evident from splitting of

XRD peaks at �35� and 60� 2h value. As mentioned above,

the XRD spectra for Gd doped samples were indexed and

refinement of the spectra have been carried out employing

the POWD code to generate the lattice parameters and the

cell volumes. It should be noted that in the present work,

Rietveld refinement has not been carried out and the index-

ing and refinement have been limited only to the lines per-

taining to the major phases. The data obtained for the

different samples have been summarized in Table II and

have also been shown in Fig. 4 as a function Gd doping con-

centration. Lattice parameter a0 is found to increase with

increasing concentration of Gd, while c0 remains constant

around 5.17 A for doping up to 11% of Gd and reduces to

5.16 A for 13% of Gd doping. The cell volume increases

monotonically with increase in Gd concentration of up to

11% Gd followed by a sharp increase for 13% of Gd. The

sharp increase in a0 and cell volume and abrupt decrease in

c0 value for 13% Gd doping, suggests toward a shift in stabi-

lization of lattice symmetry to cubic phase for higher doping

percentage. Thus, the XRD spectrum of 13% doped sample

has been indexed and refined again with cubic symmetry

(JCPDS 30-1468) having a0 value of 5.1513 6 0.0033,

though with lower figure of merit showing presence of mixed

phases which has been further investigated by probing the

local structure of the samples by EXAFS technique.

Figure 5 shows a representative experimental EXAFS

(lðEÞ versus E) spectrum of ZrO2 sample doped with 11

mol% of La and sintered at 1100 �C. In order to take care of

the oscillations in the absorption spectra, the energy depend-

ent absorption coefficient l(E) has been converted to absorp-

tion function v(E) defined as below,26

vðEÞ ¼lðEÞ ÿ l0ðEÞ

Dl0ðE0Þ; (1)

TABLE I. Crystallite size and phase identification after sintering at different

temperatures.

Sample

700 �C 1100 �C

Particle size symmetry Particle size symmetry

ZrO2:Nd (11%) 23 nm C 45 nm CþT

ZrO2:Gd(11%) 22 nm C 38 nm CþT

ZrO2:La (11%) 25 nm C 48 nm M

C ¼ Cubic, T ¼ Tetragonal, M ¼ Monoclinic.

FIG. 2. XRD spectra for rare earth doped zirconia samples after sintering at

1100 �C, (a) La-doped, (b) Gd-doped, and (c) Nd-doped ZrO2.FIG. 1. XRD spectra for rare earth doped zirconia samples after synthesis at

700 �C, (a) La doped, (b) Gd-doped, and (c) Nd-doped ZrO2.



where E0 absorption edge energy, l0ðEÞ is the bare atom

background and Dl0ðE0Þ is the step in the lðEÞ value at the

absorption edge. After converting the energy scale to the

photoelectron wave number scale (k) as defined by

k ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

2mðEÿ E0Þ

�h2

r

; (2)

the energy dependent absorption coefficient vðEÞ has been

converted to the wave number dependent absorption coeffi-

cient vðkÞ, where m is the electron mass. Finally, vðkÞ is

weighted by k2 to amplify the oscillation at high k and the

vðkÞk2 functions are Fourier transformed in R space to gener-

ate the vðRÞ versus R (or FT-EXAFS) spectra in terms of the

real distances from the center of the absorbing atom. Figures

6(a), 6(b), and 6(c) show the experimental vðRÞ versus R

spectra of the above three ZrO2 samples doped with 11

mol% of Gd, Nd, and La, respectively.

As indicated by various other workers also for the doped

ZrO2 systems,12–14 the first two peaks in the range 1-2 A

have been attributed to Zr-O bonds while the more distant

peaks in the range of 3.5-4 A are attributed to the Zr-Zr

bonds. It should be noted that as obtained by Cole et al.,14

the FT-EXAFS spectra of cubic ZrO2 is characterized by

only one peak while in case of tetragonal ZrO2, as mentioned

by Li and Chen,13 the first peak is split into two which is

characteristic of the tetragonal structure. The first Zr-O peaks

in the FT-EXAFS spectra of Gd and Nd doped ZrO2 samples

in the present case have also been found to be split into two

peaks and hence have been fitted assuming tetragonal struc-

ture of ZrO2. The fitting has been carried out assuming eight-

fold coordination for Zr-O shell and 12-fold coordination for

Zr-Zr shell. Though it was apparent from the XRD spectra of

our samples that the Gd doped samples have been stabilized

in cubic fluorite structure with a trace of tetragonal phase,

however, EXAFS spectra reveal the tetragonal structure of

the samples with certainty. It has been observed by other

workers also27 that detail structural characterization of cubic

ZrO2 reveals that oxygen atoms are displaced from their

ideal position by up to 0.05 nm along the 100 or 111 axes,

and the structure corresponds to the P43 m space group

instead of the Fm3 m space group which is a characteristic

of the cubic fluorite structure. This appearance of tetragonal

structure in our samples agrees well with the result of

Zschech et al.28 also who have found that Y doped ZrO2 pre-

pared by coprecipitation method results in tetragonally-

stabilized structure. However, for the La doped ZrO2 sam-

ples, reasonable theoretical fitting of the FT-EXAFS spec-

trum could not be achieved assuming tetragonal structure

and has been carried out successfully assuming a monoclinic

structure. XRD spectrum of the La doped samples also

clearly reveals appearance of monoclinic structure of these

samples.

FIG. 3. XRD spectra for (a) 7%, (b) 9%, (c) 11%, and (d) 13% Gd2O3 doped

zirconia samples after sintering at 1100 �C.

TABLE II. Cell parameters for generated after indexing and refinement of XRD patterns for GdSZ samples.

S. No. Sample Symmetry a0 (A) c0 (A) Volume (A3)

1. ZrO2:Gd(7%) Tetragonal 3.60236 0.0016 5.17626 0.0010 67.176 0.04




FIG. 4. Variation in lattice parameters and cell volume with extent of gado-

linium oxide doping.



It should be mentioned here that to support the experi-

mental results, we have also carried out relativistic spin-

polarized first principle calculations of the La-doped ZrO2

samples. These calculations have been performed using the

full potential linearized augmented plane wave code29 with

the generalized gradient correction (GGA) of local density

approximation for exchange correlation function.30 GGA is

used because it accounts for the density gradients, and hence,

for most of the systems, it provides better agreement with

experiment compared to local-density approximation. For

first principle calculations, an energy cut-off for the plane

wave expansion of about 25 Ry has been used. We have

assumed the cut-off for charge density (Gmax) to be 14 with

the maximum l value (lmax) for the radial expansion to be 10

and for the non-spherical part (lmax,ns) to be 4. The muffin-tin

radii of Zr and La have been taken as 2.05 a.u. and for O it

has been taken as 1.8 a.u. The convergence criterion for the

total energy E has been taken to be about 0.1 mRy per atom

and the charge convergence has been set to 0.001. In the

above calculations, the tetrahedron method has been used for

k-space integration29 and the lattice constants have been

taken from the experiments.31,32 As has been discussed ear-

lier, ZrO2 is known to exist in two crystal structures, mono-

clinic at low temperature and tetragonal at high temperature.

Hence, the calculations have been performed with lattice pa-

rameters of a, b, and c ¼ 5.145, 5.2075, and 5.3107 A, a ¼ c

¼ 90� and b ¼ 99.233� for the monoclinic phase and with a

¼ b ¼ 3.64, c ¼ 5.27 A, and a ¼ b ¼ c ¼ 90� for the tetrago-

nal phase. The numbers of k-points for self-consistent field

cycles in the reducible (irreducible) Brillouin zone are about

4000 (270) for the tetragonal phase and about 2000 (430) for

the monoclinic phase. Figure 7 shows a representative

2� 2� 4 supercell used in the above calculations for the

12.5% La-doped ZrO2 sample having monoclinic structure.

From the ab initio calculations, we have found that the

undoped zirconia in the ground state has a monoclinic sym-

metry with the tetragonal phase being higher in energy by

114.682 meV/f.u. (formula unit). It has also been observed

that though with certain dopants, ZrO2 shows the tetragonal

FIG. 5. Representative l(E) vs E spectrum of ZrO2 doped with La.

FIG. 6. Experimental v(R) vs R spectra with best-fit theoretical plots for

ZrO2 doped with 11 mol% of (a) Gd, (b) Nd, and (c) La.

FIG. 7. (Color online) A 2� 2� 4 supercell in monoclinic symmetry for the

12.5% La-doped ZrO2. (The largest spheres correspond to La, second largest

to Zr and smallest spheres correspond to O.)



symmetry, La-doped ZrO2 is observed to retain the ground

state monoclinic symmetry with its energy less than that of

the tetragonal phase by 13.729 meV/f.u. The above results

agree well with the experimental findings. It should be men-

tioned here that we have carried out the calculations with

12.5% doping by La which is very close to the sample sub-

jected to our EXAFS measurements, which is having 11%

doping.

To investigate the local structure around the Zr atoms

more quantitatively, we have analyzed the best fit parameters

obtained from the fitting of the experimental vðRÞ versus Rspectra. The best fit theoretical vðRÞ versus R spectra of the

samples have been shown in Figs. 6(a)–6(c) and it should be

mentioned here that a set of EXAFS data analysis program

available within the IFEFFIT software package33 has been

used for reduction and fitting of the experimental EXAFS

data. This includes data reduction and Fourier transform to

derive the vðRÞ versus R spectra from the absorption spectra,

generation of the theoretical EXAFS spectra starting from an

assumed crystallographic structure and finally fitting of the

experimental data with the theoretical spectra using the

FEFF 6.0 code. The structural parameters for the tetragonal

and monoclinic ZrO2 used for simulation of theoretical

EXAFS spectra of the samples have been obtained from

reported values in the litearures.32,34 The bond distances,

coordination numbers (including scattering amplitudes) and

disorder (Debye-Waller) factors (r2), which give the mean-

square fluctuations in the distances, have been used as fitting

parameters and the best fit results of the above parameters

have been summarized in Figs. 8 and 9.

Figures 8(a), 8(b), and 8(c) show the variation in aver-

age Zr-O bond length for the two shells, (rZrÿO), total oxygen

coordination (NZrÿO) and average disorder parameter (r2ZrÿO)

of the Zr-O shells for different dopant cations. It can be seen

that as the size of the dopant cation is increased from Gd to

La, the bond length and disorder parameters decrease while

the coordination number increases. This implies that with

larger size dopants, the oxygen vacancy near Zr site

decreases or in other words the oxygen vacancies are created

near the dopant site leaving the host sites unperturbed. Also,

it can seen from Fig. 9 that, as the dopant size increases from

Gd to La, the average Zr-Zr bond lengths (rZrÿZr) gets con-

tracted while the total Zr-Zr coordination number (NZrÿZr)

and average disorder parameters (r2ZrÿZr) decrease. The

decrease in the disorder term as the dopants ion change from

Gd to La also confirms that with increase in the size of the

dopant cation the disorder near the host Zr cation is

decreased. However, in case of doping with Nd and La, due

to accommodation of larger size ions in the lattice, the Zr-Zr

bonds get contracted.12

The above results agree with that obtained by Cole

et al.,14 who have also observed that for doped ZrO2

FIG. 8. Variation of (a) average first shell distances, Zr-O, (b) average oxy-

gen coordination numbers, CN, (c) average Debye Waller factors.

FIG. 9. Variation of (a) average second shell distances, Zr-Zr/dopant, (b)

Zr/dopant coordination number, CN, and (c) average Debye Waller factors.



systems, with increase in ionic radius of dopants, Zr-O bond

length and the Debye-Waller (disorder) parameter decrease

and also concluded that for the larger size dopants, the oxy-

gen vacancies lie near the dopant sites rather than near Zr

sites. The slight opposite trend in the variations of the differ-

ent parameters from Nd-doped samples to La-doped samples

can be ignored since the values of the parameters for La-

doped samples have been obtained assuming a monoclinic

structure while the other two systems have been fitted assum-

ing tetragonal structure.

In order to have a further insight into the above issue

regarding the location of oxygen vacancies in doped ZrO2

system, ab inito calculations have also been performed for

La-doped ZrO2 assuming oxygen vacancies at the near

neighbor (NN) and next neighbor sites of the dopant La. For

the calculations with oxygen vacancies, the k-points in the

irreducible Brillouin zone are about 690 in number. From

the energetics, it has clearly been observed that the oxygen

vacancies are created near the dopant sites leaving the host

sites unperturbed as is observed from our experimental

results also. Oxygen ion vacancies at the NN sites leave

the system in a more stable state with an energy difference

of 10.633 meV/f.u. in comparison to vacancies at further

sites.

Since from the EXAFS measurements it has been

observed that the Gd-doped samples have the minimum oxy-

gen coordination, it can be concluded that Gd is most effec-

tive in generating oxygen vacancies among the three dopants

studied. Figure 10 shows a representative experimental

EXAFS (lðEÞ versus E) spectrum of ZrO2 sample doped

with 13% Gd and the vðRÞ versus R spectra for ZrO2 samples

doped with different (7, 9, 11, and 13%) concentrations of

Gd are shown in Figs. 11(a)–11(d). All the FT-EXAFS spec-

tra are characterized by two peaks in the range 1-2 A corre-

sponding to the Zr-O bonds and two peaks in the range 3-4

FIG. 10. Representative l(E) vs E spectrum of ZrO2 doped with 13% Gd

concentration.

FIG. 11. Experimental v(R) vs R spectra with best-fit theoretical plots for

ZrO2 doped with Gd concentrations of (a) 7 mol%, (b) 9 mol%, (c) 11

mol%, and (d) 13 mol%.

FIG. 12. Variation of (a) average first shell distances, Zr-O, (b) average ox-

ygen coordination numbers, CN, and (c) average Debye Waller factors.



A corresponding to Zr-Zr bonds. The experimental spectra

have been fitted as above with theoretical EXAFS spectra

simulated assuming tetragonal structure of ZrO2. The best fit

theoretical spectra are also shown in Figs. 11(a)–11(d) along

with the experimental spectra and the best fit results have

been summarized in Figs. 12 and 13.

From Fig. 12, it can be seen that with increase in Gd

concentration from 7% to 9%, the average Zr-O bond dis-

tance decreases along with the total oxygen coordination

while the average disorder parameter at the O site increases.

However, for doping concentration of more than 9%, Zr-O

bond distance and oxygen coordination increases with a

decrease in the disorder factor. Hence, it is observed that

maximum number of oxygen vacancies are created for an op-

timum Gd doping of 9% and the reduced effect of Gd doping

on the Zr-O shell with higher concentration of dopants may

be because of agglomeration and clustering of Gd ions inside

ZrO2 lattice. The above postulate is also supported by the

results obtained for Zr-Zr/cation shells, as shown in Fig. 13,

from where it can be seen that the Zr coordination and the

bond distances are affected most by a Gd doping concentra-

tion of 9%. The reduced effect on the Zr-O shell with

increase in concentration of Gd dopants has also been

observed by Sadykov et al.12 for their CeO2–ZrO2 samples.

IV. SUMMARYAND CONCLUSIONS

X-ray diffraction measurement with laboratory Cu Ka

radiation and EXAFS measurement with synchrotron radiation

at Zr K edge have been performed on ZrO2 samples with 11%

Nd and La doping and with 7, 9, 11, and 13% Gd doping. The

samples have been prepared by coprecipitation method fol-

lowed by sintering at two different temperatures, viz., 700 �C

and 1100 �C. It has been observed from x-ray diffraction

measurements that though all the samples prepared at 700 �C

attain cubic structure, when sintered at 1100 �C, Gd and Nd

doped samples show appearance of tetragonal structure and

La doped samples attain monoclinic structure. This had been

confirmed by EXAFS measurements which clearly reveal

splitting of the first Zr-O shell into two sub-shells for the sam-

ples sintered at 1100 �C which is a characteristic of tetragonal

and monoclinic structure. The above conclusion has been cor-

roborated by ab inito first principle calculation also which

shows that for La-doped ZrO2 the ground state energy for

monoclinic symmetry is less than that of the tetragonal phase.

The EXAFS data have further been analyzed to find out rele-

vant local structure parameters of the Zr-O and Zr-Zr shells,

viz., bond distances, coordinations, and disorder parameters. It

has been observed that though for Gd doping, the oxygen

vacancies in ZrO2 host matrix are created near the Zr site, for

Nd and La doping which are having relatively larger ionic

radii, the Zr-O shell remain more or less unperturbed and the

oxygen vacancies are located near the dopant cations. It has

also been confirmed by ab initio calculations that for La-

doped ZrO2 systems oxygen ion vacancies at the nearest

neighbor sites leave the system in a more stable state in com-

parison to oxygen vacancies at further sites. It has also been

found that Gd doping of 9% is optimum for creation of vacan-

cies near the Zr sites and hence for increasing its ionic conduc-

tivity. A further increase in Gd dopant concentration decreases

the number of oxygen vacancies possibly due to the agglomer-

ation/clustering of Gd dopants at higher concentration.

Thus, the main conclusions of the present studies are (i)

for Gd doping in ZrO2 matrix, oxygen vacancies are created

near the Zr sites, while in case of doping with ions having

larger radii like Nd and La, oxygen vacancies are created

near the dopant sites, and (ii) a Gd doping concentration of

9% is optimum for generation of oxygen vacancies in ZrO2.

ACKNOWLEDGMENTS

The authors wish to acknowledge Dr. N.K. Sahoo, Head

Applied Spectroscopy Division, BARC, Dr. D. Das, Head,

Chemistry Division, BARC and Dr. S.K. Deb, Head, Indus

Synchrotron Utilization Division, RRCAT for their encour-

agement in course of the above work.

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