Journal of Solid State Chemistry 196 (2012) 1–10

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Journal of Solid State Chemistry

0022-45

http://d

n Corr

E-m

journal homepage: www.elsevier.com/locate/jssc

Structural investigation and luminescence of nanocrystalline lanthanidedoped NaNbO3 and Na0.5K0.5NbO3

Sonia Pin a, Fabio Piccinelli b, Kagola Upendra Kumar c, Stefano Enzo d, Paolo Ghigna e, Carla Cannas f,Anna Musinu f, Gino Mariotto g, Marco Bettinelli b, Adolfo Speghini b,n

a Paul Scherrer Institute, General Energy Research (ENE), Laboratory for Bioenergy and Catalysis, CH-5232 Villigen PSI, Switzerlandb Dipartimento di Biotecnologie, Universit �a di Verona and INSTM, UdR Verona, Strada Le Grazie 15, I-37134 Verona, Italyc Grupo de Fotonica e Fluidos Complexos, Instituto de Fısica, Universidade Federal de Alagoas (UFAL), Maceio-AL, Brasild Dipartimento di Chimica, Universit �a di Sassari, 07100 Sassari, Italye Dipartimento di Chimica, Universit �a di Pavia, V.le Taramelli 16, I-27100 Pavia, Italyf Dipartimento di Scienze Chimiche, Universit �a di Cagliari, Cittadella Universitaria Monserrato, I-09042 Cagliari, Italyg Dipartimento di Informatica, Universit �a di Verona, Strada Le Grazie 15, I-37134 Verona, Italy

a r t i c l e i n f o

Article history:

Received 14 June 2012

Received in revised form

30 July 2012

Accepted 2 August 2012Available online 10 August 2012

Keywords:

Niobates

Nanoparticles

Lanthanides

EXAFS

Luminescence

96/$ - see front matter & 2012 Elsevier Inc. A

x.doi.org/10.1016/j.jssc.2012.08.003

esponding author. Fax: þ39 045 8027900.

ail address: adolfo.speghini@univr.it (A. Spegh

a b s t r a c t

Nd3þ and Eu3þ doped NaNbO3 and Na0.5K0.5NbO3 nanostructured multiferroics (nanoparticles or

nanorods) were prepared by a sol–gel route. X-Ray powder diffraction results evidence that the sodium

and mixed sodium–potassium niobates show orthorhombic (Pmc21 space group), and monoclinic

structure (Pm space group), respectively, confirmed by the Raman spectra. The local structure around

the trivalent lanthanides was investigated with Extended X-ray Absorption Fine Structure spectroscopy

at the Ln–K edge and luminescence spectroscopy. The Ln3þ ions enter the structure by substituting the

alkali metals, with a 12-fold oxygen coordination, and inducing a large amount of static disorder. The

visible emission bands of the Eu3þ ions indicate that multiple sites exist for the lanthanide ions, in

agreement with the EXAFS results showing the largest amount of static disorder in these samples. A

possible indication of clustering of oxygen vacancies around the LnNa00 defect is obtained by VBS

calculations.

& 2012 Elsevier Inc. All rights reserved.

1. Introduction

Niobate based materials are of considerable interest for theiroptical, piezoelectric, pyroelectric and photocatalytic properties,which can be exploited in many technological applications [1–8].Notable examples of non-linear optical behaviour and laserproperties of alkali niobates are those involving lithium niobate(LiNbO3), which is by far the most widely studied member of thisfamily with a high number of applications in integrated photonics[9–11]. This is essentially due its unique non-linear, acousto-optic, electro-optic properties, which make it an excellent multi-functional optical material. Similarly, strontium barium niobate(Sr0.40Ba0.60Nb2O6, SBN) and calcium barium niobate(Ca0.28Ba0.72Nb2O6, CBN) belong to a group of ferroelectric crystals(tungsten bronzes) which also present excellent pyroelectric,electro-optic, piezoelectric and non-linear optical properties[12–16]. It has been demonstrated that these materials are ableto efficiently produce multi-directional second and third

ll rights reserved.

ini).

harmonic generation in a variety of geometries and broad spectralrange, so that they can be considered as non-linear photonicglass-like crystals [17]. Lanthanide doped niobate are extremelyinteresting materials in the field of photorefractive memories andas linear and self-frequency converter solid state laser materials[18–21]. Moreover, niobate based nanopowders are used asstarting materials to prepare homogeneous ceramics under rela-tively mild preparation conditions (nanoceramics) [22] and filmdeposition [23].

Sodium niobate (NaNbO3) exhibits an antiferroelectric beha-viour at room temperature [24–26]. It is characterized by aperovskite structure and undergoes several phase transitions asa function of temperature [27–29]. Recently, alkali niobate nanos-tructures, such as NaNbO3 or KNbO3 nanoparticles or nanowires,have been studied for their promising non-linear optical proper-ties, such as second harmonic generation (SHG) [30–32]. Mixedsodium–potassium niobates, of composition NaxK1�xNbO3 (with x

varying between 0 and 1) are also valuable materials as lead freepiezoceramics, since the electrical and spectroscopic propertiescan be tuned according to the chemical composition [33–38].

Although lanthanide doped LiNbO3 is among the most studiedniobates from an optical point of view, sodium and potassium

S. Pin et al. / Journal of Solid State Chemistry 196 (2012) 1–102

niobates are gaining importance, also in nanosized form, asinteresting hosts for luminescent lanthanide ions [39,40]. Onthe other hand, identifying the crystallographic site in whichthe lanthanide dopant is accommodated is not trivial, due to boththe different charge with respect to the host cations (Naþ , Kþ ,and formally Nb5þ) and different ionic radius (Kþ and Naþ

cations are larger while Nb5þ is smaller than the Ln3þ ions)[41]. As for the spectroscopic properties, finer details of the localenvironment around the dopant lanthanide ion, in particularconcerning the first coordination shell, are of paramount impor-tance, considering also that the simultaneous occupation ofvarious substitutional sites can be possible. This behaviourinduces a significant perturbation (e.g. in terms of broadening)of the spectral features of the dopant emitting lanthanide ions inthe niobate hosts, therefore affecting in a critical way theluminescence properties [42].

Recently, some investigation on the spectroscopic properties oflanthanide doped NaNbO3 sodium has been reported in theliterature. In particular, Pr3þ doped sodium niobate, is an inter-esting material for its persistent luminescence [43]. Moreover,Er3þ/Yb3þ doped sodium niobate powders show a significantupconversion emission in the green and red regions upon a980 nm excitation [44]. On the other hand, no papers have beenappeared in the literature on the spectroscopic properties ofmixed sodium–potassium niobates, either in bulk or in nanocrys-talline form. Therefore, we think that it is important and timely toinvestigate this important class of multiferroic materials. In thepresent paper, we focus on Nd3þ and Eu3þ doped NaNbO3 andNa0.5K0.5NbO3 nanocrystalline materials, which have beenobtained as nanoparticles or nanorods by a simple heat treatmentprocedure. The Eu3þ ion is a well known optical probe that cangive valuable information on the local environment of thelanthanide ion [42,45,46], while the Nd3þ ions has a significantlarger size. Achieving a deeper understanding on this topic byinspecting the optical spectra of these materials taking intoconsideration the results of a local structural probe is then akey starting point for clarifying the properties of these materials.In this respect, a joint approach using X-ray powder diffraction,Raman, Extended X-ray Absorption Fine Structure (EXAFS), Trans-mission Electron Microscopy (TEM) and luminescence spectro-scopy is necessary to gain important information about theproperties of these materials in nanosized form.

2. Experimental section

2.1. Sample preparation

Undoped and 1% mol (with respect to the alkaline ions) Ln3þ

(Ln¼Nd, Eu), doped NaNbO3 and Na0.5K0.5NbO3 nanocrystallinepowders were prepared by a sol–gel method using citric acid as ametal-complexing agent [26,38,47,48]. Briefly, 0.1 mol of citricacid was dissolved in 10 ml of water at 90 1C. Then, appropriateamounts of Na2CO3 (Aldrich, 99.9%) or K2CO3 (Aldrich, 99.9%) andstoichiometric quantities of Nd(NO3)3 �6H2O (Aldrich, 99.99%) orEu(NO3)3 �5H2O (Aldrich, 99.99%) were dissolved in order toobtain a 1:250 metal to citric acid molar ratio. After stirring thesolution for 10 minutes, a stoichiometric amount of niobiumammonium oxalate (NAmOx, Aldrich, 99.99%) and 0.1 mol ofethylene glycol (EG) were added. The resulting transparentsolution was then stirred for 1 hour at 90 1C. A gel was obtainedafter a heat treatment at 90 1C for 3 days. The obtained gels werefirst heated at 400 1C for 2 hours and subsequently at 600 1C for2 hours or 900 1C for 2 hours. White fluffy powders were obtainedfor all the samples. Undoped NaNbO3 and Na0.5K0.5NbO3 samples

will be denoted hereafter as NN and NKN, respectively, while thedoped ones as NN:Ln and NKN:Ln, respectively.

2.2. X-ray diffraction setup

Room temperature X-Ray Powder Diffraction (XRPD) patterswere measured with a Thermo ARL X’TRA powder diffractometer,operating in Bragg–Brentano geometry equipped with a Cu-anodeX-ray source (Ka, l¼1.5418 A) and using a Peltier Si(Li) cooledsolid state detector. The spectra were collected with a scan rate of0.00331/s, time of exposure 9.0 s and 2y range of 201–1401. Thephase identification was performed with the PDF-4þ 2009database provided by the International Centre for Diffraction Data(ICDD). The instrumental X-ray peak broadening was determinedexploiting LaB6 standard reference material (SRM 660a) providedby NIST. The powder samples were ground in a mortar suspendedin a few drops of ethanol and deposited in a low-backgroundsample stage. Rietveld refinements [49] were performed on theXRPD patterns, by exploiting the MAUD software [50]. Thisprogramme has implemented a peak broadening description ofthe analytical pseudo-Voigt lines similar to the pioneering Wil-liamson Hall plot used in multiple profile analysis; it takes intoaccount precisely the instrument broadening resolution and,under the selected assumption of isotropic peak broadening as afunction of reciprocal space, performs the separation of the latticestrain contribution from the broadening originating from thereduced crystallite size. In this context it is useful to recall theequation relating the integral breadth, b, of the diffraction peakswith the sample microstructural parameters, in the case of simpleshapes of crystallites, such as spherical particles:

bhklðsÞ ¼Kb

/DSnþ2edn

hkl

where Kb is the Scherrer constant, /DSv is average value from avolumetric distribution of the diffraction domain size, e is themicrostrain parameter (e¼1.25/e2S1/2, /e2S1/2

¼r.m.s para-meter) and dn

hkl is the scattering vector. Moreover s¼1/d is thereciprocal space variable.

2.3. Transmission electron microscopy

Transmission electron microscopy (TEM) micrographs wereacquired using a JEOL 200CX microscope operating at 200 kV, inorder to determine the morphology and the size (surfaceweighted) distribution of the nanoparticles. Finely ground sam-ples were dispersed in octane using an ultrasonic bath. Thesuspensions were then dropped on carbon-coated grids for theTEM observations.

2.4. Raman microscopy setup

Unpolarized micro-Raman spectra were measured at roomtemperature on undoped niobate powders heat treated at 600 and900 1C, in full backscattering geometry and in the wavenumberregion between 10 and 1000 cm�1, by using a triple-monochromator spectrometer (Horiba-Jobin Yvon, modelT64000) set in double-subtractive/single configuration andequipped with 1800 grooves/mm grating. The spectra wereexcited in turn at several wavelengths (i.e. 488.0, 514.5 and568.2 nm) with an argon/krypton ion laser (Spectra Physics,model RM-2018). The exciting radiation was focussed onto thesample surface with a spot size of the order of 1 mm2 through along working distance objective with magnification 80� andNA¼0.75. To avoid unwanted laser induced transformations, neutralfilters of different optical densities were used, whenever necessary.The scattered radiation, filtered by the fore double-monochromator,

log

[I (Q

)*Q

] (ar

b. u

n.)

log

[I (Q

)*Q

] (ar

b. u

n.)

Pmc21

res

Pm

20

res

Scattering angle 2θ (°)40 60 80

Fig. 1. X-Ray powder diffraction patterns for the Eu3þ doped NaNbO3 (a) and the

Eu3þ doped Na0.5K0.5NbO3 (b) samples treated at 600 1C: experimental (blue line)

and Rietveld refinement (red line). Black lines: residuals from the experimental

and1–4, 6 and 7 have been submitted as colour images; however, the captions

have been reworded to ensure that they are meaningful when your article is

reproduced both in colour and in black and white. Please check and correct if

necessary. /> the calculated patterns. The obtained final agreement factors Rw are

11.7% and 12.0%, respectively. (For interpretation of the references to colour in this

figure legend, the reader is referred to the web version of this article.)

S. Pin et al. / Journal of Solid State Chemistry 196 (2012) 1–10 3

was detected at the spectrograph output by a CCD detector cryo-genically cooled by liquid nitrogen. The spectral resolution was betterthan 0.7 cm�1/pixel.

2.5. EXAFS measurements

The EXAFS spectra at the Ln–K edges were measured atdifferent temperatures at the BM29 beamline of the EuropeanSynchrotron Radiation Facility (ESRF, Grenoble, Experiment CH-2878), in fluorescence mode, using a 13 elements Ge detector forthe fluorescence signal, and a double crystal Si(511) monochro-mator. For the measurements, the samples were thoroughlymixed with cellulose in an agate mortar and then pressed topellets. The use of the K edges is required in order to have asignificant wave vector k range (k¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2mðE�E0Þ=_

p). The EXAFS

data reduction has been performed by means of the Athena code[51], and the reduced data analysis has been done in the k spaceby means of the EXCURV98 code [52]. In order to overcome theheavy correlations between the coordination numbers N and theDebye–Waller disorder term s2, the coordination numbers in theEXAFS data analysis have been restrained by the Bond–ValenceSum method [53] as widely discussed by Newville [54]. Insummary, as the coordination numbers are badly determined byEXAFS, the fits were made by fixing the coordination numbers totheir crystallographic values, and allowed to float the distancesand the respective s2. Then the consistency of the coordinationnumbers with the distances was checked by using the BVSmethod. The goodness of fit is expressed by the REXAFS parameter,defined as

REXAFS ¼XN

1

100

sið wi

expðkÞ�withðkÞ

������Þ

Being N the number of points in the EXAFS spectrum, and1=si ¼ kn

i =PN

j 9wexpj ðkjÞ9, where n is the EXAFS weighting scheme

[52]. The number of independent points, given the k and the r

range used for the fits, was never found below 11.

2.6. Luminescence measurements setup

Photoluminescence measurements were carried out using atunable dye laser pumped by a Nd:YAG laser. The emission signalwas analyzed by a half-metre monochromator (HR460, JobinYvon) equipped with a 1200 lines/mm grating and detected witha CCD detector (Spectrum One, Jobin Yvon) or with a photomul-tiplier. The spectral resolution of the emission spectra is 0.1 nm.

3. Results and discussion

3.1. X-ray diffraction

All samples under investigation are single phase, as evidencedby X-Ray powder diffraction measurements (see Figs. 1–3). Due topossible polymorphism for NaNbO3, as highlighted by Shiratoriet al. [55], we carried out Rietveld refinements on the X-Raypowder diffraction data in various orthorhombic (Pbcm, Cmcm,Pmc21, Pmma), tetragonal (P4/mbm) and cubic (Pm-3m) spacegroups. In particular, the nanocrystalline NN and NN:Ln samplespossess orthorhombic crystal structure (Pmc21 space group, no.26 [56]) for both the samples heat treated at different tempera-tures (600 or 900 1C). Looking at the distorted perovskite struc-ture of NaNbO3 (space group Pmc21), it is readily seen thatNiobium is in a distorted octahedral environment, whilst theNaþ ion is surrounded by a very distorted coordination cage of 12oxygens, with distances ranging from about 2.4–3.1 A. As repre-sentative examples, the results for the NN:Eu samples heat

treated at 600 and 900 1C are shown in Figs. 1–3. Similar XRPDpatterns have been found for the undoped NN and NN:Ndnanocrystalline samples. From the Rietveld refinements per-formed with a single phase model, it was noted that for theundoped and lanthanide doped sodium niobate samples nosignificant increase of the structural microstrain has beenobserved as a consequence of the doping or the heat treatment(see Table S1, Supporting Information). Differently, a singlemonoclinic phase with Pm space group gives the best fittingstructural model for the XRPD patterns for all the NKN andNKN:Ln (Ln¼Eu, Nd) samples, in agreement with the phasediagram of mixed alkali Na1�xKxNbO3 niobates [57], indepen-dently of the heat treatment temperature. As representativeexamples, we show the experimental and calculated XRPD pat-terns for the NKN:Eu samples in Figs. 1–3. The same results wereobtained for the Nd3þ doped samples. From the Rietveld analysis,the microstructural parameters were also obtained (see Table S1,Supporting Information). For the 600 1C heat treated NN:Eu andNKN:Eu samples, we estimate an average crystallite size of 36(1)and 33(1) nm, respectively. In the case of the samples heat treatedat 900 1C, we observed an increase of the average crystallite size,in particular we obtained 83(1) and 53(1) nm for NN:Eu andNKN:Eu, respectively. It is worth to point out that as a conse-quence of the lanthanide doping we observe an increase of themicrostrain r.m.s. parameter in the case of NKN:Eu samples withrespect to the undoped NKN one. In particular, for the sampletreated at 600 1C we obtain a microstrain parameter of

log[

I(Q)*

Q] (

arb.

un.

) lo

g[I(Q

)*Q

] (ar

b. u

n.)

Pmc21

res

Pm

20

res

Scattering angle 2θ (°)40 60 80

Fig. 2. X-Ray powder diffraction patterns for the Eu3þ doped NaNbO3 (a) and the

Eu3þ doped Na0.5K0.5NbO3 (b) samples treated at 900 1C: experimental (blue line)

and Rietveld refinement (red line). Black lines: residuals from the experimental

and the calculated patterns. The obtained final agreement factors Rw are 11.8% and

12.1%, respectively. (For interpretation of the references to colour in this figure

legend, the reader is referred to the web version of this article.)

2

I (Q

)*Q

(arb

. uni

ts)

3 4 5 6 7Q (Å-1)

Fig. 3. X-Ray powder diffraction pattern as a function of Q for the Eu3þ doped

NaNbO3 sample treated at 900 1C: experimental (blue dots) and Rietveld refine-

ment (blue line). The obtained final agreement factor Rw is 9.1% The violet lines are

reference peaks for the NaNbO3 phase [55]. Green line: residuals from the

experimental and the calculated patterns. (For interpretation of the references

to colour in this figure legend, the reader is referred to the web version of this

article.)

Table 1

Selected bond lengths (A), calculated by Rietveld refinement, for Eu3þ doped

NaNbO3 samples treated at the quoted temperature. The estimated uncertainties

on the bond lengths is 0.05 A.

Selected bond lengths (A) for Eu3þ

doped sample heat treated at 600 1C

Selected bond lengths (A) for Eu3þ

doped sample heat treated at 900 1C

Nb1–O1¼2.19 Nb1–O1¼1.97

Nb1–O2¼1.93 Nb1–O2¼1.97

Nb1–O3¼2.16, 1.88 Nb1–O3¼1.96, 2.08

Nb1–O4¼1.98, 1.90 Nb1–O4¼2.09, 1.96

Na1–O1¼2.34, 2.34 Na1–O1¼2.50, 2.53, 3.02

Na1–O3¼2.68 (�2) Na1–O3¼2.98 (�2), 2.68 (�2)

Na1–O4¼3.01 (�2), 2.43 (�2) Na1–O4¼2.92(�2)

Na2–O2¼2.70, 2.82, 2.90, 2.68 Na2–O2¼2.52, 2.66, 2.91, 3.00

Na2–O3¼2.89 (�2), 2.44(�2) Na2–O3¼2.45 (�2), 3.03 (�2)

Na2–O4¼2.56 (�2) Na2–O4¼2.67(�2)

S. Pin et al. / Journal of Solid State Chemistry 196 (2012) 1–104

0.0039(1) for the NKN:Eu sample in comparison with0.0026(3) for the undoped NKN one. This evidence is clearer inthe case of NKN samples treated at 900 1C. In fact, the NKN:Eu

sample shows a microstrain parameter of 0.0048(1), about threetimes that one of the NKN one, 0.0019(1). From a structural pointof view, in the NKN structure, the Naþ and Kþ ions have the sameoccupancy factor of 0.5 and share eight distinct crystal sites withCs point symmetry. At variance, in the NN phase, the Naþ ions arelocated in crystal sites with Cs point symmetry but in this casethey occupy only two different sites. The Rietveld structuralrefinement on the powder patterns of NKN:Eu samples revealsthat, independently of the temperature of the thermal treatment,there is no significant structural difference with respect to theundoped sample (PDF card no. 01-077-0038 [58]). On the con-trary, in the case of NN:Eu samples treated at 600 and at 900 1C,we can state that whilst the geometrical environment of theniobium is quite regular and similar to the one present in theundoped sample, we observe a significant distortion of the firstcoordination sphere of Naþ ions occupying the two availablecrystal sites, as reported in Table 1. In particular, the average Na–O bond lengths calculated for the NN:Eu sample (Na1–O¼2.72 Aand Na2–O¼2.67 A for the sample treated at 900 1C and Na2–O¼2.69 A for the sample treated at 600 1C) are larger than theones found in the undoped sample (Na1–O¼2.67 A, Na2–O¼2.58 A, PDF card no. 38-1249 [59]). In conclusion, the intro-duction of the trivalent lanthanide ions as dopants in the crystalstructure of NaNbO3 and the related charge compensatingmechanism leads mainly to a distortion of the two Naþ coordina-tion environments. As a confirm of the reliability of the X-rayresults for the diffraction patterns collected up to 2y of 901, for theNN:Eu samples heat treated at 900 1C the XRPD pattern was alsocollected with a scattering angle up to 2y of 1401, correspondingto a Q value (Q ¼ 2ps¼ 4pnsin _yl) of 7.7 A�1, in particular toverify the consistency of the calculated structural parameterssuch as the fractional atomic coordinates. From a Rietveld refine-ment on these longer patterns, we found a fairly good coincidenceof the refined structures determined from the patterns collectedup to 2y values of 901 and up to 2y of 1401, indicating that for thesamples under investigation a range of 2y of 901 is enough toobtain accurate Rietveld refinened structural parameters. In Fig. 3the pattern, the obtained Rietveld fit and residuals are shown forthe NN:Eu sample. A very similar behaviour was found for theNN:Nd sample.

3.2. EXAFS investigation

Fig. 4 and S1 (see Supporting Information) summarizes theEXAFS results for the NN:Nd sample, measured at 70 and 290 K,respectively. The corresponding Fourier Transforms of the EXAFSdata are shown in Fig. 5 and S2. Evident EXAFS oscillations arepresent above the noise level up to k¼10.5 A�1. It is clearly

-0.5

0.0

0.5

2

-0.5

0.0

0.5

-0.5

0.0

0.5

k2 χ(

k) (a

rb. u

nits

)k2

χ(k)

(arb

. uni

ts)

-0.5

0.0

0.5

4 6 8 10

2 4 6 8 10k(Å-1)

Fig. 4. Ln–K edge EXAFS spectra at 70 K for the Nd3þ doped NaNbO3 (a), Nd3þ

doped Na0.5K0.5NbO3 (b), the Eu3þ doped NaNbO3 (c), Eu3þ doped Na0.5K0.5NbO3

(d). (For interpretation of the references to colour in this figure, the reader is

referred to the web version of this article.)

0

2

4

6

0

2

4

6

0

2

4

6

|FT

[k2 χ(

k)]|

(arb

. uni

ts)

|FT

[k2 χ(

k)]|

(arb

. uni

ts)

00

2

4

6

r(Å)2 4 6

0 2 4 6

Fig. 5. Fourier transforms (modulus) of the EXAFS spectra shown in Fig. 4 (at

70 K): Nd3þ doped NaNbO3 (a), Nd3þ doped Na0.5K0.5NbO3 (b), Eu3þ doped

NaNbO3 (c), Eu3þ doped Na0.5K0.5NbO3 (d).

Table 2EXAFS fitting parameters for the spectrum of the Nd3þ doped NaNbO3 sample,

shown in Fig. 4 (at 70 K). r: distances; s2: distance variances (Debye–Waller

factors).

Shell N r (A) r2 (A2) Atom type REXAFS¼5.5%

1 5 2.44(1) 5(1)�10�3 O

2 7 2.64(2) 1.1(6)�10�2 O

3 8 3.465(7) 6.8(4)�10�3 Nb

S. Pin et al. / Journal of Solid State Chemistry 196 (2012) 1–10 5

apparent that the overall EXAFS spectra do not show largechanges with temperature. This is a clear indication that theincorporation of Nd3þ ions in the NaNbO3 structure induces alarge amount of static disorder, as also previously observed byXRD analysis. The first peak in FT, at ca. 2.5 A, is attributed tooxygen atoms, the third one, at ca. 3.5 A, to Nb atoms, while thesecond one is due to a superposition of contributions by oxygenatoms and Nb atoms, this last one being due to the non-monotonous trend of the Nb backscattering function with k. Thebest fit of the spectra, shown in Fig. 4 as red lines, has been madeby using a model which takes into account the distorted per-ovskite structure of NaNbO3, and it is composed by 2 shells ofoxygen atoms, with coordination numbers equal to 5 and 7,respectively and one additional shell of Nb atoms, with coordina-tion number equal to 8. The quality of the fit shown in Fig. 4 isrepresentative of all the fits made for this sample. It is thereforeconcluded that the dopant Nd ions occupy the Na site assubstitutional defects with a total oxygen coordination numberof 12. This conclusion is supported by the result of the EXAFSfitting that are reported in Table 2. In particular, the fitted Nd–Nbdistance, close to 3.5 A, well compares with the crystallographicNa–Nb distances, ranging from ca. 3.41 to 3.52 A, keeping intoaccount the phase-shift correction of EXAFS data and consideringthat the shortest Nb–Nb distance in the NaNbO3 structure islarger than 3.9 A.

The temperature dependence of the fitted radial distancesaround Nd3þ ions and of the corresponding standard deviations(EXAFS Debye–Waller factors) of the NN:Nd sample heat treatedat 600 1C are plotted in Fig. 6. It is well evident that the distancesare constant within the error bars, albeit for a small reduction

with increasing T of the Nd–Nb distance, that can be observedalso in the FT data shown in Fig. 5, and that the Debye–Wallerfactors show a quite weak dependence on T, if any. This behaviourconfirms the above mentioned claim of a large amount of staticdisorder around the dopant Nd3þ ions. Unfortunately, this largestatic disorder does not allow making a sensible further analysis,for example by using the Debye or Einstein model for fitting theDebye–Waller factors. We point out that the EXAFS results havebeen confirmed by Valence Bond calculations, that gave a chargeon Nd equal to 3.3, in fairly good agreement with the formalNd(III) oxidation state.

Fig. 7 Very similar results have been obtained for the NN:Eusample, and representative examples of the EXAFS spectra andthe corresponding Fourier Transforms are also shown in Figs. 4,S1, 5 and S2, measured at 70 and 290 K. The overall EXAFS spectraat the Eu–K edge are very similar to that obtained at the Nd–Kedge, and the EXAFS Fourier transforms can be interpretedanalogously. In addition, also in the case of the Eu doped sample,changes in the spectral appearance with temperature can hardlybe detected, evidencing a large disorder of static origin. Thisdisorder, mainly for what concerns the next nearest neighbour

00.00

0.01

0.02

0.03

0.04r

(Å)

2.40

2.60

2.803.35

3.40

3.45

3.50

3.55

σ2 (Å

2 )

T (K)50 100 150 200 250 300 350

0T (K)

50 100 150 200 250 300 350

Fig. 6. Temperature dependence of the fitted radial distances around Nd and of

the corresponding standard deviations (EXAFS Debye–Waller factors). Black solid

circles: Nd–O (1st shell); red solid circles: Nd–O (2nd shell); green triangles: Nd–

Nb. (For interpretation of the references to colour in this figure legend, the reader

is referred to the web version of this article.)

00.00

0.01

0.02

0.03

0.04

r (Å

2 )

2.4

2.6

2.83.4

3.6

�2 (Å

2 )

T (K)50 100 150 200 250 300 350

0T (K)

50 100 150 200 250 300 350

Fig. 7. Temperature dependence of the fitted radial distances around Eu and of the

corresponding standard deviations (EXAFS Debye–Waller factors). Black solid

circles: Eu–O (1st shell); red solid circles: Eu–O (2nd shell); green triangles: Eu–

Nb. (For interpretation of the references to colour in this figure legend, the reader

is referred to the web version of this article.)

Table 3EXAFS fitting parameters for the spectrum of the Eu3þ doped NaNbO3 sample,

shown in Fig. 4 (at 70 K). r: distances; s2: distance variances (Debye–Waller

factors).

Shell N r (A) r2 (A2) Atom type REXAFS¼6.6%

1 5 2.42(1) 5(1)�10�3 O

2 7 2.62(2) 1.4(3)�10�2 O

3 8 3.47(1) 1.4(1)�10�2 Nb

Table 4EXAFS fitting parameters for the spectrum of Nd3þ doped Na0.5K0.5NbO3 sample,

shown in Fig. 4 (at 70 K). r: distances; s2: distance variances (Debye–Waller

factors).

Shell N r (A) r2 (A2) Atom type REXAFS¼12.9%

1 5 2.46(3) 8(3)�10�3 O

2 7 2.64(3) 1.1(5)�10�2 O

3 8 3.47(1) 1.4(1)�10�2 Nb

S. Pin et al. / Journal of Solid State Chemistry 196 (2012) 1–106

shells, is even larger than that found around Nd. The EXAFSfitting, exemplified as a red line in Fig. 4 have been carried outwith a similar model, and the results are reported in Table 3. Thelargest disorder around Eu if compared to Nd is substantiated bythe largest mean values of the s2 parameter for the secondoxygen and for the Nb shells. The Valence Bond calculations gavea charge on Europium equal to 3.4, that is again in fairly goodagreement with the formal Eu(III) oxidation state. The presence ofa large amount of disorder of static origin can account for thesignificant inhomogeneous broadening of the bands observed inthe luminescence spectra of the Eu3þ doped samples (seeluminescence section).

EXAFS spectra at the Nd–K and at the Eu–K edge have beenacquired at 70 K for the doped NKN:Ln nanocrystalline materials.The spectra are shown in Fig. 4 and the fit results are reported inTables 4 and 5. The similarity between the mixed sodium–potassium perovskite and the sodium one is remarkable, andconfirms that the trivalent lanthanide ions enter the structure assubstitutional defects on the alkali site. In order to keep the fittingparameters to a minimum, we used here the same structuralmodel as for the NN:Ln materials, although these mixed niobatesactually exhibit a monoclinic structure. The Valence Bond calcu-lations gave a charge on Eu and Nd equal to 3.3 and 3.4,respectively, that is again in quite nice agreement with the formalLn(III) oxidation state.

As a final remark, we can note that the VBS calculations gave inall the investigated cases formal charges that are slightlylarger with respect to the trivalent lanthanide oxidation state.

This evidence may point to a clustering of oxygen vacancies,created by charge compensation of the LnNa

00 defects, around theLn doping ions.

3.3. TEM and SAED images

TEM images of Eu3þ doped nanostructured niobate samplesheat treated at 600 and 900 1C are shown in Figs. 8 and 9, together

S. Pin et al. / Journal of Solid State Chemistry 196 (2012) 1–10 7

with the selected area electron diffraction (SAED). TEM brightfield micrographs show mainly the presence of single ‘‘rectangu-lar-faced’’ nanoparticles for the samples treated at 600 1C and ofbundles of nanorods for the samples treated at 900 1C. Somenanorods are however present also in the NKN sample treated at600 1C, while some single nanoparticles are still present in thesamples treated at 900 1C. The ‘‘2D’’ images of the single nano-crystals have almost regular ‘‘rectangular’’ shape with averagelengths of about 25–30 nm (short side) and 28–35 nm (long side),as shown in Fig. 8. Due to the particle size and morphology, it israther difficult to calculate exact particle size distributions, but arough estimate of the average particle size is consistent with theXRD analysis reported above. The crystallinity and crystallogra-phy of the products can be extracted by the SAED images thatevidence the polycrystalline nature of the samples. It is worthnoting that the diffraction rings match well with the XRD d-reflections, confirming the presence of an orthorhombic phase forthe NN sample and a monoclinic phase for the NKN sample. Thethermal treatments up to 900 1C do not affect the crystallographicphases but for all the samples lead mostly to a progressive pilingof the single nanocrystals forming nanorods with a length of somehundred nanometres and partially to an assembly of the nano-crystals into larger particles almost cubic in shape. A deeperinsight of the features of the NKN sample treated at 900 1C isgiven by the dark field and HRTEM images shown in Fig. 9.

Table 5EXAFS fitting parameters for the spectrum of Eu3þ doped Na0.5K0.5NbO3 sample,

shown in Fig. 4 (at 70 K). r: distances; s2: distance variances (Debye–Waller

factors).

Shell N r (A) r2 (A2) Atom type REXAFS¼15.0%

1 5 2.47(2) 7(2)�10�3 O

2 7 2.63(5) 2(1)�10�2 O

3 8 3.45(1) 1.0(1)�10�2 Nb

Fig. 8. TEM images and SAED for Eu3þ doped NaNbO3 and Eu3þ

The bundle of nanorods is clearly evidenced in Fig. 9a and SAEDimage in the inset of Fig. 9b shows the polycrystalline nature ofthe sample and confirms the presence of monoclinic phase; somebrightest spots are an indication of the presence of largernanostructures. Accordingly, the dark field image in Fig. 9b showsin the bundle of nanorods either randomly piling single crystal-line nanoparticles or crystalline nanorods as disoriented orderedassembly of nanoparticles piling along the same crystalline axis.This is well consistent with the HRTEM images in Fig. 9(c) and (d),showing the peculiar structure of different nanorods with thesame lattice plane distance of 3.93 A [200] (Fig. 9c) and of 2.78 A[220] (Fig. 9d) all along each nanorod.

3.4. Raman spectroscopy

The Raman spectra of undoped NN samples heat treated at 600and 900 1C and NKN sample heat treated at 900 1C are shown inFig. 10, The spectra of the NN samples look very similar to eachother, apart from the slightly different relative intensities of somespectral bands, suggesting that no relevant structural rearrange-ments occur in the NN samples for different heat treatments at600 or 900 1C, as also evidenced by XRD results. The Raman bandsshow a significant broadening for both the heat treated NNsamples and no significant variation of the bandwidths is noticed.As for the spectra of both NN and NKN powders treated at 900 1C,at first sight, they are quite similar and consist of some broadbands of different relative intensity, extending up to about900 cm�1 far from the laser line. As it usually occurs for oxideperovskites, three main frequency regions can be distinguished inthe Raman spectrum of NaNbO3 [60]. The low frequency regionbelow about 170 cm�1 displays quite a narrow band peaked atabout 70 cm�1, which is the convolution of vibrational modesassociated to translational modes of Naþ (Kþ) and NbO6 groups,and some less intense peaks, between 100 and 150 cm�1, due tolibrational modes of NbO6 group [61]. The intermediate region

doped Na0.5K0.5NbO3 samples heat treated at 600 and 900 1C.

100 nm

Fig. 9. HRTEM and dark field images for the Eu3þ doped nanostructured Na0.5K0.5NbO3 sample heat treated at 900 1C.

(a)

Inte

nsity

(arb

. uni

ts)

(b)

250

(c)

Raman shift (cm-1)500 750 1000

Fig. 10. Raman spectra for the undoped nanostructured NaNbO3 samples heat

treated at 600 1C (a) and 900 1C (b) and for the Na0.5K0.5NbO3 sample heat treated

at 900 1C.

580

900 °C

Inte

nsity

(arb

. uni

ts)

Wavelength (nm)

7F47F3

7F27F1

7F0

600 °C

5D0

578

Inte

nsity

(a. u

.)

Wavelength (nm)

578

Inte

nsity

(a. u

.)

Wavelength (nm)

600 620 640 660 680 700 720

580 582

580 582

Fig. 11. Room temperature emission spectra for the nanostructured Eu3þ doped

NaNbO3 sample heat treated at 600 and 900 1C. Insets: enlarged views of the5D0-

7F0 transition.

S. Pin et al. / Journal of Solid State Chemistry 196 (2012) 1–108

between 200 and 450 cm�1 presents a broad bump peaked atabout 250 cm�1, with some overlapping peaks, usually referred asto the internal vibrational n5 and n6 modes of the NbO6 group.Finally, the high frequency region between 450 and 800 cm�1,shows the stretching vibrations n1 and n2 of the NbO6 octahedron.The Raman spectral features of our NN samples above 150 cm�1

look very similar to those of sodium niobate powders recentlyobtained through a hydrothermal or conventional solid-statemethod by Pan et al. [62]. The spectrum of the NKN powdersheat treated at 900 1C is quite similar to that of the NN sampletreated at the same temperature, except for the low frequencyregion below about 100 cm�1, where in the case of the NKNsample no resolved peak can be observed. On the other hand, themeasured Raman spectrum for the undoped NKN powders is

compatible with that reported by Shiratori et al. [63] for mixedsodium–potassium niobates for a monoclinic structure, in fullagreement with the obtained XRPD results.

3.5. Luminescence spectroscopy

The room temperature luminescence spectra of the NN:Eu andNKN:Eu samples heat treated at 600 and 900 1C are shown inFigs. 11 and 12, in which emission bands attributed to intraconfi-gurational transitions of the Eu3þ ion starting from the 5D0

excited level are present. In particular, the bands in the 550–750 nm range can be assigned to 5D0-

7FJ (J¼0, 1, 2, 3, 4)transitions. The strongest emission band is located around

580

Inte

nsity

(arb

. uni

ts)

Wavelength (nm)

900 °C

5D0

578

Inte

nsity

(a. u

.)

Wavelength (nm)

578

Inte

nsity

(a. u

.)

Wavelength (nm)

600 620 640 660 680 700 720 740

7F1

7F0

7F2

7F3600 °C

7F4 580 582

580 582

Fig. 12. Room temperature emission spectra for the nanostructured Eu3þ doped

Na0.5K0.5NbO3 sample heat treated at 600 and 900 1C. Insets: enlarged views of the5D0-

7F0 transition.

S. Pin et al. / Journal of Solid State Chemistry 196 (2012) 1–10 9

620 nm and is attributed to electric dipole (ED) transitionsbetween the 5D0 and 7F2 Stark levels. This band is stronger thanthat observed around 590 nm, which is due to the magneticdipole (MD) 5D0-

7F1 transition. The intensity of the latter MDtransition is independent of the local environment of the rareearth ion, differently from the hypersensitive 5D0-

7F2 transitionthat is strongly dependent on the Eu3þ environment. All theemission bands are strongly broadened, evidencing a significantstructural disorder around the lanthanide ion sites. The singlet-to-singlet 5D0-

7F0 transition is indicative of the presence ofdifferent sites accommodating the Eu3þ ions in the structure[42]. The bands attributed to this transition are located around awavelength of 580 nm and they are shown for the NN:Eu samplesas insets of Fig. 11. The band profile evidences two transitions,overlapping each other. From a fitting procedure using Gaussianprofiles of the 5D0-

7F0 band, it is possible to discriminate twobands peaking at 580.270.1 and 580.670.1 nm for both NN:Eusamples (heat treated at 600 and at 900 1C). This behaviour is ingood agreement with the X-Ray data, confirming the presence oftwo sites for the dopant lanthanide ions. The calculated FWHM ofthe two emission bands are 0.970.1 and 0.470.1 nm for theNN:Eu at 600 1C, and 0.770.1 and 0.370.1 nm for the NN:Eu at900 1C. Therefore, a small but clear narrowing of the 5D0-

7F0

bands is observed on increasing the temperature, which corre-spond to a decrease of the local disorder around the lanthanideion. This behaviour can also be noted from the slight sharpeningof all the emission bands assigned to the 5D0-

7FJ (J¼1, 2, 3, 4)transitions on increasing the annealing temperature (see Fig. 11).

The emission spectra of the NKN:Eu samples are similar to theNN:Eu ones, even if they evidence a more pronounced inhomo-geneous broadening of all the emission bands. On the other hand,this behaviour is reasonable, taking into account the multisitenature of the alkaline-earth ions for the mixed sodium–potassiumniobate in the lattice. In fact, these ions are accommodated in8 distinct, although similar for symmetry, sites in the crystalstructure. Therefore, the lanthanide ions accommodated in thedifferent sites experience a slightly different crystal field, and theobserved emission becomes a convolution of similar spectra, eachof them affected by inhomogeneous broadening due to the localdisorder. As a result, the observed bands are very broad, similar tothose observed for Eu3þ doped oxide glass hosts [64]. Thisbehaviour is also confirmed by the broad 5D0-

7F0 band, shownin the insets of Fig. 12, which exhibits no fine structure. TheFWHM of this band are 3471 and 3171 cm�1 for the niobate

samples treated at 600 and 900 1C, respectively. These value arealso larger than those found for the NN:Eu samples, for which theFWHM values are 3371 and 2771 cm�1 for the samples heattreated at 600 and 900 1C, respectively.

4. Conclusions

Undoped, Nd3þ or Eu3þ doped NaNbO3 and Na0.5K0.5NbO3

nanostructured samples (nanoparticles or nanorods) with orthor-hombic (Pmc21 space group), and monoclinic structure (Pm spacegroup), respectively, have been prepared with a facile, environ-mental friendly sol–gel route. An increase of the heat treatmenttemperature induces growth of the nanoparticles, thus formingelongated nanostructures, as nanorods. The local structure aroundthe trivalent lanthanides dopants was investigated by means ofEXAFS at the Ln–K edge and optical spectroscopy. The Ln3þ ionsenter the structure by substituting the alkali metals, with a 12-fold total oxygen coordination, with a large amount of staticdisorder. This behaviour is different from that found for Eu3þ

doped nanocrystalline LiNbO3, for which it was observed that thelanthanide ion enters into both the Liþ and Nb5þ lattice positionsof the niobate structure [65]. The optical emission bands of Eu3þ

indicate that multiple sites exist for Eu3þ , in agreement with theEXAFS results, showing the largest amount of static disorder inthese samples. In both NN:Ln and NKN:Ln materials, VBS calcula-tions give a formal charge larger than the expected value of 3þ . Apossible indication of clustering of oxygen vacancies around theLnNa

00 defect is obtained by VBS calculations.

Acknowledgments

The Authors wish to thank Erica Viviani (University of Verona,Italy) for expert technical assistance. ESRF is acknowledged forprovision of beamtime (exp. CH-2878). Particular thanks are dueto Dr. Olivier Mathon for considerable help during the XAS datacollection. This work was supported by Fondazione Cariverona(Verona, Italy), project Verona Nanomedicine Initiative.

Appendix A. Supporting information

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.jssc.2012.08.003.

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