8
Solid State Sciences 4 (2002) 1129–1136 www.elsevier.com/locate/ssscie Structural study of the cation ordering in the ternary oxide Ba 8 Ti 3 Nb 4 O 24 Nicolas Teneze a , Philippe Boullay a,, Vaclav Petricek b , Gilles Trolliard a , Danièle Mercurio a a Science des Procédés Céramiques et de Traitements de Surface, CNRS-UMR6638, Université de Limoges, Faculté des Sciences, 123 av. Albert Thomas, 87060 Limoges cedex, France b Institute of Physics, Academy of Sciences of the Czech Republic, 18221 Praha, Czech Republic Received 25 April 2002; received in revised form 5 July 2002; accepted 17 July 2002 Abstract The ternary oxide Ba 8 Ti 3 Nb 4 O 24 was synthesized and, based on a transmission electron microscopy study, its crystal structure was re- examined using single crystal X-ray diffraction. Ba 8 Ti 3 Nb 4 O 24 is a B-site deficient “hexagonal” perovskite with a (hccc) 2 -type stacking sequence of the BaO 3 layers. In addition to the previous structural studies, the existence of a superstructure is clearly demonstrated (a = 10.068(4) Å, c = 18.917(2) Å, SG: P6 3 /mcm, Z = 3). This superstructure is associated with the complex ordering of the Ti atoms, Nb atoms and vacancies on the B-sites of the h.c.p. part of the structure, which can be itself related to the necessity to minimize electrostatic repulsion between cations located in adjacent face-sharing octahedral layers. 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Hexagonal perovskite; Cationic ordering; Single crystal X-ray diffraction; Transmission electron microscopy 1. Introduction The perovskite structure ABO 3 can be described as the stacking of close packed AO 3 layers in a cubic sequence (ABC, ...). The B-site cations occupy ¼ of the octahedral cavities existing between the AO 3 layers in such a way that the BO 6 octahedra form a 3D network of corner-sharing octahedra (CSO). This octahedral framework is known for its great ability to accommodate various structural modifi- cations (crystallographic shearing, octahedral tilting, ...) in order to adapt for instance ionic substitutions or non stoi- chiometry features. This possibility combined with the phe- nomenal range of catalytic, electronic and magnetic prop- erties exhibited by transition metal perovskite undoubtedly originates the extensive work devoted to these materials (see [1] as a review). Among the perovskite-related structures, the so-called “hexagonal” perovskites exhibit a structure also character- ized by a close packing of AO 3 layers but differ from the “classical” cubic stacking of the perovskite by the introduc- tion of hexagonal stacking sequence (AB, . . . ). Hence the denomination of “hexagonal” perovskite is actually used to describe mixed hexagonal-cubic AO 3 stacking sequences. These sequences can be advantageously described using the * Correspondence and reprints. E-mail address: [email protected] (P. Boullay). Jagodzinski notation [2] where a layer in the sequence is de- noted h or c whether its neighboring layers are alike or differ- ent. The existence of such mixed sequences in “hexagonal” perovskite implies the apparition of BO 6 face-sharing oc- tahedra (FSO). The stability of the “hexagonal” perovskite is known [1] to be strongly dependent of the size and the formal charge of the B-site cations present in these adja- cent face-sharing octahedral layers. Actually, when the B- site cations have a large formal charge, some compensating mechanisms such as the occurrence of B-site vacancies can take place to reduce the electrostatic repulsion. Different type of B-site deficient “hexagonal” perovskite AB 1x O 3 can exist depending on the repartition of vacan- cies. In case of (hhc... c)-type sequences (Fig. 1a), the va- cancies are usually ordered between the hh layers resulting in a completely vacant face-sharing octahedral layer. These structures, considering the general formulation A n B n1 O 3n , possess thus only blocks of n 1 octahedral layers (CSO) separated by one vacant octahedral layer. Considering the resulting periodic shift in the stacking of CSO blocks, this (hhc...c)-type of B-site deficient “hexagonal” perovskite will be further referred as “shifted” perovskite. In case of (hc...c)-type sequences, the ordering of B-site vacancies does not result in the existence of a vacant octahedral layer but in the partial occupancy of the FSO (Fig. 1b). Consid- ering the intrinsic and periodically twinned character of the AO 3 layers stacking in these (hc...c)-type sequences, this 1293-2558/02/$ – see front matter 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. PII:S1293-2558(02)01371-7

Structural study of the cation ordering in the ternary oxide Ba8Ti3Nb4O24

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Solid State Sciences 4 (2002) 1129–1136www.elsevier.com/locate/ssscie

Structural study of the cation ordering in the ternary oxide Ba8Ti3Nb4O24

Nicolas Tenezea, Philippe Boullaya,∗, Vaclav Petricekb, Gilles Trolliarda, Danièle Mercurioa

a Science des Procédés Céramiques et de Traitements de Surface, CNRS-UMR6638, Université de Limoges, Faculté des Sciences, 123 av. Albert Thomas,87060 Limoges cedex, France

b Institute of Physics, Academy of Sciences of the Czech Republic, 18221 Praha, Czech Republic

Received 25 April 2002; received in revised form 5 July 2002; accepted 17 July 2002

Abstract

The ternary oxide Ba8Ti3Nb4O24 was synthesized and, based on a transmission electron microscopy study, its crystal structure was re-examined using single crystal X-ray diffraction. Ba8Ti3Nb4O24 is a B-site deficient “hexagonal” perovskite with a (hccc)2-type stackingsequence of the BaO3 layers. In addition to the previous structural studies, the existence of a superstructure is clearly demonstrated(a = 10.068(4) Å, c = 18.917(2) Å, SG: P63/mcm,Z = 3). This superstructure is associated with the complex ordering of the Ti atoms, Nbatoms and vacancies on the B-sites of the h.c.p. part of the structure, which can be itself related to the necessity to minimize electrostaticrepulsion between cations located in adjacent face-sharing octahedral layers. 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.

Keywords: Hexagonal perovskite; Cationic ordering; Single crystal X-ray diffraction; Transmission electron microscopy

1. Introduction

The perovskite structure ABO3 can be described as thestacking of close packed AO3 layers in a cubic sequence(ABC, . . .). The B-site cations occupy ¼ of the octahedralcavities existing between the AO3 layers in such a way thatthe BO6 octahedra form a 3D network of corner-sharingoctahedra (CSO). This octahedral framework is known forits great ability to accommodate various structural modifi-cations (crystallographic shearing, octahedral tilting,. . .) inorder to adapt for instance ionic substitutions or non stoi-chiometry features. This possibility combined with the phe-nomenal range of catalytic, electronic and magnetic prop-erties exhibited by transition metal perovskite undoubtedlyoriginates the extensive work devoted to these materials(see [1] as a review).

Among the perovskite-related structures, the so-called“hexagonal” perovskites exhibit a structure also character-ized by a close packing of AO3 layers but differ from the“classical” cubic stacking of the perovskite by the introduc-tion of hexagonal stacking sequence (AB,. . . ). Hence thedenomination of “hexagonal” perovskite is actually used todescribe mixed hexagonal-cubic AO3 stacking sequences.These sequences can be advantageously described using the

* Correspondence and reprints.E-mail address: [email protected] (P. Boullay).

Jagodzinski notation [2] where a layer in the sequence is de-noted h or c whether its neighboring layers are alike or differ-ent. The existence of such mixed sequences in “hexagonal”perovskite implies the apparition of BO6 face-sharing oc-tahedra (FSO). The stability of the “hexagonal” perovskiteis known [1] to be strongly dependent of the size and theformal charge of the B-site cations present in these adja-cent face-sharing octahedral layers. Actually, when the B-site cations have a large formal charge, some compensatingmechanisms such as the occurrence of B-site vacancies cantake place to reduce the electrostatic repulsion.

Different type of B-site deficient “hexagonal” perovskiteAB1−xO3 can exist depending on the repartition of vacan-cies. In case of (hhc. . .c)-type sequences (Fig. 1a), the va-cancies are usually ordered between the hh layers resultingin a completely vacant face-sharing octahedral layer. Thesestructures, considering the general formulation AnBn−1O3n,possess thus only blocks ofn − 1 octahedral layers (CSO)separated by one vacant octahedral layer. Considering theresulting periodic shift in the stacking of CSO blocks, this(hhc. . .c)-type of B-site deficient “hexagonal” perovskitewill be further referred as “shifted” perovskite. In case of(hc. . .c)-type sequences, the ordering of B-site vacanciesdoes not result in the existence of a vacant octahedral layerbut in the partial occupancy of the FSO (Fig. 1b). Consid-ering the intrinsic and periodically twinned character of theAO3 layers stacking in these (hc. . .c)-type sequences, this

1293-2558/02/$ – see front matter 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.PII: S1293-2558(02 )01371-7

1130 N. Teneze et al. / Solid State Sciences 4 (2002) 1129–1136

Fig. 1. Schematic representation of the two type of “hexagonal” perovskite encountered in the pseudo binary systems Ba5Nb4O15–BaTiO3.

type of B-site deficient “hexagonal” perovskite will be fur-ther referred as “twinned” perovskite.

In pseudo binary systems like Ba5Nb4O15–BaTiO3,“shifted” and “twinned” perovskite can be found depend-ing on the value of the integer m associated to the chemicalcomposition Ba5+mNb4TimO15+3m. For m = 0 and 1, the“shifted” perovskite Ba5Nb4O15 [3] – sequence (hhccc) –and Ba6Nb4TiO18 [4] – sequence (hhcccc)3 – are the stableones. Form = 2, a mixture of “shifted” and “twinned” per-ovskite is observed (detailed study in a forthcoming publi-cation). Form = 3 the compound Ba8Nb4Ti3O24 [5] – se-quence (hccc)2 – appears to be the first stable “twinned”perovskite within this system. The structure of this com-pound was shown to possess partially occupied FSO as forBa8Ta4Ti3O24 in an equivalent system [5]. In a recent struc-tural study of Ba8Ta4Ti3O24 [6] and Ba8Ta6NiO24 [7], it wasshown that a complex B-site cationic ordering actually takesplace in the FSO. Following our investigation of the pseudobinary Ba5Nb4O15–BaTiO3, we propose the structural re-examination of the crystal structure of Ba8Nb4Ti3O24 tocheck whether similar structural features can be evidenced.

2. Experimental

Single crystals of Ba8Ti3Nb4O24 were obtained usinghigh purity BaCO3, TiO2 and Nb2O5 oxides. The startingpowders weighted in stoichiometric proportions were thor-oughly mixed in an agate mortar and then fired in a platinumcrucible under ambient atmosphere. The following thermaltreatment was applied: heating up to 1873 K at 10 K/h,dwelling time 10 h, then cooling down to 1773 at 3 K/h andfinally cooling down to room temperature at 10 K/h. Trans-parent crystals in the form of flat platelets were extractedfrom the melted mixture.

Table 1Crystal data and structure refinement for Ba8Ti3Nb4O24

Crystal Flat, transparent0.22 mm×0.12 mm×0.03 mm

Wavelength KαMo: 0.71073 ÅDiffractometer Siemens P4Scan mode ω

Space group P63/mcm (n◦193)Unit cell parameters a = 10.068(4) Å

c = 18.917(2) ÅTheta range for data collection 4.69◦ to 66.91◦Reflections collected/unique 2403/561 [R(int) = 0.040]Criterion forIobs I > 3σ(I )

Reflections observed (total/supercell) 213/15Reliabily factors (observed) Robs= 2.61%

wRobs= 4.11%Reliabily factors (unique) Rall = 6.38%

wRall = 4.54%Extinction correction Type 1 Lorentzian isotropicProgram Jana 2000

Selected Area Electron Diffraction Patterns (SAEDPs)and High Resolution Transmission Electron Microscopy(HRTEM) images were obtained using a JEOL 2010 mi-croscope operating at 200 kV with a double tilting stage.Some crystals were selected from the melt and crushed inan agate mortar to obtain small fragments that were put insuspension in water. A drop of the suspension was then de-posited and dried on a copper grid, previously coated with athin film of amorphous carbon. For each examined crystal,the chemical composition was checked using the analyticalsystem mounted on the TEM (EDX) and the crystals werefound to have a composition close to the nominal one i.e.Ba8Ti3Nb4O24.

Single crystal X-ray diffraction data were collected ona P4 Siemens four-circle diffractometer using a graphitemonochromatized MoKα wavelength. The data collection

N. Teneze et al. / Solid State Sciences 4 (2002) 1129–1136 1131

Fig. 2. HRTEM image of Ba8Ti3Nb4O24 white dots represent barium atoms.

Fig. 3. Experimental (a, b) and simulated (c, d, e and f) SAED patterns of Ba8Ti3Nb4O24. Simulations with the P63/mmc space group: c (zone axis[1̄ 21̄0])and d (zone axis[11̄ 00]). Simulations with the P63/mcm space group: e (zone axis[11̄ 00]) and f (zone axis[1̄ 21̄ 0]).

1132 N. Teneze et al. / Solid State Sciences 4 (2002) 1129–1136

(a)

(b) (c)

Fig. 4. Schematic representation of the Ba8Ti3Nb4O24 structure along the[11̄ 00] (a), Oy (b) and Oz (c) directions.

conditions are given in Table 1. The intensities were cor-rected for Lorentz-polarization effects and absorption cor-rections were applied using the Gaussian method. These cor-rections and the crystal structure analysis were performedusing the JANA2000 [8] software.

3. Electron microscopy and symmetry consideration

All the examined crystals are homogeneous and presentthe same structural characteristics. They are ordered andfree of stacking faults. A typical HRTEM image observedfor the Ba8Ti3Nb4O24 crystals is displayed in Fig. 2 andrevealed the intrinsic twinned character of the structure ofthis compound. The comparison between this image and thestructural model determined by Mössner et al. [5] (insertedin Fig. 2) indicates that, in first approximation, the positionof the white dots in the image corresponds to the position ofthe barium atomic columns. Each linear part of the twin isconstituted by 5 white dots (5 barium atomic columns) and

can be associated to the existence within the structure of aslab made of 4 corner sharing octahedra (c.c.p. part of thestructure – see Fig. 1b and insert in Fig. 2).

Two typical SAEDPs observed for Ba8Ti3Nb4O24 crys-tals are shown in Fig. 3. The second SAEDPs displayed inFig. 3b is obtained from the first one (Fig. 3a) by a rotationof 30◦ around the[0 0 0 1]∗ direction (4 indexes hexagonalnotation). In order to check consistency with the previousstructural results [5], we performed SAEDPs simulations us-ing the Microdiffraction software [9] and the space groupP63/mmc witha = 5.79 Å andc = 18.85 Å. These simula-tions are purely geometrical and do not account for the ob-served relative intensities. While the SAEDPs simulated forthe[1̄ 21̄0]P63/mmc zone axis (Fig. 3c) and the experimentalone (Fig. 3a) are similar (additional spots observed along thec∗ direction in the experimental pattern are due to a multiplescattering phenomena), the situation appears quite differentfor the[11̄0 0]P63/mmc zone axis (Fig. 3b and 3d). Indeed incomparison with the simulated one, the experimental pattern

N. Teneze et al. / Solid State Sciences 4 (2002) 1129–1136 1133

Table 2Atomic coordinates, isotropic thermal parameters and occupation rates

Site Wyckoff x y z Ueq/Uiso Occupation Globalposition occupation

Ba(1) 6g 0.3325(2) 0 0.75 0.0153(6) 100% 100%Ba(2) 12k 0.3337(2) 0 0.13559(4) 0.0231(5) 100% 100%Ba(3) 2b 0 0 0 0.0174(9) 100% 100%Ba(4) 4d 1/3 2/3 0 0.0229(6) 100% 100%Nb(1) 4e 0 0 0.1877(5) 0.025(2) 50%* 50%Nb(2) 8h 2/3 1/3 0.1846(2) 0.0244(11) 24.0(7)% 87.5%Ti(2) 8h 2/3 1/3 0.1846(2) 0.0244(11) 63.5(7)%Nb(3) 12k 0.3339(3) 0 0.56181(6) 0.0149(8) 67.3(5)% 100%Ti(3) 12k 0.3339(3) 0 0.56181(6) 0.0149(8) 32.7(5)%O(1) 12j 0.520(2) 0.180(2) 0.25 0.035(4) 100% 100%O(2) 6g 0.157(2) 0 0.25 0.001(4) 100% 100%O(3) 12k 0.163(2) 0 0.622(1) 0.045(5) 100% 100%O(4) 24l 0.492(1) 0.162(1) 0.6210(4) 0.012(2) 100% 100%O(5) 6f 1/2 0 1/2 0.009(4) 100% 100%O(6) 12i 0.331(2) 0.1655(11) 1/2 0.022(3) 100% 100%

* Fixed parameter.

Table 3Anisotropic thermal parameters

Site U11 U22 U33 U12 U13 U33

Ba(1) 0.0163(6) 0.0154(10) 0.0140(5) 0.0077(5) 0 0Ba(2) 0.0242(5) 0.0249(8) 0.0206(5) 0.0124(4) 0.0037(7) 0Ba(3) 0.016(1) 0.016(1) 0.020(2) 0.0081(5) 0 0Ba(4) 0.0154(6) 0.0154(6) 0.038(1) 0.0077(3) 0 0Nb(1) 0.013(2) 0.013(2) 0.049(6) 0.007(1) 0 0Nb(2) 0.017(1) 0.017(1) 0.039(2) 0.0084(6) 0 0Ti(2) 0.017(1) 0.017(1) 0.039(2) 0.0084(6) 0 0Ti(3) 0.0156(8) 0.015(1) 0.0143(7) 0.0073(6) −0.005(1) 0Ti(3) 0.0156(8) 0.015(1) 0.0143(7) 0.0073(6) −0.005(1) 0

has rows of extra spots along[1 1 0 0]∗P63/mmc, which suggestthe existence of a superstructure along this direction.

A closer examination of the observed reflection condi-tions and the periodicity in the patterns indicate that the ex-perimental SAEDPs can be properly indexed using the spacegroup P63/mcm witha = 10.03 Å andc = 18.85 Å. In thisspace group, the new cell derives from the basic one by a ro-tation of 30◦ around the[0 0 0 1] axis which involves a per-mutation of the[1̄ 21̄0] and[11̄0 0] zone axis. Thec para-meter remains unchanged whereas the relation between thenew a parameter and the basic one is:

aP63/mcm = 3× d(1 1 0)P63/mmc

sin 120

The simulated patterns, calculated with the P63/mcm spacegroup, agree with the experimental ones as shown in Fig. 3eand 3f. Notice that this result is in agreement with theconclusion drawn in [6] for the phase Ba8Ti3Ta4O24.

4. Results and discussion

As indicated in the Table 1, the superstructure gives onlya very weak signal in X-ray diffraction and the structural

model has to account for a large number of unobserved re-flections. Nonetheless, in light of the new structural con-siderations evidenced by TEM, we decided to reinvestigatethe crystal structure of Ba8Ti3Nb4O24. In agreement withthe EDX analyses, the chemical composition was assumedto correspond to the nominal one i.e. Ba8Ti3Nb4O24. Thestarting atomic positions were calculated using the previousstructure [5] and with the transformation matrix:[

xP63/mmcyP63/mmczP63/mmc

]∣∣∣∣∣1 1̄ 01 2 00 0 1

∣∣∣∣∣ =[

xP63/mcmyP63/mcmzP63/mcm

]

As illustrated in Fig. 4a, the Ba8Ti3Nb4O24 structure canbe seen as having a framework built up eight close packedBaO3 layers (8H) with the stacking sequence ABCBACBCor (hccc)2 in the Jagodzinski notation [2]. The cubic closedpacked (c.c.p.) part of the sequence is constituted of fiveBaO3 layers and the hexagonal closed packed (h.c.p.) part ofthree BaO3 layers. The structure presents thus a successionof two CSO and two FSO. Alternatively, one can describedthe structure has been made of two perovskite slabs of fourCSO thick associated by a chemical twinning trough FSO.

As previously mentioned, the main features of the crystalstructure is the distribution of cations and vacancies on

1134 N. Teneze et al. / Solid State Sciences 4 (2002) 1129–1136

(a)

(b)

Fig. 5. Perspective views of the Ba (a) and B (b) cations polyhedra.

the B-sites. In this new setting three B-sites exist in thestructure: B(1) and B(2) located in the h.c.p. part ofthe structure (FSO) and B(3) located in the middle ofthe c.c.p. part of the structure (CSO). In the first stepof the refinement, a common isotropic thermal parameter(Uiso = 0.015) was fixed for these three B-sites and theiroccupancy where refined supposing the only presence ofNb atoms. The examination of the obtained occupanciesleads to the following result. Considering the relative weightof Nb5+ (36e−) and Ti4+ (18e−) cations and assuming afull occupancy for the B(3) site, the ratio Ti: Nb on thissite was found to be close to 1: 2, i.e., B(3) ∼ (1/3Ti +

2/3Nb). For the B-sites located in the h.c.p. part of thestructure, the “mathematical” possibilities in agreement witha composition Ba8(Ti3Nb4�1)O24 are restricted by two limitmodels: B(1) corresponds to(1/2Nb+ 1/2�) and B(2) to(1/4Nb+ 5/8Ti + 1/8�) or B(1) corresponds to 100%Tiand B(2) to(1/2Nb+ 1/8Ti + 3/8�). Both models resultin similar reliability factors but on a chemical point ofview, the very short B(1)–B(1) distance (2.36 Å) obtainedin both cases is a strong argument towards a maximizationof the vacancies on this site. The values of the first limitmodel were then assumed to be the most probable andused as starting parameters. The occupancies were refinedto their final values together with the atomic coordinatesand isotropic thermal parameters (Table 2). Notice thatrestrictions were imposed on the occupancies in order toobtain a full occupancy for the B(3) site, 50% Nb occupancyfor the B(1) site and preservation of the ratios 3: 4 : 1between Ti: Nb : �. The anisotropic thermal parametersused for the A and B cationic sites are given in Table 3.The statistical reliability factors obtained for this refinementcorrespond towRobs = 4.11% andwRall = 4.54%. Theinter-atomic distances in the coordination sphere of thecationic sites are given in Table 4.

Within the BaO3 close packed framework, the B-sitesof the perovskite existing between the BaO3 layers havea mixed Ti/Nb occupancy. Our structural reinvestigationshows that Ba8Ti3Nb4O24 has actually a B sites ordering inwhich the B(3) site is fully occupied with a mixed Ti/Nboccupancy while the B(1) and B(2) sites present vacancies(see Fig. 4a and 4b). The B(1) site is occupied at 50% by Nbatoms solely whereas the B(2) site is occupied at 87.5% byNb and Ti atoms (24% and 63.5% respectively). This is the

N. Teneze et al. / Solid State Sciences 4 (2002) 1129–1136 1135

Fig. 6. Atomic displacements connected to the ideal positions and polyhedra size.

particular cationic ordering observed for these two B(1) andB(2) sites (see Fig. 4b and 4c) that results in the change ofspace group and cell parameters compared to the previousstudy [5].

The Ba2+ cations are in a twelve-fold coordination.The average distance〈Ba–O〉 in each site is only slightlyshorter than in the theoretical one (Table 4) which expressthat the bonds are more covalent in this structure. The Baenvironments are represented in Fig. 5a. The fact that theBa(2) site is displaced from its original position towards theFSO part of the structure (see Fig. 6) can be understoodas a compensation for the cationic charge deficiency of theB(1) and B(2) sites. In Fig. 5b, the octahedral environmentof the B-type cations Ti4+ and Nb5+ are displayed. If weconsider an average distance of 2.32 Å between two closepacked BaO3 layers, the B(3)O6 octahedra are only slightlycompressed (2.30 Å) whereas the B(1)O6 and B(2)O6

octahedra are stretched along the Oz stacking direction(2.42 Å). Additionally, the cations in the B(1) and B(2) sitesare strongly displaced along the stacking direction from theirideal position in the center of the octahedra (Table 4 andFig. 6).

Regarding the B(1) sites, the Nb5+ cations are displacedtowards the shared face of two adjacent B(1)O6 octahedra,which implies a short 2.35 Å Nb–Nb distance that wouldcorrespond to a metal–metal bond. However since this site isonly occupied at 50%, this situation is most probably not re-alized. One can indeed estimate that only one of the pairs ofoctahedra is statistically occupied so that the real occupationat this position would be (0.5 Nb+ 0.5�). Nonetheless thevalue of the anisotropic thermal parameter for the Oz direc-tion (U33 = 0.049) could indicate a cationic displacement ofsome Nb atoms in order to elongate the Nb–Nb distance incases where two sites are simultaneous occupied.

The situation is somewhat complicated in the case ofthe B(2) site that is occupied at 63.5% by Ti atoms and

at 24% by Nb atoms. This site appears to be displacedtowards the middle of the perovskite slabs (see arrows onFig. 6) in order to increase the distance between two adjacentB(2) sites when two face shared octahedra are occupied.Nevertheless, this distance remains short (∼ 2.48(1) Å) anda closer examination of the occupation rate of Nb5+, Ti4+and vacancies on this site is necessary. If one considerthat Nb atoms are preferably facing vacancies this wouldcorrespond to a maximum of 25% of the B(2)–B(2) couples(since 12.5% of vacancies exist). For the rest two limitpossibilities can be considered depending whether Nb–Nbcouples are acceptable or not. In the first hypothesis, theother adjacent B(2)O6 octahedra can be occupied at 63.5%by Ti–Ti couples and at 11.5% by Nb–Nb couples. In thesecond case, this would involve 52% Ti–Ti couples and 23%Nb–Ti couples. One can then estimate that two adjacentB(2)O6 octahedra can be occupied by:

• two Ti4+ cations (52 to 63.5%),• one Nb5+ cation and one vacancy (25%),• one Nb5+ and one Ti4+ (0 to 23%) and/or one Nb5+ and

one Nb5+ (0 to 11.5%).

This last possibility is to our opinion minor but regard-ing the B(2) occupancy, one can not ensure that occasionallyNb atoms are facing Nb atoms. Nonetheless, even consid-ering the two first possibilities as the major cases encoun-tered in FSO, a distance Ti–Ti of 2.48 Å is significantlyshorter than the 2.67 Å observed in pure hexagonal BaTiO3[10] and would indicate a metal–metal bonding. However,for these B(2)O6 octahedra the O–O distances in sharedfaces is only 2.64 Å, as compared to 2.8–2.9 Å observedin B(1)O6 and B(3)O6 octahedra, suggesting that the B(2)cations are slightly shielded one from another along the c di-rection. At this point we have investigated the possibility ofa split-position for this B(2) site. Using a common isotropicthermal parameter (Uiso = 0.015(1)), we refined indepen-

1136 N. Teneze et al. / Solid State Sciences 4 (2002) 1129–1136

Table 4Inter-atomic distances in the coordination sphere of cationic sites

Site[coord.]/theoretical distance Liaisons Distance (Å)

Ba(1)[12] 2×Ba(1)–O(1) 2.97(2)

〈Ba–O〉[12]theo =3.01 2×Ba(1)–O(3) 2.96(2)

4×Ba(1)–O(4) 2.93(8)

2×Ba(1)–O(2) 2.90(2)

2×Ba(1)–O(1) 2.86(2)

〈Ba(1)–O〉 2.92

Ba(2)[12] 2×Ba(2)–O(6) 3.07(1)

〈Ba–O〉[12]theo =3.01 1×Ba(2)–O(5) 3.06(1)

2×Ba(2)–O(4) 2.95(1)

2×Ba(2)–O(3) 2.92(2)

2×Ba(2)–O(4) 2.89(1)

2×Ba(2)–O(1) 2.84(1)

1×Ba(2)–O(2) 2.80(1)

〈Ba(2)–O〉 2.85

Ba(3)[12] 6×Ba(3)–O(6) 2.89(2)

〈Ba–O〉[12]theo =3.01 6×Ba(3)–O(3) 2.83(2)

〈Ba(3)–O〉 2.86

Ba(4)[12] 3×Ba(3)–O(6) 2.93(2)

〈Ba–O〉[12]theo =3.01 3×Ba(4)–O(5) 2.906(2)

6×Ba(4)–O(4) 2.876(9)

〈Ba(3)–O〉 2.90

Nb(1)[6] 3×Nb(1)–O(3) 2.06(2)

〈Nb–O〉theo =2.04 3×Nb(1)–O(2) 1.97(2)

〈Nb(1)–O〉 2.02

Nb(2)[6] 3×Nb(2)–O(4) 2.12(1)

〈Nb–O〉theo =2.04 3×Nb(2)–O(1) 1.95(2)

〈Nb(2)–O〉 2.03

Ti(2)[6] 3×Ti(2)–O(4) 2.12(1)

〈Ti–O〉theo =2.00 3×Ti(2)–O(1) 1.95(2)

〈Ti(2)–O〉 2.03

Nb(3)[6] 1×Nb(3)–O(3) 2.06(2)

〈Nb–O〉theo =2.04 2×Nb(3)–O(6) 2.05(1)

1×Nb(3)–O(5) 2.04(2)

2×Nb(3)–O(4) 1.96(1)

〈Nb(3)–O〉 2.01

Ti(3)[6] 1×Ti(3)–O(3) 2.06(2)

〈Ti–O〉theo =2.00 2×Ti(3)–O(6) 2.05(1)

1×Ti(3)–O(5) 2.04(2)

2×Ti(3)–O(4) 1.96(1)

〈Ti(3)–O〉 2.01

dently thez-coordinates for Nb(2) and Ti(2) and obtainedthe significantly different values ofzTi(2) = 0.179(1) andzNb(2) = 0.194(1). The corresponding B(2)–B(2) distancesare then respectively of 2.69(1) Å and 2.13(1) Å. This resultis in perfect agreement with our assumptions on the occu-pancy of the FSO:

• Nb atoms are mostly associated with vacancies and aredisplaced towards the shared faces.

• Ti atoms are associated mainly with Ti atoms (occasion-ally with Nb) and are displaced towards the corner shar-ing octahedral layers.

In the part of the structure where the octahedra are linkedby CSO, the situation is different since the distance B(3)–B(3) of 4.07 Å is long enough to have successive octahedraoccupied by Nb5+ cations.

5. Conclusion

Based on TEM investigations, we have revisited the struc-ture of the ternary oxide Ba8Ti3Nb4O24 and demonstratedthe existence of a superstructure associated to a cationic or-dering on the B sites of this perovskite-related compound.Our combined SAED and single crystal X-ray study asshown, as suggested by Shpanchenko et al. [6], that the struc-ture of Ba8Ti3Nb4O24 is similar to the one proposed forBa8Ti3Ta4O24. As already observed for this last compound,the specific ordering results from the need to accommodatecationic size, formal charge and stoichiometry on the defi-cient B cationic sites. More precisely, the ordering in theFSO involving B(1)O6–B(1)O6 and B(2)O6–B(2)O6 octahe-dra appears to be governed by the necessity to avoid a con-figuration where two adjacent FSO are simultaneously oc-cupied by Nb atoms. In FSO, the configuration can thus belimited to two major situations: octahedra occupied by Nbatoms are facing empty octahedra and octahedra occupiedby Ti atoms are facing one each other.

On behalf the present study, the accurate knowledge ofthe conditions governing the cationic ordering in these B-sitedeficient “hexagonal” perovskite compounds is a key pointif one wants to understand the relative stability of “shifted”and “twinned” perovskite. Based on this accurate structuralanalysis this point will be illustrated within this samepseudo-binary system Ba5Nb4O15–BaTiO3 in a forthcomingpublication.

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