This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institution

and sharing with colleagues.

Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third party

websites are prohibited.

In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further information

regarding Elsevier’s archiving and manuscript policies areencouraged to visit:

http://www.elsevier.com/copyright

Author's personal copy

Materials Chemistry and Physics 118 (2009) 15–20

Contents lists available at ScienceDirect

Materials Chemistry and Physics

journa l homepage: www.e lsev ier .com/ locate /matchemphys

Materials science communication

Study on phase stability and ionic conductivity in HfIV-substituted bismuthvanadate

Saba Beg, Niyazi A.S. Al-Areqi ∗

Department of Chemistry, Aligarh Muslim University, Aligarh 202002, India

a r t i c l e i n f o

Article history:Received 15 January 2009Received in revised form 18 June 2009Accepted 16 July 2009

Keywords:Bi4HfxV2−xO11−(x/2)−ı

Phase stabilityX-ray powder diffractionIonic conductivity

a b s t r a c t

Influence of dopant concentration on thermal behaviour of Bi4HfxV2−xO11−(x/2)−ı was studied over com-position range 0 ≤ x ≤ 0.40 by combination of data obtained from X-ray powder diffraction, differentialscanning calorimetric and conductivity measurements. For very low dopant concentrations, the systemwas found to mimic the parent compound in exhibiting two consecutive transitions,� ↔ � ↔ �, withslightly different onset temperatures compared to that of parent, whereas the existence of � ↔ � tran-sition was well confirmed in composition range 0.15 ≤ x ≤ 0.20 and typified in Arrhenius plots to twoline regions of different activation energy. For composition range 0.25 ≤ x ≤ 0.40, no significant struc-tural changes associated with �′ ↔ � transition is visible in X-ray powder diffraction data with variabletemperature. Despite this, the existence of �′ ↔ � transition was evident by some complex incommen-surate modulations observed in Arrhenius plots and DSC/DTA thermograms. It was surprising to notethe appearance of a new destructive � ↔ �′ transition, resulting from distortion of the tetragonal phasestructure at higher temperatures. The relationship between phase stability and ionic conductivity wasalso rationalized.

Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved.

1. Introduction

Oxide ion electrolytes are of great importance due to theirexcellent applications in solid oxide fuel cells (SOFCs), water-vapour electrolytes and oxygen sensors. The BIMEVOXes, basedon metal-doped Bi4V2O11 have been discovered by Abraham etal. [1,2]. The parent compound Bi4V2O11 structurally belongs to alayered Aurivillius-type of compounds with intrinsic vacancies inpervoskite (VO3.5�0.5)2− layers sandwiched between bismuthate(Bi2O2)2+ layers, where � represents an oxide ion vacancy. Order-ing of these vacancies results in three principal polymorphic forms:� (monoclinic), � (orthorhombic) and � (tetragonal) and two suc-cessive phase transitions: � ↔ � and � ↔ � at about 445 and 567 ◦Crespectively [3]. It has been found that with increasing depar-ture from Bi4V2O11 stoichiometry, the transition temperaturesdecrease, leading to a gradual stabilization of the high tempera-ture �-phase which is predominantly responsible for high oxideion conductivity [4,5]. However, stabilization of highly conducting�-phase to room temperature can also be acquired by introducingextrinsic vacancies through solid solution formation with substi-tution of V by a large range of iso- or aliovalent cations such asCu2+, Ni2+, Zn2+, Co2+, Al3+, Ti4+, Nb5+, Ta5+, Zr4+ [6–10]. Bi4V2O11 is

∗ Corresponding author. Tel.: +91 9897283959.E-mail address: niyazi.alareqi@gmail.com (N.A.S. Al-Areqi).

a versatile host structure for accommodating significant amountsof dopants with entire range of sizes ranging from trivalent B toLa and from divalent Co to Ba cations with approximate solid solu-tion limits [4]. Thus, the degree of ionic conductivity depends onthe cation selected for substitution of V and on the dopant/cationconcentration.

In this paper, we have studied the thermal behaviour and therelationship between the phase stability and ionic conductivityin BIHFVOX, formulated as Bi4HfxV2−xO11−(x/2)−ı for 0 ≤ x ≤ .40 bycombining results obtained from X-ray powder diffraction, differ-ential scanning calorimetry and Arrhenius plots of conductivity.

2. Experimental

2.1. Sample preparations

Samples of Bi4HfxV2−xO11−(x/2)−ı for 0 ≤ x ≤ 0.40 were prepared by conventionalsolid state reactions. Accurately weighed amounts of chemically pure oxides (Bi2O3,V2O5 and HfO2) were thoroughly mixed in an agate mortar with acetone and toluenesolvent (1:1) into a paste. The resulting pastes were then heated at 700 ◦C for 20 hwith intermediate cooling, grinding and treating with acetone–toluene solvent. Thefinely grounded powders were hydraulically pelletized by applying isostatic pres-sure of 980 MPa., using Spectralab SL-89. The pellets with 2.4 cm diameter and0.1 cm thickness were sintered at 800 ◦C for 12 h and gradually cooled in air to roomtemperature.

2.2. Thermal analysis

Differential scanning calorimetric (DSC) measurements were carried out onSchimadzu DSC-60. Approximately weighed 15 mg dry powder of sample was placed

0254-0584/$ – see front matter. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved.doi:10.1016/j.matchemphys.2009.07.051

Author's personal copy

16 S. Beg, N.A.S. Al-Areqi / Materials Chemistry and Physics 118 (2009) 15–20

Fig. 1. Variation in the XRPD patterns of Bi4HfxV2−xO11−(x/2)−ı as a function of com-position.

in an alumina cell. The analysis was performed in N2 environment, flowed with rateof 35 ml min−1. The heating rate was maintained at 10 ◦C min−1 from ambient to650 ◦C. Differential thermal analysis (DTA) measurements were performed using aPerkinElmer DTA under the same conditions as in DSC. For cooling run, the systemwas slowly cooled to room temperature.

2.3. Crystallography

X-ray powder diffraction analysis for various compositions calcined at 700 ◦Cwas performed at room temperature on Rigaku/Max-B diffractometer using Ni-monochromated CuK� radiation (� = 1.54060 Å) in 2� range of 5–90◦ with steps of0.05◦ and scantime of 0.6 s/step and using SiO2 sample holder.

Variable temperature X-ray powder diffraction data for selected composi-tions were collected at 50 ◦C temperature intervals from ambient to 700 ◦C in 2�range of 5–120◦ in steps of 0.02◦ and scantime of 2 s/step, on a PhilipsX’ Pert X-ray diffractometer using Ni-monochromated CuK� radiation (�1 = 1.54060 Å and�2 = 1.54443 Å) with Anton-Paar HTK high temperature camera. Data were collectedwith the help of Pt-sample holder arranged in flate plate �/2� geometry. The unitcell parameters were refined by Powder X software program.

2.4. Electrical conductivity

Electrical measurements were performed using AC impedance analyzer LCR HI-Tester (HIOKI 3532-50) and Genrad 1659 RLC Digibridge by two-probe method inthe frequency range of 42–5 × 10−6 Hz. The sintered pellets were placed betweenboth faces of silver electrodes. The experiments were carried out in steps of 20 ◦Cfrom ambient to 800 ◦C with 15 min stabilization time per step. The heating ratewas maintained at 1 ◦C min−1 for both heating and cooling runs. Impedance spectrawere subjected to non-linear least squares fitting using a Zview software.

3. Results and discussion

Fig. 1 shows the XRPD patterns for various compositions inBi4HfxV2−xO11−(x/2)−ı system at room temperature. The presenceof BiVO4 impurities is indicated by asterisks. The results of X-raypowder diffraction analysis for various compositions at ambienttemperature are summarized in Table 1. It is clearly evident thatthe system Bi4HfxV2−xO11−(x/2)−ı exhibits the composition depen-dence of polymorphism. �-Polymorph is well characterized forvery low composition x = 0.05 [11]. However, the orthorhombic�-phase of higher symmetry, space group Aba2 is observed forrelatively low compositions 0.05 < x ≤ 0.20, whereas the stabiliza-tion of �-phase to room temperature is found to be possible in

Table 1Refined unit cell parameters of the system Bi4HfxV2−xO11−(x/2)−ı for variouscompositions.

x a (Å) b (Å) c (Å) V (Å3) Phase

0.05 5.528 5.609 15.315 474.87 �0.15 5.578 5.614 15.319 479.71 �0.20 5.582 5.608 15.323 479.32 �0.25a 5.598 5.605 15.327 480.72 � (48.7%)0.25b 3.962 15.324 240.55 � (51.3%)0.30 3.969 15.377 242.23 �0.40 3.978 15.398 243.66 �

(a and b)Refined using both orthorhombic and tetragonal model respectively.

composition range 0.25 ≤ x ≤ 0.40. In order to investigate the influ-ence of lower dopant concentrations on different polymorphicforms in the BIHFVOX system, the studied compositions have beenselected upto x = 0.40, which are generally below a theoretical solidsolution limit (x = 0.45). The diffraction pattern for x = 0.25 can beequally judged to be either tetragonal or orthorhombic with meancell (a ≈ b ≈ 3.962 Å and c ≈ 15.324 Å) and volume (240.55 Å3) withrespect to the tetragonal phase, space group I4/mmm. This composi-tion can therefore be regarded as a border between �- and �-phasestability which is also evident by the sharp increase in c-parameterbeyond x = 0.25.

It is interesting to note that on increasing the dopant concen-tration, the unit cell parameters, particularly c, increase over thecomposition range as a whole. This can however be attributed to thesubstitution of larger Hf4+(0.85 Å) for smaller V5+(0.54 Å) in 6-foldcoordination geometry [12].

These phase characterizations have been further confirmed byDSC analysis. Table 2 lists the values of transition temperatureand enthalpy (heat flow per unit mass) which are accompaniedby the possible endothermic effects on heating, detected for thesystem upon varying composition. As expected for x = 0.05, the exis-tence of two consecutive phase transitions, � ↔ � ↔ � is clearlyevident at around 436 and 561 ◦C. It is to be noted that the �Hfor � ↔ � transition is about three times greater than that for� ↔ � transition. This is in good agreement with earlier studies onother BIMEVOXes [5,13,14]. For composition range 0.15 ≤ x ≤ 0.20,the � ↔ � transition is essentially existent. It is noticed that thetransition temperature and �H for � ↔ � transition decreases sig-nificantly with increasing dopant concentration which is a typicalbehaviour of BIMEVOXes [4,15]. The existence of � ↔ � transitionhas also been evidenced by variable temperature X-ray powderdiffraction analysis. Fig. 2 shows the XRPD patterns for x = 0.20 onheating run from ambient to 700 ◦C. It can be observed that thediffraction patterns collected upto 350 ◦C still exhibit the sublat-tice doublet (2 0 2) and (2 0 0) around 2�, 32.48◦ [16,17], confinedto the existence of orthorhombic �-phase. However, splitting ofthis diffraction peak is found to decrease gradually with increas-ing temperature which is ultimately converged at 450 ◦C to singlet(1 1 0), characteristic to tetragonal �-phase. This is also reflectedin the variation of refined unit cell parameters defined in themean orthorhombic cell (Fig. 3). For the sake of the appropriate

Table 2Transition temperatures and enthalpies determined from DSC analysis for variouscompositions of the system Bi4HfxV2−xO11−(x/2)−ı .

x � ↔ � � ↔ � �′ ↔ �

T (◦C) �H (J g−1) T (◦C) �H (J g−1) T (◦C) �H (J g−1)

0.05 436 63.58 561 24.710.15 452 22.260.20 426 18.050.25 381 10.320.30 422 7.140.40 362 5.82

Author's personal copy

S. Beg, N.A.S. Al-Areqi / Materials Chemistry and Physics 118 (2009) 15–20 17

Fig. 2. Variation in the XRPD patterns of Bi4HfxV2−xO11−(x/2)−ı for x = 0.20 as a func-tion of temperature.

representation of graphs, the tetragonal a-parameter (at) has beenconverted to the corresponding orthorhombic a-dimension (ao)using ao = at

√2.

It is noticed that the diffraction data obtained at 400 ◦C couldbe judged equally well to be either orthorhombic or tetragonalphase with mean cell (a ≈ b ≈ 3.988 Å and c ≈ 15.428 Å) and vol-

Fig. 3. Variation in unit cell parameters of Bi4HfxV2−xO11−(x/2)−ı for x = 0.20 as afunction of temperature.

Fig. 4. Combined Arrhenius plot of conductivity and DSC thermogram ofBi4HfxV2−xO11−(x/2)−ı for x = 0.20. Inset shows details of thermal hysteresis in hightemperature region.

ume (245.37 Å3) with respect to �-phase. This temperature whichrecognizes two line regions in Fig. 3 can therefore be regardedas � ↔ � transition temperature for x = 0.20. Similar results havebeen reported earlier for trivalent [14] and tetravalent [9,15] metal-doped BIMEVOXes. It is also noticed that the cell volume exhibitssignificant change in �-stability region which is clearly evidentfrom the similar variation in a- and c-parameters. Conductivitymeasurements also confirmed the existence of � ↔ � transition.Fig. 4 shows the Arrhenius plot of conductivity on heating–coolingcycle for x = 0.20 along with DSC thermogram. The two line regionsof different activation energy with significant thermal hysteresisobserved at the discontinuity during cooling run are directly asso-ciated with �- and �-stability region, as already revealed by X-raypowder diffraction analysis. It is noticed that the stability region of�-phase on cooling run is highly pronounced compared to the sameon heating, suggesting slow kinetics of � ↔ � transition [18]. The� ↔ � transition temperature deduced from the Arrhenius plot isfound to be in the range of 360–380 ◦C, while the endothermic peakin DSC thermogram is observed around 426 ◦C. However, this smalldifference in the transition temperature as measured by the twotechniques is attributed to the difference in the timescales of thetwo experiments [5,18,19], as the DSC heating rate is much fasterthan that used in the conductivity measurements.

It is interesting to note that the Arrhenius plots, in compositionrange 0.25 ≤ x ≤ 0.40, exhibit two line regions but with no signif-icant discontinuity between low and high temperature regions.Arrhenius plot of conductivity for x = 0.40 is presented in Fig. 5. Thetwo line regions are well judged by point at which the change inactivation energy (slope) is observed and somewhat accompaniedwith very low thermal hysteresis during the subsequent coolingrun. It is evident that such behaviours are clearly indicative of�-BIMEVOXes [15,18,20,21]. Thus, the conductivity of low temper-ature region can be referred to a subtle incommensurate orderingof oxide ion vacancies in vanadate layers, while that of high tem-peratures is entirely associated with fully disordered �-phase. Thereproducibility in conductivity between heating and cooling run ishowever clear evidence of relatively fast kinetics and reversibilityof �′ ↔ � transition, i.e. relatively fast quenching from high tem-peratures preserves the fully disordered �-phase structure [22].Study of �′ ↔ � transition has been further completed with vari-able temperature X-ray powder diffraction. Fig. 6 shows the XRPD

Author's personal copy

18 S. Beg, N.A.S. Al-Areqi / Materials Chemistry and Physics 118 (2009) 15–20

Fig. 5. Combined Arrhenius plot of conductivity and DSC thermogram ofBi4HfxV2−xO11−(x/2)−ı for x = 0.40. Inset shows details of differential thermal effect.

patterns for x = 0.40 against temperature. It can be seen that alldiffraction patterns exhibit the tetragonal phase in temperaturerange 25–600 ◦C, as evident by the existence of sublattice singlet(1 1 0) peak around 2�, 32.54◦. As expected, no significant struc-tural variations associated with �′ ↔ � transition could be seen inthis temperature range, even in the temperature dependence ofrefined unit cell parameters shown in Fig. 7. It has been found thatjust few satellite intensities related to tetragonal sublattice peaksvanish in the diffraction pattern at 350 ◦C as noted by Joubert etal. [14] and the appearance of a shoulder on the feet of sublatticepeak at 2�, 34.86◦ for temperature above 350 ◦C, corresponding to

Fig. 6. Variation in the XRPD patterns of Bi4HfxV2−xO11−(x/2)−ı for x = 0.40 as a func-tion of temperature.

Fig. 7. Variation in unit cell parameters of Bi4HfxV2−xO11−(x/2)−ı for x = 0.40 as afunction of temperature.

the point on Arrhenius plot at which the activation energy changes,and the very weak thermal event observed in DSC thermogram. It isworthwhile to point that the stabilized tetragonal, denoted by �′ is arather order phase and is differentiated from the high temperature-fully disordered �-phase by ordering of oxygen sublattices in theperovskite-like slabs [20,22,23]. Thus, It can be generalized that the�′ ↔ � transition involves a change in oxygen stoichiometry. Thischange associated with increased ordering in the oxygen sublatticeis invisible in X-ray diffraction but can only be detected well usingneutron diffraction which exclusively shows weak sublattice peaksarising from the stronger relative diffraction of oxygen sublatticedue to highly accelerated massive neutrons [24,25]. However, anarbitrary border of �′ ↔ � transition shown in Figs. 6 and 7 havebeen rationalized, based on the change in the activation energyand the weak thermal effect observed in Arrhenius plot and DSCthermogram, respectively.

By accident, it was noticed that the tetragonal �-phase structuresurprisingly starts to distort/collapse to orthorhombic �′-phase at600 ◦C which is clearly evident by splitting of (1 1 0) sublattice peakinto doublet which could be indexed in orthorhombic cell as (2 0 2)and (2 0 0). Fig. 8 shows the XRPD pattern for x = 0.40 at 650 ◦C. Theweak peak observed around 2�, 27.41◦ is entirely accounted forthe existence of pure Bi2O3 which completely disappears in lowtemperature diffraction patterns. This actually reflects the temper-ature dependence of solid solution formation. It is clear that thehigh temperature orthorhombic phase, denoted by �′ symmetri-cally resemblances the �-domain, appearing either for the parentcompound or for BIMEVOX systems with low value of x, exceptfor the coexistence of traces of Bi2O3 phase with �′-phase. In addi-tion, the lowering of second-order �′ ↔ � transition temperature forx = 0.40 as shown in Table 2 reflects the thermal instability of thetetragonal phase. This can also be regarded as a further evidencefor occurrence of � ↔ �′ transition.

Although, the limited temperature scale available for DSCmeasurements could not enable us to detect any thermal effectassociated with � → �′ transition, DTA measurements were carried

Author's personal copy

S. Beg, N.A.S. Al-Areqi / Materials Chemistry and Physics 118 (2009) 15–20 19

Fig. 8. XRPD pattern of Bi4HfxV2−xO11−(x/2)−ı for x = 0.40 at 650 ◦C.

out on a freshly prepared sample with composition x = 0.40 (Fig. 9).An exothermic peak is clearly observed in the first cooling run ofthe thermogram, corresponding to the reversible � ↔ �′ transition.Like � ↔ � transition, the destructive � ↔ �′ transition exhibitedfor x = 0.40 is also thermodynamically of a first-order transition.However, the difference in forward and backward transition tem-peratures is generally attributed to the different rates of heating andcooling runs. Moreover, the Arrhenius plot of conductivity on cool-ing run shows relatively small thermal hysteresis around 660 ◦C,evident for such destructive transition. It is noted that no significantdiscontinuity corresponding to this transformation is observed inthe Arrhenius plot. The disappearance of discontinuity is accountedfor two reasons; first, the mobility of oxide ion vacancies in bothfully disordered �- and high temperature �′-phases are comparabledue to which the relative conductivity of each polymorph cannotbe resolved from net conductivity at high temperatures [4]. Sec-ond, the relatively fast kinetics of the progressive � → �′ transitionwhich is clearly reflected in low thermal hysteresis observed in theArrhenius plot on cooling run [22].

Thermal conductivity parameters for Bi4HfxV2−xO11−(x/2)−ı sys-tem as a function of composition are summarized in Table 3.The variation of low temperature conductivity measured at 340 ◦C(�340) with composition exhibits a clear maximum at x = 0.25(7.31 × 10−5 S cm−1). The gradual increase in �340 in composi-tion range, corresponding to �- and �-phase stability region isalso reflected in gradual decrease in the low temperature activa-tion energy, Ea(1). However, the conductivity behaviour shownin variation of �340 in this composition range is associated withthe increase in concentration of oxide ion vacancies, migrating

Fig. 9. DTA thermograms on first heating–cooling cycle for x = 0.40.

Table 3Values of electrical conductivity and activation energy in the Bi4HfxV2−xO11−(x/2)−ı .

x �340 (S cm−1) Ea(1) (eV) �560 (S cm−1) Ea(2) (eV)

0.05 1.93 × 10−5 0.63 3.53 × 10−2 0.310.15 3.51 × 10−5 0.58 2.05 × 10−2 0.360.20 5.22 × 10−5 0.55 7.48 × 10−3 0.370.25 7.31 × 10−5 0.49 5.83 × 10−3 0.390.30 1.48 × 10−5 0.48 3.66 × 10−3 0.410.40 1.26 × 10−5 0.51 2.26 × 10−3 0.39

Ea(1) and Ea(2) are obtained by the line regression fitting to Arrhenius equation inthe temperature ranges (200–440 ◦C) and (540–740 ◦C) respectively.

between Hf-tetrahedra and Hf-octahedra in the pervoskite layers[26,27], while the significant decay in �340 observed in �-phaseregion is attributed to the defect pair formation (e.g. Hf4+ and V••

O)where both vacancy concentration and pathways for O2− motionare optimized [13,28]. The high temperature conductivity at 560 ◦C(�560) shows sharp decay followed by levelling off until x = 0.40which is typical for BIMEVOXes. This conductivity behaviour shownin variation of �560 is also mirrored in the reverse variation of hightemperature activation energy, Ea(2) with composition.

4. Conclusion

The present work is an extension of our studies on BIH-FVOX material, formulated as Bi4HfxV2−xO11−(x/2)−ı. The thermalbehaviour of polymorphism and electrical conductivity againstcomposition has been carefully examined. The relationshipbetween phase stability and ionic conductivity is found to becompositionally dependent, except for that in low temperatures,where the conductivity behaviour in the composition range, cor-responding to �-phase region exhibits a strong dopant–vacancyinteraction which is significantly pronounced and associated withthe drop in low temperature conductivities and accompanied alsowith increasing low temperature activation energies.

It is interesting to note that the increased concentration of largesized Hf4+ dopant in the composition range of �-phase stabilityhas an effective role in distortion of the fully disordered tetragonalstructure at higher temperature. The destructive � → �′ transitionhas been clearly evidenced by variable X-ray powder diffractiondata for x = 0.40 and also typified in Arrhenius plot at 560 ◦C. How-ever, such unexpected transition has not been reported yet for anymember of BIMEVOX family.

Acknowledgments

Authors gratefully acknowledge Prof. I. Abrahams, Departmentof Chemistry, Queen Mary, University of London, UK for providingvariable temperature XRD analysis and Prof. Alimuddin, Depart-ment of Applied Physics, Aligarh Muslim University for providingAC impedance analyzer and other research facilities available in thelab. of Materials Science.

References

[1] F. Abraham, M.F. Debreuille-Gresse, G. Mairesse, G. Nowogrocki, Solid StateIonics 28–30 (1988) 527.

[2] F. Abraham, J.C. Boivin, G. Mairesse, G. Nowogrocki, Solid State Ionics 40–41(1990) 934.

[3] G. Mairesse, in: B. Scrosati, A. Magistris, C.M. Mari, G. Mariotto (Eds.), Fast IonicTransport in Solids, Kluwer Academic Publishing, Dordreckt, 1993, p. 271.

[4] C.K. Lee, D.C. Sinclair, A.R. West, Solid State Ionics 62 (1993) 193.[5] C.K. Lee, A.R. West, Solid State Ionics 86–88 (1996) 235.[6] R. Essalim, B. Tanouti, J.P. Bonnet, J.M. Reau, Mater. Lett. 13 (1992) 382.[7] V. Sharma, A.K. Shukala, J. Gopalakrishnan, Solid State Ionics 58 (1992) 359.[8] J.B. Goodenough, A. Manthiram, M. Paranthaman, Y.S. Zhen, Mater. Sci. Eng. B12

(1992) 357.[9] F. Krok, I. Abrahams, D. Bangobango, W. Bogusz, J.A.G. Nelstrop, Solid State

Ionics 111 (1998) 37.

Author's personal copy

20 S. Beg, N.A.S. Al-Areqi / Materials Chemistry and Physics 118 (2009) 15–20

[10] F. Krok, I. Abrahams, W. Wrobel, S.C.M. Chan, M. Malys, W. Bogusz, J.R. Dygas,Solid State Ionics 154–155 (2002) 511.

[11] M. Huve, R.N. Vannier, G. Nowogrocki, G. Mairesse, G.V. Tandeloo, J. Mater.Chem. 6 (1996) 1339.

[12] R.D. Shannon, C.T. Prewitt, Acta Crystallogr. B 26 (1970) 1046.[13] J. Yan, M. Greenblatt, Solid State Ionics 81 (1995) 225.[14] O. Joubert, M. Ganne, R.N. Vannier, G. Mairesse, Solid State Ionics 83 (1996)

199.[15] W. Wrobel, I. Abrahams, F. Krok, A. Kozanecka, S.C.M. Chan, M. Malys, W.

Bogusz, J.R. Dygas, Solid State Ionics 176 (2005) 1731.[16] C.J. Watson, A. Coats, D.C. Sinclair, J. Mater. Chem. 7 (10) (1997) 2091.[17] S.N. Achary, M.D. Mathews, S.J. Patwe, A.K. Tyagi, J. Mater. Sci. Lett. 18 (1999)

355.[18] I. Abrahams, F. Krok, M. Malys, W. Wrobel, Solid State Ionics 176 (2005) 2053.[19] M. Alga, A. Ammar, R. Essalim, B. Tanouti, F. Mauvy, R. Decourt, Solid State Sci.

7 (2005) 1173.

[20] I. Abrahams, J.A.G. Nelstrop, F. Krok, W. Bogusz, Solid State Ionics 110 (1998)95.

[21] F. Krok, I. Abrahams, A. Zadrozna, M. Malys, W. Bogusz, J.A.G. Nelstrop, A.J. Bush,Solid State Ionics 119 (1999) 139.

[22] F. Krok, I. Abrahams, D.G. Bangobango, W. Bogusz, J.A.G. Nelstrop, Solid StateIonics 86–88 (1996) 261.

[23] S. Lazure, R.N. Vannier, G. Nowogrocki, G. Mairesse, C. Muller, M. Anne, P.Strobel, J. Mater. Chem. 5 (1995) 1395.

[24] C. Muller, M. Anne, M. Bacmann, Solid State Ionics 111 (1998) 27.[25] C. Muller, M. Anne, M. Bacmann, M. Bonnet, J. Solid State Chem. 141 (1998)

241.[26] I. Abrahams, F. Krok, M. Malys, A.J. Bush, J. Mater. Sci. 36 (2001) 1099.[27] I. Abrahams, F. Krok, Solid State Ionics 157 (2003) 139.[28] R. Kant, K. Singh, O.P. Pandey, Int. J. Hydrogen Energy 33 (2008) 455.