Synthesis, phase stability and electrical characterisation of BINAVOX solid solutions Craig J. Watson, Alison Coats and Derek C. Sinclair Chemistry Department, University of Aberdeen, Meston Walk, Aberdeen, UK AB24 3UE The compositional range of ‘Bi4V2O11’ solid solutions containing Na has been determined by means of a phase diagram study using X-ray diVraction and electron probe microanalysis. The locus of the solid solution indicates that Na replaces V in the crystal structure and the BINAVOX family has the overall formula Bi4+yV2-y-xNaxO11-y-2x. The solid solution limits are temperature dependent and for samples prepared and air-cooled from 830 °C the limits are defined as 0.00<x<0.18 and 0.05<y<0.19.BINAVOX compositions with x>0.10 are thermodynamically unstable below ca. 720 °C but can be stabilised kinetically by rapid cooling from temperatures above ca. 750 °C. ac Impedance measurements demonstrate that BINAVOX materials are electrically inhomogeneous, exhibit temperature- and time-dependent conductivities and do not oVer any significant advantages over the parent solid solution. The bismuth vanadate, Bi4+yV2-yO11-y has attracted a lot of to use.Reaction mixtures totalling 3–4 g were weighed and attention as the parent phase for a family of oxide ion mixed into a paste with acetone using an agate mortar and conductors known as BIMEVOX.1–4 Bi4+yV2-yO11-y is a pestle. In order to limit volatilisation of the reagents, approxisolid- solution phase whose compositional limits are highly mately two thirds of the powder for each sample was coldtemperature dependent, covering a compositional range from pressed into pellets, placed in Au foil boats and covered with ca. 0<y0.22 at 880 °C.5 Three polymorphs are known to the remaining powder. A heating sequence of 500 °C for 2 h, exist and the high-temperature c polymorph exhibits high 650 °C for 2 h, 800 °C for 12 h, regrinding and repelletising, conductivity mainly due to oxide ions.The explanation for the then 750 °C for 2 h and 830 °C for 12 h was found to be high conductivity is that the c polymorph has a layered adequate to obtain equilibrium. Pellets for EPMA and ac structure of alternating Bi2O22+ and VO3.52- layers and the impedance measurements were cold-pressed, covered with perovskite-like layers VO3.5%0.52- are oxygen deficient. powder of similar composition and sintered at 830 °C for a Transformation to the b and a polymorphs at lower tempera- further 12 h.tures results in ordering of the oxygen vacancies and the Phase purity was determined by X-ray diVraction using a conductivity decreases markedly. Partial substitution of Cu or Hagg Guinier camera or a Stoe-Stadi diVractometer using Cu- Ni for V suppresses the c�b/a transitions thus stabilising the Ka1 radiation.The final composition of the BiNaVOX matehigh- temperature c polymorph of Bi4+yV2-yO11-y, giving con- rials was determined using a Cameca SX51 electron microprobe ductivities as high as 1×10-3 V-1 cm-1 at 240 °C.1 analyser with an incident beam energy of 20 kV and a current The crystal chemistry of Bi4+yV2-yO11-y is still not clearly of 50 nA.All samples were polished to <1 mm and carbon resolved owing to problems associated with stoichiometry, coated. The standards used were Bi2CuO4 for Bi-La, V2O5 for polymorphism, twinning6 and incommensurate supercells.3 V-Ka and NaAlSi3O8 for Na-Ka. Oxygen was calculated by Recently, Huve et al.7 reported that the symmetry of the a stoichiometry.ac Impedance measurements from ca. 25 to polymorph depends on the purity of the V2O5 employed in 800 °C were conducted using a Hewlett Packard 4192A the synthesis. High-purity V2O5, free from Na/K contamiimpedance analyser over the frequency range 5 Hz–5 MHz nation gave rise to a monoclinic cell, space group A2/m, with an applied voltage of 100 mV. Electrodes were fabricated whereas reagents with trace amounts of Na/K result in the from Pt organopaste; electroded pellets were fired at 800 °C commonly reported orthorhombic cell, space group Amam.overnight to decompose the paste and harden the Pt residue. Despite the complexities associated with the crystal chemistry Pellets were attached to the Pt measuring leads of a conduc- of Bi4+yV2-yO11-y the number of BINAVOX materials has tivity jig and placed in a horizontal tube furnace whose grown considerably with over seventeen diVerent families being temperature was controlled and measured within ±3 °C.reported. In this paper we describe, for the first time, the BINAVOX family. The solid-solution limits at 830 °C are determined via a combination of X-ray diVraction (XRD) results and electron probe microanalysis (EPMA) and the thermal Results and Discussion stability and electrical properties of the BINAVOX materials The compositions studied have general formula are also discussed. Low levels of Na-doping, ca. 3 mol%, are Bi4+yV2-y-xNaxO11-y-2x. Although XRD was adequate to eVective for kinetic stabilisation of the c polymorph on rapid determine whether any given composition was single phase, cooling from 830 °C, but these materials are metastable and EPMA was employed to determine the final composition(s) of transform slowly to the a polymorph on post-annealing at, e.g.the phases present in selected samples. Combining both sets 350 °C. On slow cooling or annealing at ca. 650–720 °C for ca. of results enabled the solid-solution area at 830 °C in air to be 48 h the c-BINAVOX solid solutions decompose into a threedetermined, as shown on the composition triangle phase mixture containing a-BINAVOX.ac Impedance spec- Bi2O3–Na2O–V2O5 in Fig. 1. Closed circles represent the final troscopy measurements show the bulk conductivity behaviour compositions of single-phase BiNaVOX samples, half-filled of BINAVOX materials to be electrically inhomogeneous and circles represent the starting composition of samples which very dependent on thermal history.were determined to be phase mixtures and filled triangles represent the final composition of the BINAVOX phase within Experimental phase mixtures ( half-filled circles). EPMA established that there were no significant problems associated with volatility Bi2O3 (99.99%), V2O5 (99.6%) and Na2CO3 (99.99%) reagents were dried at 300 °C overnight and stored in a desiccator prior of the reagents.For example, a sample of nominal starting J. Mater. Chem., 1997, 7(10), 2091–2095 2091Fig. 1 Compositional extent of BINAVOX by EPMA for samples prepared and air-cooled from 830 °C. Closed circles represent phase- Fig. 2 X-Ray diVractograms of Bi4+yV2-x-yNayO11-x-2y solid solupure compositions, half-filled circles represent phase mixtures and tions prepared and air-cooled from 830 °C closed triangles represent the composition of BINAVOX in phase mixtures. The inset shows the loci of solid solutions for diVerent Bi4.10(2)V1.90(2)O10.90 (x=0, y=0.1). This is consistent with the substitution mechanisms. suggested substitution mechanism of replacing small V ions by larger Na ions.Compositions with 0.05<x<0.15 could not composition, Bi4.08V1.77Na0.15O10.62 was analysed to have a be indexed satisfactorily on any single polymorph, owing to final composition of Bi4.10(2)V1.76(2)Na0.15(2)Od. The results variations in peak intensities and problems associated with clearly indicate that Na has been incorporated into the crystal peak convergence/asymmetry, such as shown for the doublet lattice and that the solid-solution limits can be defined as at ca. 32° for x=0.06 and 0.10, Fig. 2(b) and (c), respectively. 0<x<0.18 and 0.05<y<0.19 for samples prepared and air- Compositions with x0.05 indexed as a polymorphs and cooled from 830 °C. diVerential thermal analysis (DTA) showed the presence of an Several interesting features emerge from the results. First, exotherm at ca. 450 °C on heating and an endotherm at ca. phase-pure, stoichiometric Bi4V2O11 (y=0) cannot be prepared 350 °C on cooling, consistent with a reversible a=c transition. in air at 830 °C. The parent phase of the samplwith the No thermal event was detected by DTA between 25 and 700 °C stoichiometric composition, y=0, was found by EPMA to be for samples with x>0.05. These results indicate a gradual Bi-rich with y=0.05 and trace amounts of a secondary phase change in polymorphism from a to c on increasing x.BiVO4 were detected. This confirms the work of Lee et al.4 XRD patterns obtained for BINAVOX solid solutions with who established that excess Bi is always required to prepare intermediate values of x, ca. 0.05<x<0.15 depend on thermal single-phase Bi4+yV2-yO11-y below ca. 850 °C. history, as shown for Bi4.13(2)V1.78(2)Na0.09(2)O10.69 (x=0.13, Secondly, on the basis of the solid solution locus, Fig. 1, the y=0.09) in Fig. 3. Rapid cooling from elevated temperatures phase diagram results suggest that Na is predominantly incorenables the c polymorph to be ‘preserved’ at ca. 25°C, as porated onto V rather than Bi sites within the crystal lattice. shown by the single peak at ca. 32°, Fig. 3(a). Annealing the There is considerable disorder in the coordination of oxygen same sample for 5 days at 350 °C leads to a clear doublet at ions around the V-sites within the lattice and the possibility ca. 31–33°, Fig. 3( b), consistent with the existence of the a exists that the large Na ions are located in distorted tetrahedral polymorph. No change in phase assemblage was detected by sites within the perovskite-like layers. Nevertheless, this result EPMA for any BINAVOX material prepared at 830 °C and is rather surprising given the large diVerence in ionic radii of subsequently annealed below ca. 600 °C.These results clearly V5+ and Na, 0.54 and 1.13 A ° ,8 respectively and needs confirdemonstrate that the c polymorph of BINAVOX materials mation by crystallographic studies. The low solubility limit of with x>0.05 in samples quenched from high temperature Na in Bi4+yV2-yO11-y makes it is diYcult to use the phase transform to the more stable a polymorph on low-temperature diagram to establish the precise substitution mechanism, howannealing, typically at ca. 350 °C. Without undertaking extens- ever, it is clear that the BINAVOX phase diagram diVers ive crystallographic studies on samples subjected to a variety significantly from those obtained with other large dopant of heat treatments, it is diYcult to determine whether the cations such as Sr2+ and Pb2+.Such diagrams9 show extensive change from a to c on increasing x is continuous or if a two- solid solutions extending in the -y direction, inferring that phase region exists at intermediate x which separates a and c large divalent cations substitute onto the Bi sites. regions at low and high x values, respectively. The a and c polymorphs can usually be distinguished by The above comments on polymorphism refer to materials XRD in the range 2h 31–33°.The presence of a doublet at ca. 32° is ascribed to (020) and (200) reflections of the lower symmetry, orthorhombic a polymorph whereas a singlet at ca. 32.5° is assigned to the (110) reflection of the higher symmetry, tetragonal c polymorph. For samples with y=0.1, it is clear from XRD that increasing the Na content (x value) is eVective in stabilising the c polymorph on air-cooling from 830 °C, as shown by the convergence of the (020)–(200) doublet at ca. 31–33° for x=0, Fig. 2(a), into a singlet for x=0.15, Fig. 2(d). In general, samples with high x values could be satisfactorily indexed as c polymorphs and the unit cell expands in the c direction compared with undoped materials. For example, the lattice parameters of c-Bi4.10(2)V1.75(2)Na0.15(2)O10.60 (x=0.15 and y=0.1) are a=3.941(9) and c=15.349(5) A ° , compared Fig. 3 X-Ray diVractograms for Bi4.13(2)V1.78(2)Na0.09(2)O10.69 quenched from 850 °C (a) and post-annealed for five days at 350 °C (b) with a=5.518(2), b=5.593(2) and c=15.239(5) A ° for a- 2092 J. Mater. Chem., 1997, 7(10), 2091–2095quenched from high temperature and subsequently annealed c polymorph and for the decomposition products including a- BiNAVOX.It is clear that Na is exsolved from c- at relatively low temperature, 350 °C. At intermediate temperatures, diVerent behaviour occurs. Bi4.10(2)V1.75(2)Na0.15(2)O10.60 on annealing at 650–720 °C to form a phase mixture including a-Bi4.02(5)V1.88(4)Na0.10(4)O10.78. Unlike many other BIMEVOX families, c-BINAVOX solid solutions with x>0.10 decompose on annealing between 650 Good quality analysis of the impurity phases was restricted because of their small grain size, Fig. 5, however, both are Bi- and 720 °C into a mixture of phases containing a-BINAVOX of low x values. XRD results for Bi4.10(2)V1.75(2)Na0.15(2)O10.60 rich with respect to the BINAVOX materials.Further details of these phases are discussed elsewhere.10 (x=0.15) quenched onto a brass block from 830 °C, Fig. 4(a), and annealed at 650 °C for 72 h, Fig. 4(b), clearly demonstrate The decomposition of BINAVOX materials with x>0.10 takes place only over a narrow temperature range, ca. a mixture of a-BINAVOX and peaks associated with secondary phases. 650–720 °C and is fully reversible: reheating above ca. 720 °C for 1 h results in reformation of single-phase BINAVOX. This EPMA revealed the presence of three phases in the decomposed sample as shown by the back-scattered image in Fig. 5. suggests that BINAVOX materials with x>0.10 are thermodynamically stable only above ca. 720 °C but may be preserved Table 1 lists the atom% of Bi, V and Na for the single-phase to room temperature, where they are kinetically stable, by rapid cooling.BINAVOX materials with x<0.10 did not decompose on prolonged annealing between ca. 650 and 720 °C, suggesting that a-BINAVOX materials are thermodynamically stable under the conditions tested. Given the obvious complexity associated with stability and polymorphism in the BINAVOX system, we chose not to indicate the polymorphs on the phase diagram, Fig. 1 and stress that this diagram is relevant only for materials which have been aircooled from 830 °C. Like many other BIMEVOX systems, the BINAVOX solid solution limits are highly temperature dependent with the maximum solubility in the c polymorph occurring close to melting temperatures. Three types of conductivity data set were collected for the BINAVOX solid solutions; (i) above 720 °C where all samples were thermodynamically stable c polymorphs, (ii) between ca. 150 and 650 °C for samples air-cooled from 800 °C, and (iii ) as a function of time at ca. 250 °C for samples quenched from Fig. 4 X-Ray diVractograms for Bi4.10(2)V1.75(2)Na0.15(2)O10.60 800 °C. quenched from 830 °C (a) and post-annealed at 650 °C for 72 h ( b).In general, impedance plane plots below 400 °C consist of a ×indicate unindexed peaks associated with secondary phases. single, semicircular arc and a low-frequency ‘spike’. There was no evidence of a grain boundary arc and the low-frequency eVects are attributable to ionic polarisation and diVusionlimited phenomena at the electrode and support the idea that conduction is mainly by means of ions.The inclined spike is similar to that expected for a Warburg impedance with an ideal slope of 45°. (i) Conductivity data above 720 °C Total conductivities for single-phase BINAVOX solid solutions were extracted from the inverse of the low-frequency intercept of the electrode spike with the real axis of impedance plane plots at 725, 750, 775 and 800 °C. The results demonstrated that all single-phase compositions had similar but slightly lower conductivity values and similar activation energies to the c polymorph in undoped materials, as shown for a variety Fig. 5 A back-scattered electron image of the composition of compositions in Fig. 6. Thus, Na-doping does not enhance Bi4.10(2)V1.75(2)Na0.15(2)O10.60 after being annealed at 650 °C for 72 h.Dark regions are a-Bi4.02(2)V1.88(2)Na0.10(2)O10.78; grey and white the high-temperature conductivity of c-Bi4+yV2-yO11-y regions are the minor phases 1 and 2, respectively. materials. Table 1 Composition analysis (atom%) before and after the decompo- (ii) Conductivity data between 150 and 650 °C sition of a c-BINAVOX solid solution at 650 °Ca The conductivity changes irreversibly on thermal cycling before between 150 and 600 °C, Fig. 7. The high conductivity values decomposition after decomposition obtained below ca. 400 °C on the initial heating cycle of c- BINAVOX materials are not reproduced on subsequent coolc- BINAVOX a-BINAVOX minor 1 minor 2 ing and reheating. Given the metastable nature of BINAVOX element av. s av. s av. av. materials with x>0.10 and the tendency to transform from the c to the a polymorph at 350 °C, as shown by the XRD Bi 24.68 0.07 23.98 0.25 25.63 26.87 V 10.56 0.04 11.20 0.18 9.75 9.35 results in Fig. 3, it is not surprising that conductivities depend Na 0.89 0.05 0.56 0.04 1.20 0.06 on thermal treatment. O 63.87 0.03 64.26 0.08 63.42 63.71 Conductivities of Bi4.13(2)V1.78(2)Na0.09(2)O10.69 and c- Bi4.10(2)V1.75(2)Na0.15(2)O10.60 are shown in Fig. 7 (conductivities aatom% shown here is normalised to give total %=100. Actual total for undoped a-Bi4.06(2)V1.94(2)O10.94 are indicated as a dotted mass% was 99.64 and 100.83 for the c- and a-BINAVOX phases, line for comparison). It is clear from the Arrhenius plots in respectively. Data of ten points were averaged in determining the composition of the BINAVOX phases; s is standard deviation.Fig. 7 that the samples have similar conductivities and acti- J. Mater. Chem., 1997, 7(10), 2091–2095 2093(iii) Conductivity data at ca. 250 °C for samples quenched from 800 °C The bulk conductivity at ca. 250 °C for a pellet of c- Bi4.10(2)V1.81(2)Na0.15(2)O10.60 which had been rapidly quenched from 800 °C was extracted from the inverse of the low-frequency intercept of the asymmetric semicircular arc on the real axis of impedance plane plots, Fig. 8. The associated capacitance of the arc was calculated to be 33 pF cm-1 using the relationship vRC=1 (where v=2pf and is the angular frequency) at the arc maximum and is consistent with a bulk or intragranular response. The bulk arc resistivity increased as a function of time and Fig. 9 shows the smooth decrease in bulk conductivity from 20 to 4.5 mS cm-1 over a period of ca. 150 h. Conductivity measurements are clearly very sensitive to oxygen ordering as the highly conducting metastable c polymorph transforms, albeit rather slowly, to the a polymorph. This decrease in conductivity is consistent with the observed line broadening/ splitting in XRD patterns when annealing rapidly quenched c-BINAVOX materials at moderate temperatures, as shown in Fig. 3 for Bi4.13(2)V1.78(2)Na0.09(2)O10.69. Fig. 6 Arrhenius plots for various BINAVOX materials above 725 °C In order to further investigate the bulk conductivity characteristics of BINAVOX materials, ac impedance data were replotted in the form of combined spectroscopic plots of the imaginary components of the complex impedance and electric modulus formalisms, Z and M, respectively. Such plots are well established for probing the electrical homogeneity of many electroceramics11 and reveal that the bulk characteristics of BINAVOX materials are inhomogeneous.Fig. 10 shows a Fig. 7 Arrhenius plots for various BINAVOX materials over the Fig. 8 Complex impedance plane plot for Bi4.10(2)V1.75(2)Na0.15(2)O10.60 temperature range 150 to 650 °C (–, cooling cycle: x=0, y=0.06; quenched from 800 °C and annealed at 253 °C for 143 h.Closed circles $, cooling cycle: x=0.09, y=0.13; #, initial heating cycle: x=0.15, identify selected frequencies on a logarithmic scale, e.g. 5=105 Hz. y=0.10) vation energies over the temperature range ca. 320–650 °C.Similar results were obtained for all single-phase BINAVOX compositions. Initial heating-cycle data for c-Bi4.10(2)V1.75(2)Na0.15(2)O10.60 below 300 °C demonstrate a characteristic feature of BINAVOX materials with x>0.10, namely enhanced, lowtemperature conductivity values which are time dependent and irreproducible on thermal cycling. For ac impedance measurements, samples are allowed to equilibrate for ca. 1 h between successive temperatures and are therefore maintained at elevated temperatures for long periods during thermal cycling.The conductivity of c-Bi4.10(2)V1.75(2)Na0.15(2)O10.60 below ca. 300 °C was over an order of magnitude higher on the initial heating cycle compared with that on any subsequent heating or cooling cycle and at 305 °C on the initial heating cycle decreased from 0.11 to 0.018 mS cm-1 overnight, ca. 12 h, Fig. 7. This time dependent decrease is clearly associated with the metastable nature of these materials and is probably associated with their tendency to undergo oxygen re-ordering at low temperatures. In order to further investigate this phenomenon, samples were Fig. 9 Variation in conductivity as a function of time for rapidly quenched from 800 °C and variations in the bulk Bi4.10(2)V1.75(2)Na0.15(2)O10.60 quenched from 800 °C and annealed at ca. 250 °C conductivity monitored as a function of time at ca. 250 °C. 2094 J. Mater. Chem., 1997, 7(10), 2091–2095extremely broad M spectra but such plots clearly show the bulk conductivity characteristics in these materials to be complex and inhomogeneous.In conclusion, although Na can be incorporated into the Bi4+yV2-yO11-y lattice the BINAVOX family exhibit complex thermal stability behaviour and are unsuitable for producing stable, high oxide-ion conducting materials for practical applications. The authors would like to thank the University of Aberdeen for an M.Sc. studentship for C.J.W., EPSRC for financial support for the EPMA facility and Professor Tony West for helpful discussions.References 1 F. Abraham, J. C. Boivin, G. Mairesse and G. Nowogrocki, Solid Fig. 10 A combined Z and M spectroscopic plot for State Ionics, 1990, 40/41, 934. Bi4.10(2)V1.75(2)Na0.15(2)O10.60 quenched from 800 °C and annealed at 2 T. Iharada, A. Hammouche, J. Fouletier, M. Kleitz, J. C. Boivin 253 °C for 143 h and G.Mairesse, Solid State Ionics, 1991, 48, 257. 3 K. B. R. Varma, G. N. Subbanna, T. N. Guru Row and C. N. R. Rao, J.Mater. Res., 1990, 5, 2718. combined Z and M plot for c-Bi4.10(2)V1.81Na0.15O10.60 after 4 C. K. Lee, B. H. Bay and A. R. West, J.Mater. Chem., 1996, 6, 331. ca. 143 h at 250 °C on quenching from 800 °C. For an ideal 5 C. K. Lee, D. C. Sinclair and A.R. West, Solid State Ionics, 1993, Debye response, the frequency maxima of Z and M peaks 62, 193. 6 F. Abraham, M. F. Debreuille-Gresse, G. Mairesse and should be coincident and the half-height peak widths 1.14 G. Nowogrocki, Solid State Ionics, 1988, 28/30, 529. decades on a log( f ) scale. Although it is not uncommon for 7 M. Huve, R. N. Vannier, G. Nowogrocki, G. Mairesse and the fmax values of Z and M to be separated by up to one G. V. Tendeloo, J.Mater. Chem., 1996, 6, 1339. order of magnitude in good solid electrolytes such as Na- 8 R. D. Shannon, Acta Crystallogr., Sect. A, 1976, 32, 751. b-Al2O3 12 it can clearly be seen that the M peak is exception- 9 C. K. Lee, G. S. Lim and A. R.West, J.Mater. Chem., 1994, 4, 1441. ally broad, with a half-height peak width of ca. 2.56 decades 10 C. J.Watson, M.Sc. Thesis, Aberdeen University, 1997. 11 D. C. Sinclair and A. R. West, J. Appl. Phys., 1988, 66, 3850. on a log( f ) scale. Such a response could be associated with 12 I. M. Hodge, M. D. Ingram and A. R. West, J. Electroanal. Chem., the metastability of the BINAVOX materials as they transform 1976, 74, 125. from the disordered c to the ordered a polymorph on low- 13 E. Pernot, M. Anne, M. Bacmann, P. Strobel, J. Fouletier, temperature annealing or it may be associated with the intrin- R. N. Vannier, G. Mairesse, F. Abraham and G. Nowogrocki, Solid sic, anisotropic nature of the conduction properties of State Ionics, 1994, 70/71, 259. BIMEVOX materials, as shown in single-crystal studies.13 Further studies are in progress to clarify the origin of the Paper 7/03629I; Received 27thMay, 1997 J. Mater. Chem., 1997, 7(10), 2091–2095 2095