New oxide ion conducting solid electrolytes, Bi4V2011: M; M = B, Al, Cr, Y, LaP

Chnoong Kheng Lee,' Boon Hong Bay" andAnthony R. Westb "Chemistry Department, Universiti Pertanian Malaysia, 43400 Serdang, Selangor, Malaysia bChemistry Department, University of Aberdeen, Meston Walk, Aberdeen, U K AB9 2 U E

The compositional ranges of Bi4V2011 solid solutions containing trivalent cations: B, Al, Cr, Y and La have been determined by means of a phase diagram study at solidus temperatures, ca. 850 "C. At least three mechanisms for accommodating variable cation contents are required. These are nominally V+Bi, V+M, Bi-+M; the first two also involve the formation of anion vacancies. The detailed mechanisms by which ions as different in size as B and La can enter the Bi,V2011 structure cannot, however, be inferred from the phase diagrams. Phase diagrams at 500 "C have much smaller solid-solution areas, indicating the stability and extent of the solid solutions to be very temperature-dependent. Conductivity studies show that La-, Y- and Al-doped materials, with a composition around Bi4Vl~8Mo~2010~8, are the best conductors at 300 "C with a conductivity value of up to 1.4 x Q-' cm-'. At 600 "C, Al-doped materials have the highest conductivity, ca. 1 x lo-' R-' cm-'. Conductivity Arrhenius plots show changes of slope associated with the phase transitions: a+P+y for B, Al, Cr and P 3 y for Y; data for La (y-polymorph only) are almost linear.

Solid electrolytes obtained by doping Bi4V2OI1 are a new family of oxide ion conductors known as BIMEVOX.1-3 The highest conductivity has been reported for Cu and Ni doped materials with values as high as 3 . 2 ~ lop3 R-' an-' at 300 "C., The compound Bi,V201, exhibits three crystallo- graphic polymorphs, a, p and y, in the temperature range 25-895"C.3,5 The structure of the different phases has been described as consisting of alternating sheets of constitution Bi2022+ and vo3.5 Uo.52-, where 0 refers to oxide ion vacancies.' High levels of oxide ion conductivity occur in the high-temperature y-form, which can be stabilised to room temperature by the partial substitution of V by Cu or Ni.

Doped Bi,V,O,, materials have been under intensive study in several laboratories because of their attractive electrical properties. In addition to being good oxide ion conductors, they have also been shown to exhibit ferroelectric and pyro- electric properties.6-8 Most of the studies concentrated on partial substitution of V by lower-valent cations such as Co, Ni, Cu, Zn, Al, Ti19299*10 and Ge," resulting in the stabilisation of the y-polymorph to room temperature. Substitution of Mo leads to stabilisation of the P-polymorph.12 The possibility of other substitution mechanisms involving Bi sites as well as V sites has been less studied. Following on from the phase diagram study of the Bi203-V205 join showing a range of Bi-rich solid solution^,^ similar solid solutions were found in Pb-,I3 Ge-I' and divalent cation-doped material^.'^ In our previous study of bismuth vanadate solid solutions doped with a series of divalent cations, it was shown, with the help of phase diagrams, that at least three mechanisms were necessary to accommodate variable cation content in the solid solutions. These are: V+Bi, V+M, Bi+M and possibly also the forma- tion of interstitial M. There also appeared to be a clear correlation between dopant M2+ size and the locus of the solid-solution area. With increasing ionic size, the solid solu- tions extend to progressively more Bi-deficient compositions, indicating a greater readiness for the larger ions to substitute directly into Bi sites. Conductivity studies indicated that the best oxide ion conductors were obtained for the composi- tions with x=0.2 and y=O in the general formula Bi4+yV2-y-xMxOll -y-3x/2, where M is the divalent cation."

Apart from a single composition with A13+ as the dopant, no systematic studies of trivalent doping of Bi,V,O,, have

?Presented at the Second International Conference on Materials Chemistry, MC2, University of Kent at Canterbury, 17-21 July 1995.

been reported. We therefore report here the results of an overview into doping Bi4V2011 with a selection of trivalent cations, ranging from the smallest, B to the large La; remark- ably, extensive doping occurs for all cation sizes.

Experimental Bismuth vanadate solid solutions doped with trivalent cations were prepared by solid-state reaction in gold foil boats. Reagents used were Bi203 (99.9Y0, Aldrich), V 2 0 5 (99.8%, Aldrich), B203 (99.98 YO, Aldrich), Al,03 (99.99%, Johnson Matthey), Cr203 (99.999%, Johnson Matthey), La203 (99.9%, Aldrich) and Y 2 0 3 (99.99Y0, Aldrich). They were dried at 300 "C prior to weighing. Compositions were weighed (ca. 3 g total), mixed with acetone in an agate mortar, dried, fired at temperatures in the range of 820-880°C for 20 h, depending on composition, and air-cooled to room temperature (ca. 1-2 min). Samples were analysed by X-ray powder diffraction (XRD, Philips diffractometer, Cu-Ka, radiation). For determi- nation of the phase diagram at 500"C, selected samples were heated at 800°C for 2 h, cooled to 500°C at 50°C h-', annealed at 500 "C overnight, and air-quenched. For differential thermal analysis (DTA), a DuPont 991 instrument with a 1200°C cell and a heating rate of 10°C min-l was used.

Pellets for electrical property measurement were cold-pressed and sintered at 820-880 "C overnight; Au paste electrodes were then fired on at 200-600°C. In order to ensure that the materials were in the y-form, the pellets, with electrodes attached, were refired at 820-850 "C overnight and air quenched, immediately before conductivity measurements. Ac impedance measurements were made over the range 150-800 "C using a Hewlett-Packard 4192A impedance analyser over the frequency range 10-lo6 Hz.

Samples were equilibrated at constant temperature for 30 min prior to each set of measurements.

Results and Discussion Solid solutions of Bi4V2OI1 with M3+

For each trivalent cation, the relevant region of the phase diagram Bi203-V205-M,03 was investigated in order to establish the locus and compositional extent of the single- phase Bi,V,O,, solid solutions. In each case, the effect of temperature had to be considered since the solid solutions may be more extensive at high temperatures, as found on the

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Bi203-V205 join Cooling rate was also important, since sometimes the various y-+P+a transitions may be suppressed by rapid cooling The phase diagrams presented here (Fig 2-6, later) refer to final reaction temperatures in the range 820-900 "C, depending on composition (these temperatures were found by trial and error to be close to the solidus), after which samples were removed from the furnace and allowed to cool naturally in air In addition, the phase diagrams were determined at an arbitrary and lower temperature, 500°C, as described in the Experimental The solid-solution limits at 500°C are indicated using dotted lines in Fig 2-6

In order to facilitate description and discussion of the results, and in particular to assess the likely substitution mechanism(s), the directions of possible solid-solution formation for Bi4VZOli doped in various ways are shown in Fig 1 Since the trivalent dopant is of different charge to V, additional compensation mechanisms are required other than for (7) which would involve M substituting directly for Bi The various schemes shown in Fig 1 cover the range of possible ionic compensation mechanisms, including formation of interstitial cations (3) or anion vacancies ( 1, 2, 4-6)

It is important to recognize that proper crystallographic studies are required to determine the precise structural details of each substitution as these cannot be inferred from the phase diagram The general formula of the solid solutions may be written as Bi4+yV2--y--xMx01~ - x - y The phase diagram results are presented on a triangular compositional grid with x and y as the variables We refer to mechanism (2) ( I e y=O, variable x) in which M substitutes for V as the 'stoichiometric join'

In B-doped materials (Fig 2) the solid solution is quite

'2'5 *'2'3 0 20 40 1 00

x (mol %)

Fig. 1 Possible doping mechanisms for trivalent cations in bismuth vanadate ( BI~VZO,,) solid solution

-02 - 0 1 0 0 1 02

y '" B14 + yv2 - yol 1 - y

Fig.2 Composition range of the Bi4+,,V2 ,,BxOll ,, solid s o h tion, (A) a, (A) a+, (0) Y+

extensive in direction (2), corresponding to the mechanism V+B [z e for y=O, x(max)=O 31 However, the maximum solid-solution extent is displaced towards negative values of y , with the V-rich limit of the solid-solution area running roughly parallel to direction (5) All the B-doped materials give the a- polymorph, it was not possible to stabilise the y-polymorph to room temperature with B as a dopant

In Al- and Cr-doped materials (Fig 3 and 4) the solid- solution area was much more extensive than for B substitution with, for instance, a limit of x=O 5 at y=O compared with x= 0 3 at y=O for B Maximum x values of ca 0 7 and 0 6, respectively, were obtained at negative values of y for Al- and Cr-doped phases In contrast to the B-doped materials, most compositions gave the y-polymorph Thus, at y = 0, y-phase was obtained for all solid solutions with x > O 2, the a-poly- morph was obtained only for lower values of x and a few compositions with negative values of y Sharma et a1 lo reported that Bi4Vl 2010 8 was an a-polymorph at the preparation temperature of 650 "C In this study, composition x = 0, y = 0 2 gave the a-polymorph at 500°C (see later) and so our results are consistent with those in ref 10

In Y- and La-doped materials (Fig 5 and 6) the solid- solution areas are much smaller than those obtained for Al- and Cr-substitution, but tend to extend to more negative values of y The difference between Y-and La-doped solid solutions is that for Y-doped materials the P-polymorph was most commonly obtained while the a- and y-polymorphs were obtained for La-doped materials Stabilization of the 1-poly-

- 0 4 -02 0 02 yl" B14+ y"2-yoll - y

Fig.3 Composition range of the Bi4+,,VZ yAlxO,, ,, solid solu- tlon, (A) a, (A) a+, (0 ) Y, (0) Y+

43-02-01 0 0 1 0 2 03 Y'nB'4+yV2-y011-y

Fig.4 Composition range of the BI~+ , ,VZ-~ ,,CrXOll ,, solid solu- tlon, (A) a, (A) a+, (0 ) Y , (0) Y+

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-0.2 -0.1 0 0.1 0.2 Y in Bi, + yv2-yol, - y

-0.3 -0.2 4 . 1 0 0.1 0.2 0.3 Y i " B i ~ + y V 2 - y 0 1 , - y

Fig. 6 Composition range of the Bi4+yV2-x-yLaxOll-x-y solid solu- tion; (A) a, (a) a+, (0 ) Y, (0) Y+

morph is not common but it has been reported previously in Bi4V2Ol1 doped with Mo.12

The phase diagrams at 500°C give very much smaller solid solution areas for all the materials (Fig. 2-6, dotted curves). In addition, all the solid solutions give a-polymorphs on cooling from 500°C to room temperature. These results show that the stability of the trivalent-doped materials is very temperature dependent, as has been noted in the parent bismuth vanadate solid solution^.^ In the divalent cation- doped materials, however, the solid solution areas at 500"C, though smaller,16 are not reduced in size by nearly as much as those found here with the trivalent dopants.

The phase diagrams for the trivalent cation-doped systems show that several mechanisms are required to account for the wide range of single-phase compositions that form, as is the case also for the divalent-doped systems.14 The three most simple mechanisms are: V+Bi (1); V-+M (2); Bi+M (7) . The phase diagrams indicate, however, that neither of mechanisms (2) or (7) is particularly dominant for any of the trivalent dopants, in contrast to the divalent systems in which mechan- ism (2) predominates for small dopants and mechanism ( 7 ) for larger ones. The trivalent systems are generally most extensive in directions (4)-( 6) (Fig. 1) which could indicate that some kind of double substitution mechanism, involving substitution of M for both Bi and V, is preferred. Since the range of trivalent dopants studied varies greatly in size (Table 1) it is difficult to imagine that atoms as small as B could substitute directly for Bi in the same way that La could. The distorted nature of both Bi and V sites in the crystal

Table 1 Octahedral ionic radii"

ionic radius/pm ionic radius/pm

Bi3 + 103 v5 + 54 Y3 + 90 ~ 1 3 + 53.5 La3 + 103.2 Cr3 + 61.5

B3+ 27

a From ref. 18.

structure of Bi4V201, may allow considerable flexibility of the structure towards atoms of different size; thus B could perhaps occupy off-centre positions on substituting for Bi, in the same way that Li is able to substitute for Ca in the perovskites (Ca, -xLix)(Zrl -xTax)03.17 Clearly, structural studies to eluci- date the doping mechanisms are required since these cannot be deduced from the loci of the solid solutions in the phase diagrams.

Conductivity of doped Bi4V2OI1

Conductivity measurements were carried out for single-phase materials obtained on the stoichiometric join ( y=O) as well as for selected compositions with varying y and x (with reference to the general formula Bi4+yV2--y-xMxOll - x - y ) . Conductivity values were extracted from impedance complex plane plots. In general, for temperatures below 400 "C, a broadened semicircle with a low-frequency spike was obtained; at higher tempera- tures the spike became more pronounced. Typical impedance data are shown in Fig. 7 for an La-doped sample ( Bi4.10V1~70La0.20010~70); the associated capacitance of the semi- circle [Fig. 7(u)] has a value of 3.9 x F cm-' after correcting for jig capacitance, which is typical of a bulk component. There was no sign of any lower frequency, second semicircle that might be attributable to grain boundary impedances. Grain boundary resistances appear to be negligible in comparison with bulk resistances, therefore. At higher temperatures, the low-frequency spike inclined at ca. 45" to the horizontal axis was the predominant feature [Fig. 7(b)]; its associated capacitance of ca. F cm-' is characteristic of ionic polarization phenomena at the blocking electrodes, and a diffusion-limited Warburg impedance, thus supporting the idea that conduction was purely or predominantly ionic. There was no sign of partial collapse of the low-frequency spike, such as would occur if significant levels of electronic conduction were present. As in the case of Bi4V2Ol1 solid solutions5 and other doped Bi4V201, phase^,^,^," for some of which transport number2 and dc polarisation measurement^^-^ have been carried out, the main conducting species in these materials appears to be oxide ions.

Arrhenius plots of the conductivity of representative doped materials are shown in Fig. 8. All have the same overall composition, x=O.2, y=O, which is the composition close to that of highest conductivity with dopants such as Co, Ni and Ti. The pellets for the Al- and Cr-doped Bi4V2OI1 were initially the y-polymorph but the Arrhenius plots show three different

&O 470 490 510 530

~'1105 n Fig. 7 Impedance data for Bi4~,oV,.70Lao.20010.70 at (a) 200 "C (b) 500°C

and

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* B-doped 0

-1 -

-2 -

h p- r

1 -

C b 4- m \

v

- .

-5 -

-6

-7 0 8 1 1 2 1 4 1 6 1 8 2 2 2 2 4 2 6

lo3 KIT

Fig. 8 Arrhenius plots of Bi,V1 8Mo 20i0 8 M =B, Al, Cr, Y, La

slopes corresponding to a-, #I- and y-polymorphs and similar to that of the B-doped pellet which was the a-polymorph at the beginning of the conductivity measurements This further indicates that the y-polymorph in Al- and Cr-doped solid solutions is metastable at low temperatures and readily reverts to the a-polymorph during the heating cycle of conductivity measurements

The p- and y-polymorphs were present in the Y- and La- doped pellets and their conductivity behaviour was as expected with one and zero changes of slope, respectively (Fig 8) There was, however, some evidence of slight curvature in the plot for the La-doped sample at high temperature Companng the different materials in Fig 8, conductivity at e g 300°C is highest with La as the dopant, closely followed by Y, that of A1 is somewhat lower and that of B, Cr is much lower In terms of possible applications, the La-doped materials in particular are attractive because their conductivity data are reversible on cooling and do not show the same scatter and hysteresis which is often seen in materials that undergo the a + j 3 y transitions

The effect on conductivity of varying composition was investigated For A1 and Cry the conductivity was measured for samples with different x at y=O, both sets showed a conductivity maximum at x = 0 20, as shown for the Al-doped materials in Fig 9 Solid solutions on this stoichiometric join are much less extensive for B, Y and La dopants but neverthe- less the conductivity increased to a maximum at x z 0 2

The effect of varying y at a constant x value of 0 2 was also studied For Al- and La-dopants, the conductivity decreased by a factor of 2 to 3 between y=O and y=O 1, for both systems (Table 2) This trend is similar to that observed in the parent, undoped materials5 and divalent-doped materials Is

Conclusions Bi4V2OI1 is an extremely versatile host structure for doping and is able to accept large amounts of trivalent ions, indepen- dent of their size Thus, remarkably, ions as different in size as B and La are able to enter the Bi4V2OI1 structure in large amounts

The mechanism of doping is complex but can nominally be readily interpreted in terms of the three simple cation substi- tution mechanisms V+Bi, Bi+M and V+M Some dopants

*>

2 -

h F

3 - Y

I C \ g -4- 0 -

5 -

6-

600 "C

400 "c

3CO "C

250 O C

7 ' I I I I I I 0 1 015 0 2 025 03 035 0 4 045

x in BI& xAlxO,,

Fig. 9 Conductivity isotherms for Al-doped materials with varying x

M=Al M=La

T/"C y = o y = o 1 y = o y = o 1

200 250 300 3 50 400 450 500 550 600

199x10 832x10 576x10 215x10 129x10 ' 543x10 303x10 ' 106x10 ' 648x10 294x10 138x10 473x10 269x10 134x10 487x10 165x10 967x10 520x10 138x10 466x10 327x10 246x10 333x10 108x10 180x10 590x10 664x10 207x10 800x10 229x10 124x10 363x10 111x10 406x10 238x10 575x10

favour retention of the a-structure (B), whereas others more readily stabilise the y-structure (e g La)

The conductivity for all five dopants studied appears to be a maximum around the composition x = 0 2, y = 0 that appears also to be favoured by other dopants Impedance data indicate the conduction mechanism to be ionic, and presumably there- fore due to 02- ions, similar to the conclusions reached for other doped y-structures

The highest conductivity at 300°C was obtained for La as the dopant, 1 4 x R-' cm-I Although this value is about one order of magnitude lower than for the best BIMEVOX matenals, the La-doped material may be attractive for appli- cations due to the reproducibility and reversibility on heat/cool cycles of its conductivity data

We thank Mr Azali Md Sab, Soil Science Department, UPM for assistance with the X-ray diffraction analysis C K L is grateful to the Majlis Penyelidikan Kemajuan Sains Negara for financial support, grant no 2-07-05-009

References

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3 F Abraham, M F Debreuille-Gresse, G Mairesse and G Nowogrocki, Solid State Ionccs, 1988 28-30,529

4 E Pernot M Anne M Bacmann P Strobe1 J Fouletier, R N

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C. K. Lee, M. P. Tan and A. R. West, J. Muter. Chem., 1994,4,525. R. N. Vannier, G. Mairesse, F. Abraham and G. Nowogrocki, J. Solid State Chem., 1993,103,441. R. N. Vannier, G. Mairesse, G. Nowogrocki, F. Abraham and J. C. Boivin, Solid State Ionics, 1992,53-56, 713. C . K. Lee, G. S. Lim and A. R. West, J. Muter. Chem., 1994,4,1441. C . K. Lee, G. S. Lim, K. S. Low and A. R. West, Solid State Ionic Materials, World Scientific Pub. Co., 1994, p. 211. C. K. Lee, G. S. Lim and A. R. West, unpublished results. R. I. Smith and A. R. West, J. Solid State Chem., 1994,108,29. R. D. Shannon, Acta Crystallogr., Sect. A , 1976,32,751.

Paper5/04832J; Received 21st July, 1995

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