- Email: [email protected]
Thermal behaviour and polymorphism of BIMEVOX oxide ion conductors including the new materials: Bi4V2O11: M; M = La, Y, Mg, B
__ __ !!!I!!
a
SOLID STATE
EL!!EYIER
Solid State Ionics 86-88
IONICS
(1996) 235-239
Thermal behaviour and polymorphism of BIMEVOX oxide ion conductors including the new materials: Bi,V,O, 1 : M; M = La, Y, Mg, B C.K. Lee”‘“, A.R. Westb “Chemistry
Department,
hChemistry
Univeristi
Department,
Pertanian
University
Malaysia,
of Aberdeen,
43400
Meston
Serdang,
Selangor,
Walk, Aberdeen
Malaysia
AB9 ZUE, UK
Abstract The BIMEVOX materials can be prepared nature/concentration of dopant. Polymorphic variations in conductivity Arrhenius plots. BIMEVOX materials. Some conductivity cm-‘; B&V&a,, 2010.H’ 1.4X 10m4 0-l cm-‘. Keywords:
BIMEVOX;
Ion conductors;
modifications: cy, p, y, y ‘, depending on Bi:V ratio and the transitions take place on heat/cool cycles and can be detected by DTA and by A survey of thermal behaviour is given, using as examples several new values at 300°C are: Bi,V,,,Mg,,,O,,,,,,,, 1.1 X IO--’ R- ’ cm- ‘; Bi 4V LXY o* O,,,,, 1.0X 10m4 0-l cm-r; Bi 4V I.8B 0 zO,,,,, 2.0X IO-’ a-~’ in four polymorphic
Polymorphism
1. Introduction The BIMEVOX materials, based on metal-doped bismuth vanadate, Bi,V,O,, are a recently discovered family of materials with very high levels of oxide ion conductivity e.g. 2 X 10e3 0-l cm-’ for B&V, .8Cuo20 ,0.X at 300°C [l]. The parent phase forms with B&V20 I I exists in three polymorphic transition temperatures: 450°C
56O’C
ffPP@Y.
Transition temperatures vary with both Bi:V ratio [2] and with dopant type/concentration [3]. The general trend is for transition temperatures to decrease with *Corresponding OI67-2738/96/$15.00 PII
author. Copyright
SO167-2738(96)00132-4
01996
increasing departure from the Bi,V,O, , stoichiometry, leading to a gradual stabilisation of the high temperature y polymorph, which is the one primarily responsible for high oxide ion conductivity. In addition to the well-established (Y, /3 and y polymorphs, the existence of a high temperature y ’ polymorph was referred to in the early literature [4] but this may, instead, be associated with partial melting [2]. More recently, a different low-temperature y’ polymorph has been proposed to exist in some heavily-doped materials [3,5,6]. Structural information on the various BIMEVOX phases is limited. In general terms, they are layered structures with alternating sheets of nominal stoichiometry (Bi,O,)z”+ and (VO,,)rbut the details are not clear, especially for the y polymorph which exhibits much structural disorder in all three atomic
Elsevier Science B.V. All rights reserved
236
C.K. Lw, A.R. Wr.rt I Solid Starr lonics 86-88
positions [1,5]. The oxide ion conductivity appears to be associated with vacancies in the vanadate layer, written schematically as which may be where 0 refers to an oxide va(Vo,.,o,,s)n2”-~ details are unclear. cancy, although mechanistic Consistent with this is the observation that the conductivity is markedly anisotropic with the inplane conductivity value 100 times higher than that perpendicular to the layers IS]. The occurrence of polymorphic transitions on heat/cool cycles leads to complicated conductivity vs. temperature behaviour, often with considerable hysteresis between cooling and heating. One advantage of stabilising the y polymorph to low temperatures is that much of this hysteresis disappears and the conductivity data are more nearly reversible, although may still show curvature in the Arrhenius plots. Most of the work on the BIMEVOX materials has focused on a composition in which 10% of the V is nominally replaced by ions such as Cu, Ni Co, Al, Ti, Nb and Ta [ 1.7-91. Recently, we have carried out exploratory phase diagram studies to determine the stoichiometry ranges for a whole range of di-, triand tetravalent dopants [lo-121. These give the startling results that Bi,V,O, , is an enormously versatile host structure for doping. Thus, with trivalent dopants, atoms spanning the entire range of sizes possible, from B to La, are able to enter the Bi,V,O, , structure in significant amounts; similarly with divalent dopants, with atoms ranging in size from Co to Ba. It is also apparent from the phase diagrams and the loci of the solid solutions, that the mechanism of doping is complex. In ‘simple stoichiometric’ materials such as Bi,V, 8Cu, *O ,“,,, structural studies by X-ray/neutron powder diffraction and EXAFS indicate that Cu substitutes onto the V sites [ 1,131. Nevertheless, to account for solid solutions that are rich in either Bi or V/M, additional substitution mechanisms are required, such as substitution of Bi onto V sites or substitution of M onto Bi and/or V sites or substitution of M onto previouslyunoccupied interstitial sites. The phase diagrams show clearly that mechanisms such as these are necessary to account for the wide ranges of solid solution formation. Crystallographic studies are, however, required to elucidate the structural details. An additional advantage of phase diagram studies
(1996) 2.7.5-239
is that well-characterised new materials are generated which could be missed in trial-and-error attempts at doping and these lend themselves to systematic studies of stoichiometry/electrical conductivity correlations. For instance, it was found that with La as a dopant, an extensive range of solid solutions forms that are good oxide ion conductors at low temperatures, e.g. 1.4X 10e4 a-’ cm-’ at 300°C. With Al as a dopant, however, an extensive area of solid solutions forms that give superior conducting properties at higher temperatures, e.g. 1.1 X IO-’ 12-l cm-’ at 600°C [Ill. In this paper, we attempt a rationalisation of the thermal behaviour of BIMEVOX phases, through a comparison of conductivity Arrhenius plots and differential thermal analysis, DTA, data linked to polymorphic identification by X-ray powder diffraction, XRD. We use examples from previously unstudied BIMEVOX systems, containing in particular, Mg, Y and La as the dopants.
2. Experimental Reagents used, preparation and XRD of the materials are as reported elsewhere [ 10,111. DTA was carried out using a Du Pont 991 instrument with a 1200°C cell and a heating rate of 10 C” min- ‘. AllO, was used as the reference material and heat/ cool cycles were carried out at in a N, atmosphere. Conductivity measurements were carried out using a.c. impedance method as described elsewhere [12]; the purpose of the data shown here is to complement those obtained by DTA for identifying the various phase transitions.
3. Results and discussion A general scheme that illustrates our present understanding of BIMEVOX polymorphism is shown in Fig. 1, in the form of free energy against temperature. At any temperature, the most stable polymorph is the one of lowest free energy; metastable polymorphs arise when a high temperature polymorph fails to undergo an expected transition on cooling, probably because the cooling rate is too high. These undercooled materials may undergo
C.K. Lee, A.R. West I Solid State Ionics 86-88
(1996) 235-239
237
I
A6
T
Fig. I. Schematic representation of the relative free energies of different polymorphs as a function of temperature.
transitions at some lower temperature, or on prolonged annealing, to give either the equilibrium polymorph or some other metastable polymorph. Thus hysteresis between heat and cool cycles of DTA or conductivity measurements is associated with the sluggishness of certain transitions on cooling; such effects are very composition-dependent. On surveying the literature, and our own unpublished results, a wide range of thermal behaviours is seen, including the following:
Fig. 2. Assembly transitions.
a
-8-y
---+8-r
I
u+p+y+j?+cy
showing a+P+y+P
sitions. (c) Bi,V, xnC~,, ,20,,, l(L showing
(Y+Y-+~+LY
trantransi-
transitions. (e) tions. (d) Bi,V, XHMgc,,,(I,,, xz showing (~+y-tp transitions. showing (f) P-lY-+P B&V, ,Y,, 2O /o MI transitions. (g) showing Y”Y’Y’ Bi,V, ,Mg,, G,, ,,, 4O ,,,41) showing y+y’+y
transitions.
-t-v
HFN-
cl
plots showing various polymorphic
transitions. (b) Bi,V, 8xCr,, ,20,,,,,
Bi,V, Jn,,
f---
of DTA
(a) Bi,V, xxB,, ,,O ,(,xx showing
-IX
-8
-8 CI
-----+I
-fl
m
-I
-8
8
-Y
Y’
-
-a
-8 I
-
1’
-I’-7
Many of these are illustrated in the compilation of DTA results in Fig. 2 and in conductivity Arrhenius plots, Figs. 3 and 4. Although the timescales of DTA runs ( 10 C” mini ’ ) and conductivity measurements (isothermal anneals for 30 mins in steps of 2%50°C) differ greatly, similar phase transitions are seen by the two techniques. It is important to clarify that
DTA alone is insufficient to assign, with certainty, peaks to particular polymorphic transitions. Additional information, particularly from XRD, is necessary. A general pattern of behaviour is observed with varying composition; the cy # /3 and p # y transition temperatures are highest in Bi,V,O,, and show a general decrease on departing from this stoichiometry, either by varying the Bi:V ratio or by substitution of dopant, M. In some cases, the rate of change of the two transition temperatures with composition is similar and for instance, as with Bi-rich Bi,V,O, , , first the p polymorph and then the y polymorph becomes progressively stabilised and can be preserved intact to room temperature by quenching 121. In other cases, as with Cu-, Co- and Mg-doped
C.K. Lee, A.R. West I Solid State lonics
238
_,~ 0.u
1
1.2
1.4
1.6
1.8
2
2.2
2.4
2.6
1000/T (i(-‘) Fig.
3.
Examples
a+p+y+P+~~ P+y+P
of
Arrhenius
transitions transitions (Bi,V,
(Bi,V,
plots
showing
,,B,, ,?O,,, x,)
and
(i) (ii)
xY,,,O,,,,,,).
l
Mg-doped(H)!
- La-doped(C) * La-doped(H)
1000/T (K-‘) Fig, 4. Examples
of Arrhenius
plots showing (i)
y’+y+y’
transitions (B&V, ,Mg,, 20,,1 ,(, ) and (ii) y phase with no obvious transition (Bi,V,
*La,, ?O,,, xI,) but slight curvature
in the Arrhenius
plot.
materials, the two transition temperatures change in such a way that the stability field of the intermediate temperature /3 polymorph becomes gradually squeezed out leading to direct (Y+ y transitions (Fig. 2c-d). Each of the transitions (Y# p, p # y and CY# y is characterised by a sharp discontinuity in the conductivity Arrhenius plots, since each polymorph has a characteristic activation energy. Interpretation of the conductivity data is, however, complicated for
86-88
(1996)
2-75239
two reasons; first, the conducting species in p and y polymorphs are primarily oxide ions but that in the (Y polymorph appears not to be oxide ions, at least for undoped bismuth vanadate [2]; second, since most data are reported on polycrystalline materials, the relative contributions of the conductivity in the two crystallographic orientations cannot be separated easily from the net conductivity data. This separation may be feasible by a.c. impedance measurements at low temperatures where the conductivities are low, but not in the highly conducting, high temperature region. In spite of these uncertainties over the interpretation of conductivity data, the general pattern of behaviour associated with the different polymorphs and phase transitions is clear, Figs. 3 and 4. The status of the y’ polymorph is still uncertain, but there does appear to be a clear pattern of thermal behaviour (Mg-doped material in Fig. 2f and Fig. 4), with a fairly sharp transition to the high temperature y polymorph. For the y, y’ polymorphs, the conductivity data are much more nearly reversible on heat/cool cycles. There is a major difference in activation energy between low and high temperatures, with a region of curvature, but no discontinuity, at intermediate temperatures. The crystallographic identity of y’ is not established: by routine powder XRD, no distinction between y and y’ polymorphs is apparent, but supercell reflections have been reported in y samples annealed at 510°C [5]. Whether these are an indication of a y’ polymorph that is better ordered has not been established. The y polymorph, when stable over a wide temperature range, shows no sign of thermal activity by DTA; conductivity Arrhenius plots may, however, show some slight curvature, e.g. La-doped material in Fig. 4. The above survey refers to materials that are single phase by XRD. An additional complication arises since the solid solution limits with most BIMEVOX systems are markedly temperature dependent, especially over the range 500-850°C (approximate melting temperature). Consequently, heavily non-stoichiometric materials that are single phase at high temperatures may precipitate secondary phases on cooling at the same time as the BIMEVOX solid solution limits contract. In practice, these precipitation reactions may be prevented if cooling rates are sufficiently rapid and metastable,
C.K. Lee, A.R. West I Solid State Ionics
supersaturated BIMEVOX solid solutions are then free to undergo various phase transitions to give low temperature products which are also metastable but still single phase. The thermal behaviours associated with such metastable phases are indistinguishable from those given in Fig. 4, care is required, however, to check whether precipitation reactions do occur during thermal cycles or annealing at intermediate temperatures, which may lead to either DTA heat effects or conductivity changes independently of those associated with polymorphic changes. One characteristic of metastable phases is that they may revert to a more stable low temperature phase during the heating cycle of DTA; such transitions are characterised by an exotherm on heating, e.g. Fig. 2g.
Acknowledgments We thank Mr. Mat Department, UPM, for surement. C.K. Lee Penyelidikan Kemajuan financial support.
Kamal Margona, Chemistry assistance with DTA meais grateful to the Majlis Sains Negara, Malaysia for
86-88
(1996) 2X-239
239
References 111F. Abraham, J.C. Boivin, G. Mairesse and G. Nowogrocki, Solid State Ionics 40/41
(1990) 934.
121C.K. Lee, D.C. Sinclair and A.R. West, Solid State lonics 62 (1993) 193. [31 C.K. Lee and A.R. West, unpublished results. G. Mairesse and G. 141 F. Abraham, M.F. Debueuille-Gresse, Nowogrocki, Solid State Ionics 28-30 (1988) 529. PI E. Pemot, M. Anne, M. Bacmann, P. Strobel, J. Foulatier, R.N. Vannier, G. Mairesse, F. Abraham and G. Nowogrocki, Solid State Ionics 70/71 (1994) 59. M Ganne, R N Vannier. G [61 0. Joubert, A Jouanneaux, Mairesse, Solid State Ionics 73 (1994) 309. 171 R. Essalim, B. Tanouti, J.P. Bonnet and J.M. Reau, Mater. Lett. 13 ( 1992) 382. Solid State PI V. Sharma, A.K. Shukala and J. Gopalakrishnan. Ionics 58 (1992) 359. A. Manthiram, M. Paranthaman, Y.S. 191 J.B. Goodenough, Zhen, Mater. Sci. Eng. B 12 (1992) 357. Cl01 C.K. Lee, G.S. Lim and A.R. West, J. Mat. Chem. 4 (1994) 1441. [Ill C.K. Lee, B.H. Bay and A.R. West, J. Mat. Chem. 6 ( 1996) 331. [I21 C.K. Lee, M.P. Tan and A.R. West, J. Mat. Chem. 4 (1994) 525. [I31 I. Abraham, F. Krok and J.A.G. Nelstrop, 2nd Int. Mat. Chem. Conf., U.K., 1995.
a
SOLID STATE
EL!!EYIER
Solid State Ionics 86-88
IONICS
(1996) 235-239
Thermal behaviour and polymorphism of BIMEVOX oxide ion conductors including the new materials: Bi,V,O, 1 : M; M = La, Y, Mg, B C.K. Lee”‘“, A.R. Westb “Chemistry
Department,
hChemistry
Univeristi
Department,
Pertanian
University
Malaysia,
of Aberdeen,
43400
Meston
Serdang,
Selangor,
Walk, Aberdeen
Malaysia
AB9 ZUE, UK
Abstract The BIMEVOX materials can be prepared nature/concentration of dopant. Polymorphic variations in conductivity Arrhenius plots. BIMEVOX materials. Some conductivity cm-‘; B&V&a,, 2010.H’ 1.4X 10m4 0-l cm-‘. Keywords:
BIMEVOX;
Ion conductors;
modifications: cy, p, y, y ‘, depending on Bi:V ratio and the transitions take place on heat/cool cycles and can be detected by DTA and by A survey of thermal behaviour is given, using as examples several new values at 300°C are: Bi,V,,,Mg,,,O,,,,,,,, 1.1 X IO--’ R- ’ cm- ‘; Bi 4V LXY o* O,,,,, 1.0X 10m4 0-l cm-r; Bi 4V I.8B 0 zO,,,,, 2.0X IO-’ a-~’ in four polymorphic
Polymorphism
1. Introduction The BIMEVOX materials, based on metal-doped bismuth vanadate, Bi,V,O,, are a recently discovered family of materials with very high levels of oxide ion conductivity e.g. 2 X 10e3 0-l cm-’ for B&V, .8Cuo20 ,0.X at 300°C [l]. The parent phase forms with B&V20 I I exists in three polymorphic transition temperatures: 450°C
56O’C
ffPP@Y.
Transition temperatures vary with both Bi:V ratio [2] and with dopant type/concentration [3]. The general trend is for transition temperatures to decrease with *Corresponding OI67-2738/96/$15.00 PII
author. Copyright
SO167-2738(96)00132-4
01996
increasing departure from the Bi,V,O, , stoichiometry, leading to a gradual stabilisation of the high temperature y polymorph, which is the one primarily responsible for high oxide ion conductivity. In addition to the well-established (Y, /3 and y polymorphs, the existence of a high temperature y ’ polymorph was referred to in the early literature [4] but this may, instead, be associated with partial melting [2]. More recently, a different low-temperature y’ polymorph has been proposed to exist in some heavily-doped materials [3,5,6]. Structural information on the various BIMEVOX phases is limited. In general terms, they are layered structures with alternating sheets of nominal stoichiometry (Bi,O,)z”+ and (VO,,)rbut the details are not clear, especially for the y polymorph which exhibits much structural disorder in all three atomic
Elsevier Science B.V. All rights reserved
236
C.K. Lw, A.R. Wr.rt I Solid Starr lonics 86-88
positions [1,5]. The oxide ion conductivity appears to be associated with vacancies in the vanadate layer, written schematically as which may be where 0 refers to an oxide va(Vo,.,o,,s)n2”-~ details are unclear. cancy, although mechanistic Consistent with this is the observation that the conductivity is markedly anisotropic with the inplane conductivity value 100 times higher than that perpendicular to the layers IS]. The occurrence of polymorphic transitions on heat/cool cycles leads to complicated conductivity vs. temperature behaviour, often with considerable hysteresis between cooling and heating. One advantage of stabilising the y polymorph to low temperatures is that much of this hysteresis disappears and the conductivity data are more nearly reversible, although may still show curvature in the Arrhenius plots. Most of the work on the BIMEVOX materials has focused on a composition in which 10% of the V is nominally replaced by ions such as Cu, Ni Co, Al, Ti, Nb and Ta [ 1.7-91. Recently, we have carried out exploratory phase diagram studies to determine the stoichiometry ranges for a whole range of di-, triand tetravalent dopants [lo-121. These give the startling results that Bi,V,O, , is an enormously versatile host structure for doping. Thus, with trivalent dopants, atoms spanning the entire range of sizes possible, from B to La, are able to enter the Bi,V,O, , structure in significant amounts; similarly with divalent dopants, with atoms ranging in size from Co to Ba. It is also apparent from the phase diagrams and the loci of the solid solutions, that the mechanism of doping is complex. In ‘simple stoichiometric’ materials such as Bi,V, 8Cu, *O ,“,,, structural studies by X-ray/neutron powder diffraction and EXAFS indicate that Cu substitutes onto the V sites [ 1,131. Nevertheless, to account for solid solutions that are rich in either Bi or V/M, additional substitution mechanisms are required, such as substitution of Bi onto V sites or substitution of M onto Bi and/or V sites or substitution of M onto previouslyunoccupied interstitial sites. The phase diagrams show clearly that mechanisms such as these are necessary to account for the wide ranges of solid solution formation. Crystallographic studies are, however, required to elucidate the structural details. An additional advantage of phase diagram studies
(1996) 2.7.5-239
is that well-characterised new materials are generated which could be missed in trial-and-error attempts at doping and these lend themselves to systematic studies of stoichiometry/electrical conductivity correlations. For instance, it was found that with La as a dopant, an extensive range of solid solutions forms that are good oxide ion conductors at low temperatures, e.g. 1.4X 10e4 a-’ cm-’ at 300°C. With Al as a dopant, however, an extensive area of solid solutions forms that give superior conducting properties at higher temperatures, e.g. 1.1 X IO-’ 12-l cm-’ at 600°C [Ill. In this paper, we attempt a rationalisation of the thermal behaviour of BIMEVOX phases, through a comparison of conductivity Arrhenius plots and differential thermal analysis, DTA, data linked to polymorphic identification by X-ray powder diffraction, XRD. We use examples from previously unstudied BIMEVOX systems, containing in particular, Mg, Y and La as the dopants.
2. Experimental Reagents used, preparation and XRD of the materials are as reported elsewhere [ 10,111. DTA was carried out using a Du Pont 991 instrument with a 1200°C cell and a heating rate of 10 C” min- ‘. AllO, was used as the reference material and heat/ cool cycles were carried out at in a N, atmosphere. Conductivity measurements were carried out using a.c. impedance method as described elsewhere [12]; the purpose of the data shown here is to complement those obtained by DTA for identifying the various phase transitions.
3. Results and discussion A general scheme that illustrates our present understanding of BIMEVOX polymorphism is shown in Fig. 1, in the form of free energy against temperature. At any temperature, the most stable polymorph is the one of lowest free energy; metastable polymorphs arise when a high temperature polymorph fails to undergo an expected transition on cooling, probably because the cooling rate is too high. These undercooled materials may undergo
C.K. Lee, A.R. West I Solid State Ionics 86-88
(1996) 235-239
237
I
A6
T
Fig. I. Schematic representation of the relative free energies of different polymorphs as a function of temperature.
transitions at some lower temperature, or on prolonged annealing, to give either the equilibrium polymorph or some other metastable polymorph. Thus hysteresis between heat and cool cycles of DTA or conductivity measurements is associated with the sluggishness of certain transitions on cooling; such effects are very composition-dependent. On surveying the literature, and our own unpublished results, a wide range of thermal behaviours is seen, including the following:
Fig. 2. Assembly transitions.
a
-8-y
---+8-r
I
u+p+y+j?+cy
showing a+P+y+P
sitions. (c) Bi,V, xnC~,, ,20,,, l(L showing
(Y+Y-+~+LY
trantransi-
transitions. (e) tions. (d) Bi,V, XHMgc,,,(I,,, xz showing (~+y-tp transitions. showing (f) P-lY-+P B&V, ,Y,, 2O /o MI transitions. (g) showing Y”Y’Y’ Bi,V, ,Mg,, G,, ,,, 4O ,,,41) showing y+y’+y
transitions.
-t-v
HFN-
cl
plots showing various polymorphic
transitions. (b) Bi,V, 8xCr,, ,20,,,,,
Bi,V, Jn,,
f---
of DTA
(a) Bi,V, xxB,, ,,O ,(,xx showing
-IX
-8
-8 CI
-----+I
-fl
m
-I
-8
8
-Y
Y’
-
-a
-8 I
-
1’
-I’-7
Many of these are illustrated in the compilation of DTA results in Fig. 2 and in conductivity Arrhenius plots, Figs. 3 and 4. Although the timescales of DTA runs ( 10 C” mini ’ ) and conductivity measurements (isothermal anneals for 30 mins in steps of 2%50°C) differ greatly, similar phase transitions are seen by the two techniques. It is important to clarify that
DTA alone is insufficient to assign, with certainty, peaks to particular polymorphic transitions. Additional information, particularly from XRD, is necessary. A general pattern of behaviour is observed with varying composition; the cy # /3 and p # y transition temperatures are highest in Bi,V,O,, and show a general decrease on departing from this stoichiometry, either by varying the Bi:V ratio or by substitution of dopant, M. In some cases, the rate of change of the two transition temperatures with composition is similar and for instance, as with Bi-rich Bi,V,O, , , first the p polymorph and then the y polymorph becomes progressively stabilised and can be preserved intact to room temperature by quenching 121. In other cases, as with Cu-, Co- and Mg-doped
C.K. Lee, A.R. West I Solid State lonics
238
_,~ 0.u
1
1.2
1.4
1.6
1.8
2
2.2
2.4
2.6
1000/T (i(-‘) Fig.
3.
Examples
a+p+y+P+~~ P+y+P
of
Arrhenius
transitions transitions (Bi,V,
(Bi,V,
plots
showing
,,B,, ,?O,,, x,)
and
(i) (ii)
xY,,,O,,,,,,).
l
Mg-doped(H)!
- La-doped(C) * La-doped(H)
1000/T (K-‘) Fig, 4. Examples
of Arrhenius
plots showing (i)
y’+y+y’
transitions (B&V, ,Mg,, 20,,1 ,(, ) and (ii) y phase with no obvious transition (Bi,V,
*La,, ?O,,, xI,) but slight curvature
in the Arrhenius
plot.
materials, the two transition temperatures change in such a way that the stability field of the intermediate temperature /3 polymorph becomes gradually squeezed out leading to direct (Y+ y transitions (Fig. 2c-d). Each of the transitions (Y# p, p # y and CY# y is characterised by a sharp discontinuity in the conductivity Arrhenius plots, since each polymorph has a characteristic activation energy. Interpretation of the conductivity data is, however, complicated for
86-88
(1996)
2-75239
two reasons; first, the conducting species in p and y polymorphs are primarily oxide ions but that in the (Y polymorph appears not to be oxide ions, at least for undoped bismuth vanadate [2]; second, since most data are reported on polycrystalline materials, the relative contributions of the conductivity in the two crystallographic orientations cannot be separated easily from the net conductivity data. This separation may be feasible by a.c. impedance measurements at low temperatures where the conductivities are low, but not in the highly conducting, high temperature region. In spite of these uncertainties over the interpretation of conductivity data, the general pattern of behaviour associated with the different polymorphs and phase transitions is clear, Figs. 3 and 4. The status of the y’ polymorph is still uncertain, but there does appear to be a clear pattern of thermal behaviour (Mg-doped material in Fig. 2f and Fig. 4), with a fairly sharp transition to the high temperature y polymorph. For the y, y’ polymorphs, the conductivity data are much more nearly reversible on heat/cool cycles. There is a major difference in activation energy between low and high temperatures, with a region of curvature, but no discontinuity, at intermediate temperatures. The crystallographic identity of y’ is not established: by routine powder XRD, no distinction between y and y’ polymorphs is apparent, but supercell reflections have been reported in y samples annealed at 510°C [5]. Whether these are an indication of a y’ polymorph that is better ordered has not been established. The y polymorph, when stable over a wide temperature range, shows no sign of thermal activity by DTA; conductivity Arrhenius plots may, however, show some slight curvature, e.g. La-doped material in Fig. 4. The above survey refers to materials that are single phase by XRD. An additional complication arises since the solid solution limits with most BIMEVOX systems are markedly temperature dependent, especially over the range 500-850°C (approximate melting temperature). Consequently, heavily non-stoichiometric materials that are single phase at high temperatures may precipitate secondary phases on cooling at the same time as the BIMEVOX solid solution limits contract. In practice, these precipitation reactions may be prevented if cooling rates are sufficiently rapid and metastable,
C.K. Lee, A.R. West I Solid State Ionics
supersaturated BIMEVOX solid solutions are then free to undergo various phase transitions to give low temperature products which are also metastable but still single phase. The thermal behaviours associated with such metastable phases are indistinguishable from those given in Fig. 4, care is required, however, to check whether precipitation reactions do occur during thermal cycles or annealing at intermediate temperatures, which may lead to either DTA heat effects or conductivity changes independently of those associated with polymorphic changes. One characteristic of metastable phases is that they may revert to a more stable low temperature phase during the heating cycle of DTA; such transitions are characterised by an exotherm on heating, e.g. Fig. 2g.
Acknowledgments We thank Mr. Mat Department, UPM, for surement. C.K. Lee Penyelidikan Kemajuan financial support.
Kamal Margona, Chemistry assistance with DTA meais grateful to the Majlis Sains Negara, Malaysia for
86-88
(1996) 2X-239
239
References 111F. Abraham, J.C. Boivin, G. Mairesse and G. Nowogrocki, Solid State Ionics 40/41
(1990) 934.
121C.K. Lee, D.C. Sinclair and A.R. West, Solid State lonics 62 (1993) 193. [31 C.K. Lee and A.R. West, unpublished results. G. Mairesse and G. 141 F. Abraham, M.F. Debueuille-Gresse, Nowogrocki, Solid State Ionics 28-30 (1988) 529. PI E. Pemot, M. Anne, M. Bacmann, P. Strobel, J. Foulatier, R.N. Vannier, G. Mairesse, F. Abraham and G. Nowogrocki, Solid State Ionics 70/71 (1994) 59. M Ganne, R N Vannier. G [61 0. Joubert, A Jouanneaux, Mairesse, Solid State Ionics 73 (1994) 309. 171 R. Essalim, B. Tanouti, J.P. Bonnet and J.M. Reau, Mater. Lett. 13 ( 1992) 382. Solid State PI V. Sharma, A.K. Shukala and J. Gopalakrishnan. Ionics 58 (1992) 359. A. Manthiram, M. Paranthaman, Y.S. 191 J.B. Goodenough, Zhen, Mater. Sci. Eng. B 12 (1992) 357. Cl01 C.K. Lee, G.S. Lim and A.R. West, J. Mat. Chem. 4 (1994) 1441. [Ill C.K. Lee, B.H. Bay and A.R. West, J. Mat. Chem. 6 ( 1996) 331. [I21 C.K. Lee, M.P. Tan and A.R. West, J. Mat. Chem. 4 (1994) 525. [I31 I. Abraham, F. Krok and J.A.G. Nelstrop, 2nd Int. Mat. Chem. Conf., U.K., 1995.