Phase transitions and dielectric properties in some compounds with bismuth oxide layer structure

Mat. Res. Bull., Vol. 27, pp. 123-128, 1992. Printed in the USA. 0025-5408/92 $5.00 + .00. Copyright © 1991 Pergamon Press pie.

PHASE TRANSITIONS AND DIELECTRIC PROPERTIES IN SOME COMPOUNDS WITH BISMUTH OXIDE LAYER STRUCTURE J.P. Mercudo, A. SouirU, M. Manier and B. Frit Laboratoire de Cdramiques Nouvelles, URA CNRS n°320 Facuitd des Sciences, 123, Avenue Albert Thomas 87060 LIMOGES CEDEX, FRANCE.

(Received June 17, 1991; Communicated b y P. Hagenmuller)

ABSTRACT Solid solutions dedving from terroelectdc Bi3TiNbO9 by cationic substitutions of La3+ for Bi3+ and (Ti4+,W 6+) for Nb5+ have been synthesized using a standard ceramic method. Evolutions of unit cell parameters and dielectric properties as a function of the substitution are given. All compounds show a ferroelectdc phase transition, the temperature of which decreases with increasing substitution rate. The relations between the crystal chemical data and the dielectric behavior are discussed. MATERIALS INDEX: bismuth, oxides, ceramics Introduction A number of layer bismuth compounds with general formula Bi2Am-lBmO3m+3 where A is a mono-, di- or trivalent element (or a combination of them) allowing dodecahedral coordination, B a transition element suited to octahedral coordination (e.g. FeII, Ti IV, NbV, Ta V, W VI) and m an integer usually lying in the range 1-8, are known to be ferroelectdc (1-4). The structure of these compounds can be regarded as a regular intergrowth of (Bi202) 2+ layers and (Am-1BmO3m+l)2" perovskite-type layers (5). Ceramic materials prepared from these oxides are characterized by (i) lower room temperature dielectric permittivity (~r = 100-200), (if) higher Curie temperatures, (iii) lower temperature coefficient of the resonant frequency ('q= 0-20.106K -1), (iv) smaller ageing rate and (v) larger anisotropy of the electromechanical factor with respect to the classical lead titanozirconate ceramics. They can be used, therefore, as piezoceramics in devices operating at high temperature and high frequency (6). However they are quite difficult to pole mainly because of their high coercive fields. Among these compounds, Bi3TiNbO9, Bi2PbNb209 (m=2) and mostly Bi4Ti3012 (m=3), have been thoroughly investigated from the point of view of both structural and dielectdc properties, either as pure phases, or as solid solutions obtained by simple or double substitution of homo- or heterovalent cations (2,7-10). The present work is concerned with the crystallochemical study and the dielectdc charactedzation of some solid solutions derived from Bi3TiNbO9 by two different types of substitutions : La for Bi and ('i3+W) for Nb.

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The polycrystalline compounds have been prepared by solid state reaction of the corresponding oxides. Stoichiometric mixtures of reagent grade Bi203, "1302, Nb205, WO3 and La203, were thoroughly mixed and calcined at 1123 K, for 15 hours, in alumina or platinum crucibles. After crushing, the powders were heated at higher temperatures chosen on head of the composition, in order to assure complete reaction. X-ray diffraction data were obtained with a Siemens D5000 powder diffractometer fitted with a graphite monochromator and using the CuKct radiation (~. = 0.15406 nm) and silicon (NBS grade, a = 0.54309 nm) as an internal standard. The cell parameters were refined by a least-squares method. Disk-shaped samples (-10 mm in diameter, -1-2 mm thick) were sintered at appropriate temperatures, polished and coated with a low temperature gold paste (DPC 523911,880K, 10 min), aged overnight at 380K and left for two days at room temperature before measurements. Low frequency dielectric measurements were carried out between room temperature and 1280K (at increasing and decreasing temperature), under weak a.c. fields, using a Wayne-Kerr B 905 automatic bridge. Results and discussion Two kinds of modifications have been investigated. They result from cationic substitutions either in the large cubooctahedral cavities or in the octahedral sites.

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FiG. 1 Bi3.xLaxTiNbO9 : Lattice parameters vs x

Armstrong and Newnham have already shown that the 12-coordinated site in the perovskite layer of these phases can be occupied by large cations like alkali, alkaline earth or large size lanthanides (e.g,. La to Gd)(7). On the contrary the (Bi202) ~+ layers cannot accomodate any substitution, excepted those involving cations with lone pairs, such as Pb2+. So the maximum substitution rate to be expected is one La atom per formula unit. Only cations with radii in the range 0.058 0.070 nm can occupy the octahedral sites. This range is somewhat narrower than that observed in the perovskites, probably because of the mismatch between the lateral dimensions of both (Bi202) 2+ and perovskite-like layers. As expected the maximum substitution rate for Bi3.xLaxTiNbO 9 corresponds to x = 1 .The evolutions of the lattice parameters, which logically result (rLa 3+ > rBi 3+) in a very slight increase of the unit cell volume (dV/V = 0.65%), are given in Fig. 1 as a function of x. In the whole substitution range, the a and b parameters increase (da/a = 0.3%, d b lb = 1%) while the c parameter decreases (d~'c = - 0.7%). Within the range of error, one can estimate that the symmetry changes from the orthorhombic to a tetragonal one when x reaches 0.4. Such an evolution can be clearly understood by considering the distortions induced in the ideal layers of the perovskite structure by the presence of the lone-pair Bi3+ cations within the cubooctahedral cavities (as shown in Fig. 2).

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Among the twelve equivalent Bi(1)-O distances of the close-packed coordination, only six are short enough to be considered (Table 1): five normal Bi(1)-O(1), Bi(1)-O(4) and Bi(1)-O(5) distances ranging between 0.250 and 0.258 nm, plus one very short distance Bi(1)-O(1) = 0.216 nm(11). Two kinds of atomic movements may result: * Atomic movements parallel to (001) (Fig. 2a) : The (Nb,'l'i) atoms are shifted along the polar axis a 0.042 nm away from the centre of the octahedra, and the octahedra rotate about c. It leads to a global decrease of the a and b parameters and to a slight orthorhombic distortion (b
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FIG. 2 Comparison of the distorted perovskite layers of the Bi3l]NbO9 structure with an ideal cubic layer (SrTiO3). Only haft of the structure is shown. The very short Bi(1) - O(1) distance and the fifth strong Bi(2) - 0(2) bond are indicated by dotted lines. Interatomic distances are in nm.

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Because of the very short Bi(1)-O(1) distance (i) the (Nb,'l])O 6 octahedra rotate around the polar axis a. This rotation is enhanced by the fifth strong bond (Bi(2)-O(2) ,. 0.243 nm) contracted by the Bi(2) atoms of the (Bi202) 2+ layers with one of the apex 0(2) atoms of the parovskite-like layers, in addition to the four Bi(2)-O(3) bonds of the ideal square pyramid configuration. (ii) the corresponding (Nb,Ti)-O(1) distance is increases strongly ((Nb,l'i)-O(1) = 0.231 nm). This results in a strong elongation of the octahedra along c (the mean (Nb,Ti)-O distance along cis 0.207 nm against 0.201 nm for the mean (Nb,Ti)-O distance along (001)) and, despite tilting around a, in an increasing thickness of the perovsklte-type layer, i.e. of the c parameter. In conclusion, the distortions caused by the asymmetric Bi3+ cations result in an increase of the c parameter and a decrease of both a and b parameters along with an orthorhombic distortion. The replacement of the asymmetric Bi3+ cations by symmetric La 3+ ions should logically cancel the distortions and, therefore, should result in an opposite evolution. It is actually observed.

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The temperature dependence of the dielectric constants of the solid solutions is given in Fig. 4 and 5. For both matedais the chemical modifications not only lower the temperature of the ferroelectric transition but also drastically reduce the value of the dielectric constant in the vicinity of this transition.

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For the Bi-La substitution, the transition becomes more and more diffuse as x increases : for x > 0.8, it has almost disappeared (Fig. 4). This result is surely related to the change of symmetry, the evolution of the lattice parameters shown in Fig. 2 indicating that the structure is nearly tetragonal when x > 0.4.

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Fig. 3 gives the evolutions of the lattice parameters of the Bi3Til+x/2Nbl-xWx/209 solid solutions as a function of x. The central cation remains unchanged, the substitution involving only the octahedral sites . The observed variations are closely related to the average ionic radius which changes (as Nb 5+ is replaced by Ti4+ and W 6+) according to the relation I Nb5+ = 0.5 Ti4+ + 0.5 W6+(12). As x increases from 0 to 1, the mean ionic radius decreases by 0.0024 nm and the lattice parameters a and b by 0.0035 nm each, as they should behave in a simple perovskite structure(7).

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It is in agreement with previous data obtained e.g. for Bi4_xLaxTi3012 (10,13). As for other modified bismuth titanates (3,10,13) (Tc (K) = 948 - 375 x for Bi4_xLax'ri3012 and Tc (K) = 948 - 425 x for Bi4.xPbxTi3-xNbxO12) the transition temperature T c decreases almost linearly, but the evolution, for the same substitution range is stronger : Tc (K) = 1203 610 x .

Previous studies have shown that the prevailing factor leading to the decrease of Tc is rather the bismuth content in the cubooctahedral cavities than the nature of the substituting cation (e.g. La3+, Sr2+ or pb2+)(3,9,10,13). Our present results confirm these observations. On the contrary, in the case of the octahedral modification, the maximum of the dielectric constant remains sharp within the whole substitution range and the decrease of the transition temperature Tc,

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TABLE 1 Interatornic Distances (in nm) in Bi3"13NbO9 (after (11)) Bi(1)-0(1) Bi(1)-0(4) Bi(1)-0(5) Bi(2) - 0(2) Bi(2) - 0(3) Bi(2) - 0(4) Bi(2) - 0(5) (Nb,'li) - 0(1) (Nb,'13) - 0(2) (Nb,Ti) - 0(4) (Nb,'ri) - 0(5)

0.216 0.250 0,258 0.243 0,220 0,371 0.323 0.231 0.183 0.173 0.177

0.254 0.250 0,258 0.268 0.228 0.378 0.378

0.296 0.272 0,312 0,314 0.230

0,325 0.272 0,312 0.339 0.243

0,365

0.220 0.225

although linear, is not so important (Tc (K) = 1203 - 180 x). This emphasized the leading influence of the Bi 3+ cations on the dielectric properties of this kind of phases.

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Conclusions This study has shown that the ferroelectdc properties of solid solutions deriving from Bi3TiNbO9 are strongly dependent on both the nature and the rate of the substituting elements. A similar work is now in progress concerning some higher members of these layered compounds, especially those

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containing two kinds of perovskite layers, e.g. solid solutions derivated from Bi7Ti4Nb021 in which alternate stacking of m=2 and m=3 perovskite layers is present. References 1. G.A. Srnolenskii, V.A. Isupov and A.I. Agranovskaya, Fiz. Tverd. Tela, 1, 169 (1959). 2. E.C. Subbarao, Phys. Rev., 122(3), 804 51961). 3. E.C. Subbarao, J. Phys. Chem. Solids, 23, 665 (1962). 4. R.E. Newnham, R.W. Wolfe and J.F. Dordan, Mat. Res. Bull., 6, 1029 (1971). 5. B. Audvillius, Ark. Kemi, 1, 463 (1949); 1. 499 (1949); 2, 519 (1950). 6. S. Ikegami and I. Ueda, Jpn. J. Appl. Phys., .1~ 1572 (1974). 7. R.A. Armstrong and R.E. Newnham, Mat. Res. Bull., Z, 1025 (1972). 8. R.W. Wolfe and R.E. Newnham, J. Electrochem. Soc., 116. 832 (1969). 9. L.A. Shebanov and L.V. Korzunova, Mat. Res. Bull., 20, 781 (1985). 10. J.P. Mercurio and B. Frit, Silicates Ind., 9-10. 143 (1989). 11. R.W. Wolfe, R.E. Newnham, D.K. Smith and M.I. Kay, Ferroelectdcs, 3. 1 (1971). 12. R.D. Shannon and C.T. Prewitt, Acta Cryst.,B25.925 (1969). 13. M. Shimazu, J. Tanaka, K. Muramutsu and M. Tsukloka, J. Solid State Chem., 35, 402 (1980).