Synthesis and characterization of layered bismuth vanadates
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T. N. Guru Row and C. N. R. Raoa) Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India (Received 26 April 1990; accepted 14 May 1990)
Bi2VO5.5 (Bi4V2On), which is the vanadium analog of the first member of the Aurivillius family of oxides of the general formula Bi2An iBnO3n+3, has been prepared and characterized. The vanadate has the expected layered structure and is ferroelectric with a Curie temperature of 720 K. While we have not been able to synthesize the vanadium analog of the n - 2 member of the Aurivillius family, we have examined the structure and properties of a vanadate of the composition Bi2V3O9.
I. INTRODUCTION
II. EXPERIMENTAL PROCEDURE
Layered bismuth oxides of the general formula Bi2An_!BnO3n+3 discovered by Aurivillius1 are well known.2 In this family of oxides, the A cations are generally Bi3+, Ba2+, Pb2+ etc., while the B cations are Ti4+, Nb5+, Fe3+ etc. Crystal structures of these oxides consist of Bi2O2 layers interleaved with ra-perovskite layers of the composition An-iBnO3n+i. Many of the Aurivillius family of oxides exhibit ferroelectricity with fairly high Curie temperatures. Thus, the n = 1 member, Bi2WO6, is a ferroelectric with Curie temperature of 1220 K. Although some of the well-known oxides of this family contain Ti and Nb in the B-site, studies of the vanadium analogs are limited to two preliminary reports in the literature.3'4 A study of the relative stabilities of bismuth-containing oxides of layered perovskite and pyrochlore structures3 has shown that d° cations (which favor the ferroelectric distortion of the BO6 octahedra) stabilize the layered structure of the Aurivillius family of oxides. On the other hand, partially filled d cations seem to favor the pyrochlore structure. If this is so, there is no reason why layered oxides containing V5+ (d°) should not be stable. In this paper, we describe the synthesis and characterization of the vanadium analog of the n = 1 member of the Aurivillius family of oxides, Bi2VO55 or Bi4V2On. We have studied the structure of this oxide by the Rietveld refinement of x-ray powder diffraction data. More interestingly, Bi2VO55 is ferroelectric with a high Curie temperature just like Bi2WO6. Efforts to prepare the vanadium analog of the n = 2 and three members of the Aurivillius family have not been successful. We have, however, carried out a study of the bismuth vanadate of the composition Bii.33V2O6 or Bi2V3O9, which is supposed to occur as a stable phase.3
Polycrystalline samples of Bi2VO5.5 (Bi 4 V 2 O n ) could be prepared by heating a stoichiometric mixture initially up to 770 K and then at 1070 K in air for 24 h, by repeating the grinding and heating operation twice. Samples of Bii.33V2O6 were prepared by heating an appropriate mixture of Bi2O3, V2O3, and V2O5 in a sealed, evacuated (~KT4 Torr) silica tube at 1070 K.3 The oxides were examined by x-ray powder diffraction (CuK a radiation, STOE powder diffraction system). Powder data were collected using the transmission mode at 45 kV, 30 mA. Th
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