Layered Perovskite-like Oxides Research Articles

Structure and Dielectric Properties of Bi4Pb1.5Ti4.5O16.5 and Bi5Ca0.5GaTi3.5O16.5

New polycrystalline Bi-containing layered perovskite-like oxides (Aurivillius phases) Bi4Pb1.5Ti4.5O16.5 and Bi5Ca0.5GaTi3.5O16.5 were synthesized, and their lattice parameters were determined by x-ray powder diffraction. Both compounds have orthorhombic structures (sp. gr. Cmm2 = D 2v 11 , no. 35), with a = 5.4434(1) A, b = 5.4506(1) A, and c = 45.527(2) A in Bi4Pb1.5Ti4.5O16.5 and a = 5.3868(7) A, b = 5.4185(8) A, and c = 45.267(8) A in Bi5Ca0.5GaTi3.5O16.5. Their dielectric properties were studied between room temperature and 1200 K at frequencies from 1 to 50 kHz. Both compounds were shown to be ferroelectrics. Their Curie temperatures are 805 and 844 K, respectively.

Synthesis and structure of new phases of the A m −1Bi2 B m O3m+3 (m = 2) type

Three polycrystalline bismuth-containing layered perovskite-like oxides are synthesized by high-temperature solid-state reactions. One of these compounds was described previously, namely, Bi3Ti1.5W0.5O9, for which the unit cell parameters a = 5.372(5) A, b = 5.404(4) A, and c = 24.95(2) A are determined in this study. The other two compounds, namely, Na0.75Bi2.25Nb1.5W0.5O9 with the unit cell parameters a = 5.463(1) A, b = 5.490(7) A, and c = 24.78(0) A and Ca0.5Bi2.5Ti0.5Nb1.5O9 with the unit cell parameters a = b = 3.843(2) A and c = 24.97(6) A, are synthesized for the first time. The compositions of these compounds are based on the composition of the well-known compound Bi3TiNbO9 with a high Curie temperature (T C = 1223 K), in which bismuth, niobium, and titanium atoms are partially or completely replaced by other atoms. The experimental and calculated interplanar distances determined from the X-ray diffraction patterns of the studied compounds are presented.

Synthesis and structure of new A m − 1Bi2B m O3m + 3 (m = 3) phases

Six new polycrystalline bismuth-containing layered perovskite-like oxides described by the general formula A m − 1Bi2B m O3m + 3 (m = 3) were synthesized by high-temperature solid-state reactions. The unit-cell parameters of these compounds were determined as a = 5.438(9) A, b = 5.462(7) A, and c = 33.08(6) A for Bi4Ti2Nb0.5Fe0.5O12; a = 5.408(6) A, b = 5.425(9) A, and c = 32.85(6) A for Ca0.5Bi3.5Ti2.75W0.25O12; a = 5.457(0) A, b = 5.482(2) A, and c = 32.75(2) A for NaCaBi2Nb3O12; a = b = 3.826(2) A and c = 32.82(6) A for Ca0.25Th0.25Bi3.5Ti3O12; a = b = 3.816(2) A and c = 32.81(5) A for Nd0.5Gd0.5Bi3Ti3O12; and a = b = 3.826(4) A and c = 32.95(8) A for Th0.5Bi3.5Ti2.5Ga0.5O12. The compositions of these compounds are based on the composition of the basic Bi-containing layered perovskite-like Bi4Ti3O12 oxide in which BiIII and TiIV atoms are partially or completely replaced with other atoms. The experimental and calculated interplanar spacings determined from X-ray diffraction patterns and the reliability factors of their indexing are reported.

Morphotropic series of Bi-containing layered perovskite-like oxides and principles of their systematics

The well-known bismuth-containing layered perovskite-like oxides (BLPO) of composition Am − 1Bi2BmO3m + 3 are divided into morphotropic series. In these series, the a, b, and c parameters of the orthorhombic unit cells and the Curie temperature (Tc) change monotonically with increasing unstrained interatomic A-O bond lengths, when A are s elements. For compounds in which A are p elements (PbII or BiIII), the a and b unit-cell parameters and Tc, unlike the c unit-cell parameter, deviate substantially from these correlations, with the deviations decreasing with an increase of the m parameter. These facts must be taken into consideration in the search for new BLPO, whose compositions are predicted with the use of the systematics.

On problems of search for novel Bi-containing layered perovskite-like high-Tc ferroelectrics

A crystallochemical analysis of structural parameters of Bi-containing layered perovskite-like oxides (BLPO) of the type Am-1Bi2BmO3m + 3 is given. The morphotropic series, the technique of calculations of unit cell parameters and existence conditions are considered for the first time.