Synthesis, Structure, Transport and Magnetic Properties of the Perovskite-Related Oxides.

A series of perovskite-related rm R_{1-x}A_{x}TiO_3 (R = rare-earth; A = alkaline-earth), rm LaTi_{1-x}V_{x}O _3 and rm (Ba_{1 -x}Sr_{x})_2(Sr_{0.67 }Bi_{0.33})(Pb_{1-y} Bi_{y})O_{6-delta} compounds have been prepared. The substitution of Sr and Ba into RTiO_3 results in the structural phase transitions from orthorhombic (Pbnm) -to-cubic (Pm3m). The Ca substituted phases remain orthorhombic throughout the range studied. The substitutional ranges of the solid solutions are dependent upon the size mismatch between R^{3+} and A ^{2+} ions. The range of miscibility between RTiO_3 and BaTiO _3 decreases from La to Er. Single phase rm Y_{x}Ba_{1 -x}TiO_3 compounds can be obtained by introducing oxygen vacancies (0 <= x <= 0.4; 0<=delta<= 0.15). The orthorhombic to cubic phase transitions in the rm La_{1-x}Ba_ {x}TiO_3 phases appear to be first order. High resolution electron microscopy and electron diffraction studies showed no evidence for A-site ordering, microdomain ordering, or superstructure formation in the cubic regions of the rm R_{1 -x}A_{x}TiO_3 series. The insulating rm R_{1-x}A_ {x}TiO_3 phases order as canted antiferromagnets. The disruption of this magnetic ordering results in metal-insulator transitions and large Pauli susceptibilities. It has been shown that the perovskite tolerance factor is a useful parameter to sort insulating and metallic phases in the rm R_{1 -x}A_{x}TiO_3 compounds. The ionic model using the Ti-O bond distance and the Ti-O-Ti angle has been applied to determine the interrelationships between structure and properties in these compounds. Single phase rm LaTi_{1-x}V_ {x}O_3 have been obtained throughout the range 0<= x <= 1. The rm LaTi_{1-x}V _{x}O_3 series are the unique examples of B-site substituted compounds exhibiting insulator -to-metal transitions. The structural chemistry of the rm (Ba_{1-x}Sr_{x })_2(Sr_{0.67}Bi_{0.33 })(Pb_{l-y}B_{ y})O _{6-delta} has been studied by powder X-ray and neutron diffraction techniques. These phases adopt the cubic rm (NH_4)_3FeF _6 type structure (space group Fm3m) with Sr^{2+} ions occupying the B sites. Thermogravimetric analysis and neutron refinements show oxygen vacancies and a large anisotropy associated with the O atoms. The anisotropy is discussed in terms of a vacancy model. Superconductivity was not observed in any of these samples.