STRUCTURAL, MAGNETIC, AND TRANSPORT-PROPERTIES OF RXBA1-XTIO3-DELTA SOLID-SOLUTIONS, WHERE R = LA, ND, GD, ER, AND Y - RARE-EARTH-DEPENDENT METAL-TO-SEMICONDUCTOR TRANSITIONS

Compositionally homogeneous solid solutions of formula RxBa1-xTiO3-delta (where R = La, Nd; 0 less-than-or-equal-to x less-than-or-equal-to 1) have been prepared by the dc arc-melting method. The La system showed a negative deviation from Vegard's Law due to BaO deficiencies, whereas the members of the Nd series were stoichiometric and showed a linear relationship between x and the lattice constant a. Limited substitution of the smaller rare earths Gd and Er were also achieved at low values of x (for Gd, 0 less-than-or-equal-to x less-than-or-equal-to 0.4; Er, 0 less-than-or-equal-to x less-than-or-equal-to 0.3) with virtually no substitution at high values of x. Single-phase oxygen deficient cubic YxBa1-xTiO3-delta phases were obtained (where 0 less-than-or-equal-to x less-than-or-equal-to 0.4 and 0 < delta less-than-or-equal-to 0.15). Two compositionally dependent phase transitions occur in the La and Nd systems: tetragonal (P4mm)-to-cubic (Pm3m) at very low values of x (<0.05) which is second order and cubic (Pm3m)-to-orthorhombic (Pbnm) at high values of x which appears to be first order. HREM and electron diffraction studies showed no evidence for A-site ordering, microdomain ordering, or superstructure formation in the cubic regions. The single-phase samples disproportionate into BaTiO3 and R2Ti2O7 phases upon heating to 1000-degrees-C in air with the exception of La0.33Ba0.67TiO3, Which remains cubic upon oxidation with only a 0.8% increase in cell volume. Four-probe resistivity measurements showed metallic conductivities for a majority of the samples between R300 and 4 K with R300 almost-equal-to 10(-2)-10(-4) OMEGA-cm. Metallic conductivity correlates with the perovskite tolerance factor [t = (r(A) + r(O))/square-root 2(r(B) + r(O))] and occurs when 0.93 < t < 1.02 with semiconducting or insulating behavior occurring outside this range. Dc susceptibility studies showed large Pauli susceptibilities associated with the metallic samples with a correlation between the magnitude of the susceptibility and the number of d electrons. Superconductivity was not observed above 4 K in any of the samples. The effective magnetic moments of the Nd3+ ions in the NdxBa1-xTiO3 phases (ca. 3.3 mu(B)) for x less-than-or-equal-to 0.9 were close to the free ion value (3.62 mu(B)). Electron localization is discussed in terms of structural parameters and electronic effects.