Structural, Magnetic, and Transport Properties of R_xBa_1-xTiO_3-δ Solid Solutions, Where R = La, Nd, Gd, Er, and Y: Rare-Earth-Dependent Metal-to-Semiconductor Transitions

Compositionally homogeneous solid solutions of formula R_xBa_1-xTiO_3-δ (where R = La, Nd; 0 ≤ x ≤ 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 ≤ x ≤ 0.4; Er, 0 ≤ x ≤ 0.3) with virtually no substitution at high values of x. Single-phase oxygen deficient cubic Y_xBa_1-xTiO_3-δ phases were obtained (where 0 ≤ x ≤ 0.4 and 0 ≤ δ ≤ 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 °C in air with the exception of La_0.33Ba_0.67TiO₃, 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 ≈ 10^-2-10^-4 Ω ∙ cm. Metallic conductivity correlates with the perovskite tolerance factor [t = (r_A + r_O)√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 Nd_xBa_1-xTiO₃ phases (ca. 3.3 µB) for x ≤ 0.9 were close to the free ion value (3.62 µ_B). Electron localization is discussed in terms of structural parameters and electronic effects.