METAL-INSULATOR TRANSITIONS IN LA1-XTIO3, ESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-0.33. STRUCTURE-PROPERTY CORRELATIONS

The solid-solution system La1-xTiO3 was investigated over the range 0 less-than-or-equal-to x less-than-or-equal-to 0.33, i.e., involving en d members LaTiO3 and La2/3TiO3. X-ray single-crystal and powder neutro n diffraction studies have established the structure of La2/3TiO3 for the first time which is described in Pban with a = 5.461(1) angstrom, b = 5.482(1) angstrom, c = 7.759(3) angstrom. La vacancies are ordered largely in the z = 1/2 plane and Ti atoms are displaced from the cent ers of Ti-O octahedra toward this plane giving rise to c-axis chains c onsisting of two short Ti-O bonds (1.906 angstrom) alternating with tw o long Ti-O bonds (1.975 angstrom). The Pban structure is found for 0. 25 < x less-than-or-equal-to 0.33, an Ibmm structure for 0.20 < x less -than-or-equal-to 0.25 and the Pbnm (GdFeO3 type) structure for 0 less -than-or-equal-to x < 0.20. The compounds La2/3TiO3 (x = 0.33) and La0 .70TiO3 (x = 0.30) are insulators, La0.75TiO3 (x = 0.25) undergoes a m etal-insulator transition and La0.80TiO3 (x = 0.20) to La0.92TiO3 (x = 0.08) are metallic. Electrical, magnetic and structural properties ar e compared with those of the closely related La1-xSrxTiO3 and SrTiO3-x systems. It is also argued that oxidized forms of LaTiO3, described p reviously LaTiO3+x, are better understood as La1-xTiO3.