Neutron powder diffraction study of a phase transition in La0.68(Ti0.95Al0.05)O3

Crystal structures and structural changes of the compound La-0.68(Ti0.95Al0.05)O-3 have been studied using neutron powder diffraction data and the Rietveld method in the temperature range from 25 degrees to 592 degrees C. The Rietveld profile-fitting analyses of the neutron data and the synchrotron diffraction profile revealed that the crystal symmetry of the low-temperature phase of La-0.68(Ti0.95Al0.05)O-3 is orthorhombic Cmmm (2a(p) x 2a(p) x 2a(p); p: pseudo-cubic perovskite). The unit-cell and structural parameters were successfully refined with the orthorhombic Cmmm for the intensity data measured at 25 degrees, 182 degrees, and 286 degrees C, and with the tetragonal P4/mmm (a(p) x a(p) x 2a(p)) for intensity data obtained at 388 degrees and 592 degrees C. The P4/mmm-to-Cmmm phase transition was found to be induced by tilting of the (TiAl)O-6 octahedron. The tilt angle decreased with increasing temperature, reaching 0 degrees at the Cmmm-P4/mmm transition temperature.