Structural behavior and Raman spectra of perovskite-like solid solutions (1-x)LaMg0.5Ti0.5O3-xLa2/3TiO3

Structural behavior and phase transitions in the A-cation deficient perovskite-like solid solutions (1 - x)LaMg0.5Ti0.5O3-xLa2/3TiO3 (0 <= X <= 0.5) were studied to reveal crystal-chemical factors associated with the marked variation of dielectric properties with x-value. Structures and phase transisitons were characterized using variable-temperature transmission electron microscopy (TEM) and X-ray powder diffraction combined with ambient temperature neutron powder diffraction measurements. The solid solutions feature a series of octahedral-tilting phase transitions and a B-site Mg/Ti order-disorder phase transition; the temperatures of these transitions decreased with increasing x. Additionally, short-range ordering of A-site vacancies was detected for x > 0.40 using high-resolution TEM. The reported compositional dependence of the dielectric response was found to correlate with changes in octahedral tilting, B-site ordering, and short-range ordering of A-site vacancies. A phase diagram summarizing ordering and displacive phase transitions was determined from variable-temperature diffraction and imaging experiments. Among the tilting transitions, a relatively uncommon first-order phase transition between structures exhibiting orthorhombic (a(+)b(-)b(-) and a(0)b(-)c(-)) and rhombohedral (a(-)a(-)a(-)) tilt systems was observed; the rhombohedral tilt was stabilized at room temperature for x = 0.50. Raman spectra of the solid solutions revealed previously unreported anomalous asymmetric broadening of certain peaks associated with B-site ordering. This broadening, which occurred even for the samples exhibiting significant long-range Mg/Ti order and large anti-phase ordered domains, was attributed to a decreasing phonon correlation length caused by strong heterogeneity of the B-site order parameter within each anti-phase domain.