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| - Structural Behavior and Raman Spectra of Perovskite-Like SolidSolutions (1 − x)LaMg0.5Ti0.5O3-xLa2/3TiO3
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| - 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 associatedwith the marked variation of dielectric properties with x-value. Structures and phase transisitons werecharacterized using variable-temperature transmission electron microscopy (TEM) and X-ray powderdiffraction combined with ambient temperature neutron powder diffraction measurements. The solidsolutions feature a series of octahedral-tilting phase transitions and a B-site Mg/Ti order−disorder phasetransition; the temperatures of these transitions decreased with increasing x. Additionally, short-rangeordering of A-site vacancies was detected for x> 0.40 using high-resolution TEM. The reportedcompositional dependence of the dielectric response was found to correlate with changes in octahedraltilting, B-site ordering, and short-range ordering of A-site vacancies. A phase diagram summarizingordering and displacive phase transitions was determined from variable-temperature diffraction and imagingexperiments. Among the tilting transitions, a relatively uncommon first-order phase transition betweenstructures exhibiting orthorhombic (a+b-b- and a0b-c-) and rhombohedral (a-a-a-) tilt systems wasobserved; the rhombohedral tilt was stabilized at room temperature for x = 0.50. Raman spectra of thesolid solutions revealed previously unreported anomalous asymmetric broadening of certain peaksassociated with B-site ordering. This broadening, which occurred even for the samples exhibiting significantlong-range Mg/Ti order and large anti-phase ordered domains, was attributed to a decreasing phononcorrelation length caused by strong heterogeneity of the B-site order parameter within each anti-phasedomain.
- (1 − x)LaMg0.5Ti0.5O3-xLa2/3TiO3 perovskite-like solid solutions exhibit a series of octahedral-tilting phase transitions (including uncommon a+b-b- → a-a-a) and a B-site order-disorder phase transition. Short-range ordering of A-site vacancies (VLa) occurs for x > 0.40 as revealed by the high-resolution transmission electron microscopy and image simulations; this ordering generates weak diffuse reflections in the electron diffraction patterns.
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