High-pressure structural evolution of a perovskite solid solution (La1−x,Ndx)GaO3

The structural evolution with pressure of six perovskites in the system La1−xNdxGaO3 with x=0.00, 0.06, 0.12, 0.20, 0.62 and 1.00 have been determined by single-crystal diffraction. At room pressure, all six samples have Pbnm symmetry. The room-pressure bulk moduli vary only slightly with composition, between K0T=169(4) and 177(2) GPa, with K0′=(dK/dP)P=0∼6.5. As pressure is increased there is significant compression of the octahedral Ga–O bonds, the tilts of the GaO6 octahedra decrease and the structures evolve towards higher symmetry. At room conditions the average Ga–O bond length increases with increasing compositional parameter x. However, the GaO6 become stiffer with increasing x; the Ga–O bonds thus become stiffer as they become longer. Bond strengths in the octahedra in perovskites are therefore not a simple function of bond lengths but depend also upon the extra-framework cation.Phase transitions to R-3c symmetry occur at 2.2 GPa in end-member LaGaO3, at ∼5.5 GPa in the x=0.06 sample, at ∼7.8 GPa for x=0.12, and at ∼12 GPa for x=0.20. No evidence of the transition in the x=0.62 or 1.00 samples was found by X-ray diffraction to 9.4 or 8.0 GPa, respectively, or by Raman measurements of NdGaO3 up to 16 GPa. The transition pressure therefore increases with increasing Nd content (increasing x) at approximately 0.45 GPa per 0.01 increment in x, at least up to x=0.20. Compression of the R-3c phase of LaGaO3 above the transition results in no significant changes in the tilt angle of the octahedra. The structural behavior of all six samples at high pressures is the result of the GaO6 octahedra being softer than the extra-framework (La, Nd)O12 site. The results therefore demonstrate that the evolution of solid-solution perovskites at high pressures follow the same general principles recently elucidated for end-member compositions.

High-pressure structure determinations of six perovskites on the La1−xNdxGaO3 join show that at La-rich compositions the GaO6 octahedra have much lower bulk moduli, as shown in the figure. This softening of the octahedra drives the high-pressure phase transition from Pbnm to R-3c structures.Figure optionsDownload as PowerPoint slide