High-pressure structural evolution of a perovskite solid solution (La_1-x,Nd_x)GaO₃

The structural evolution with pressure of six perovskites in the system La_1-xNd_xGaO₃ 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 K_0T=169(4) and 177(2) GPa, with K₀^′ = (d K / d P)_{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.