High-pressure structural evolution of a perovskite solid solution (La{sub 1-x},Nd{sub x})GaO{sub 3}

• Published in: Journal of solid state chemistry Vol. 180; no. 12
• Main Authors: Angel, R.J., Zhao, J., Ross, N.L., Jakeways, C.V., Redfern, S.A.T., Berkowski, M.
• Format: Journal Article
• Language: English
• Published: United States 15-12-2007
• Subjects: BOND LENGTHS; COMPRESSION; GALLIUM COMPOUNDS; INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; LANTHANUM COMPOUNDS; MONOCRYSTALS; NEODYMIUM COMPOUNDS; ORTHORHOMBIC LATTICES; OXIDES; PEROVSKITE; PHASE TRANSFORMATIONS; PRESSURE RANGE GIGA PA; SOLID SOLUTIONS; SYMMETRY; X-RAY DIFFRACTION
• ISSN: 0022-4596; 1095-726X
• DOI: 10.1016/j.jssc.2007.09.019

The structural evolution with pressure of six perovskites in the system La{sub 1-x}Nd{sub x}GaO{sub 3} 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{sub 0T}=169(4) and 177(2) GPa, with K{sub 0}{sup '}=(dK/dP){sub P=0}{approx}6.5. As pressure is increased there is significant compression of the octahedral Ga-O bonds, the tilts of the GaO{sub 6} 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 GaO{sub 6} 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 LaGaO{sub 3}, at {approx}5.5 GPa in the x=0.06 sample, at {approx}7.8 GPa for x=0.12, and at {approx}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 NdGaO{sub 3} 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 LaGaO{sub 3} 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 GaO{sub 6} octahedra being softer than the extra-framework (La, Nd)O{sub 12} 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. - Graphical abstract: High-pressure structure determinations of six perovskites on the La{sub 1-x}Nd{sub x}GaO{sub 3} join show that at La-rich compositions the GaO{sub 6} 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.