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Anomalous thermal expansion in rare-earth gallium perovskites: a comprehensive powder diffraction study

Crystal structures of rare-earth gallium perovskites LaGaO3, PrGaO3, NdGaO3 and Pr1−xNdxGaO3 (x = 0.25, 0.50, 0.75) solid solutions were investigated in the temperature range 12–300 K by high-resolution powder diffraction using synchrotron or neutron radiation. The previously reported negative thermal expansion in the b direction of the PrGaO3 lattice has been found to be persistent in Pr1−xNdxGaO3 solid solutions and its magnitude has been revealed as proportional to the amount of praseodymium. Evaluation of the obtained temperature evolution of cell dimensions indicated a weak anomalous behaviour of the b lattice parameter in NdGaO3, and its origin is supposed to be the same as in PrGaO3, i.e. a coupling of the crystal electric field levels with phonon excitations of about 23–25 meV energy. The performed bond length analysis revealed an anomalous behaviour of both LnO12 (Ln—rare-earth) and GaO6 coordination polyhedra, which can be a structural manifestation of anomalous thermal expansion in the considered compounds.