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Anomalous thermal expansion in rare-earth gallium perovskites: a comprehensive powder diffraction study
A Senyshyn1, D M Trots, J M Engel
1Institute for Materials Science, Darmstadt University of Technology, D-64287 Darmstadt, Germany. Lviv Polytechnic National University, 12 Bandera Street, 79013 Lviv, Ukraine.
Abstract:
Crystal structures of rare-earth gallium perovskites LaGaO(3), PrGaO(3), NdGaO(3) and Pr(1-x)Nd(x)GaO(3) (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 PrGaO(3) lattice has been found to be persistent in Pr(1-x)Nd(x)GaO(3) 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 NdGaO(3), and its origin is supposed to be the same as in PrGaO(3), 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 LnO(12) (Ln-rare-earth) and GaO(6) coordination polyhedra, which can be a structural manifestation of anomalous thermal expansion in the considered compounds.
