Thermal behaviour of the NdGaO₃ crystal structure has been precisely investigated by means of the high-resolution powder diffraction technique using synchrotron radiation in the wide temperature range 12-1223 K. The diffraction experiments were carried out at the powder diffractometer at B2 line (HASYLAB/DESY). It was shown that NdGaO₃ possesses orthorhombically distorted GdFeO₃ type of structure (sp. group Pbnm, Z=4) in the whole temperature range investigated. Temperature variations of the lattice parameters and cell volume were found to be anisotropically and nonlinearly increasing with the temperature.
Thermal expansion was calculated by two independent theoretical methods-quasiharmonic lattice dynamics using GULP software and Debye model. Deviations between calculated and observed thermal expansion at low temperatures (50-200 K) [1] can be explained by the partially non-applicability of the Debye approximation for perovskites and by the influence of phonon-phonon interaction. Observed deviations at high temperatures might be associated with the increasing of anharmonicity contribution and with the possible processes of the vacancies formation.
[1] D. Savytskii, L. Vasylechko, A. Senyshyn et al., Low-temperature structural and Raman studies on rare-earth gallates, Physical Review B (accepted for publication)
Acknowledgement. This work was supported by WTZ (UKR-012-97), Ukrainian Ministry of Science (Project "Ion"). We would like to thank Dr. A. Oganov for assistance with the analysis of the data.

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