Perovskite-type rare-earth gallates attracted much attention because of their interesting physical qualities. Most important properties of RGaO3 compounds are used in substrate materials, either for high-temperature superconductors (HTSC), for doped manganese perovskite LnMnO3 films with colossal magnetoresistance (CMR) and for GaN films. Recently it was proposed to use rare-earth gallates as superionic conductors. Among such rare-earth perovskite-type gallates, PrGaO3 is outstanding. Our recent studies of the PrGaO3 structure using high-resolution powder diffraction techniques and synchrotron radiation revealed negative thermal expansion in the temperature range 12 - 80 K. Nevertheless, no negative expansion could be seen in isostructural LaGaO3, CeGaO3 and NdGaO3.
Considering the structure of RGaO3 (R=La-Nd) and its solid solutions R1-xRxGaO3 allows us to conclude that many of such compositions are promising candidates for negative expansion at low temperatures. This is because a significant contribution of the overall thermal expansion is caused by rotations of the GaO6-octahedra. There is, however, a subtle balance between the rotations and distortions of the polyhedra. The negative thermal expansion of PrGaO3 seems to be related to the violation of this balance caused by interactions between octahedral rotational modes and Pr-ion excitations.
Negative thermal expansion has also been observed in Pr-containing solid solutions (La0.19Pr0.81GaO3, La0.4Pr0.6GaO3) [Aleksiyko et al]. Hence, we performed semiclassical simulations of the thermal expansion of PrGaO3 using the GULP code based on pairwise interactions. We did, however, not observed traces of negative expansion, which indicates possible electronic effects involved in the nature of such negative thermal expansion of Pr-containing rare-earth gallates.
This work was supported by WTZ (UKR-012-97),UMS (M/85-2003) and PCSR (N7 T08A 00520).