A high-resolution neutron powder diffraction study of the low-temperature structural phase transitions in RbCaF3 perovskite

The temperature dependence of the crystal structure and thermoelastic properties of RbCaF3 perovskite are reported from a high-resolution neutron powder diffraction investigation in the temperature interval 2-300 K. Two phase transitions have been identified from these data, the first from space group Pm & Unknown;3m to I4/mcm, at 200 K, has been well characterized in earlier studies, the second, from space group to I4/mcm to Pbnm has previously been less well understood. New results from the former transition show the order parameter evolves with a critical exponent of 0.284(5), and the phase is characterized by an Einstein temperature of 192(8) K, and a Debye temperature of 270(10) K. The temperature variation of the tetragonal spontaneous strain in this phase is consistent with an additional fourth order invariant in the Landau potential. The second phase transition is found to be strongly first order and a temperature dependent, and heating rate/cooling rate dependent phase coexistence is found at all temperatures below similar to 80 K on heating, and similar to 65 K on cooling. The evolution of the orthorhombic unit cell is atypical in the region of the phase transition, with a and b exhibiting strong negative linear thermal expansion, whilst the c axis behaves in the expected manner. The crystal structure of the Pbnm phase at 2 K is reported from an unconstrained Rietveld refinement for the first time.