The structural phase transitions in LiK0.50Rb0.50SO4 mixed crystal

The structural phase transitions of the LiK0.50Rb0.50SO4 mixed crystal were investigated by Raman spectroscopy and neutron powder diffraction up to 80 0 K. The results show two first order structural transitions, quite analogo us to the transitions in LiKSO4. Raman spectra and diffraction profiles wer e compatible with average symmetries C-3v(4), C-6(6) and D-2h(16) for the t hree observed phases, with increasing temperature. The transitions, both re constructive, are smeared over large temperature intervals, leading to the coexistence of Raman bands and neutron peaks of the different phases and to an anomalous behavior of the lattice parameters into these intervals. In t he hexagonal phase, the cell Volume presents a hysteretic behavior dependin g on the sample thermal history. Raman spectra of the orthorhombic phase sh ows the high rotational disorder of the SO4 ions, associated with the high Li positional disorder in this high conducting phase.