Neutron scattering studies of vibrational spectra and structural transformations in the superionic conductors CsHSO₄and CsHSeO₄

Abstract

The results of neutron diffraction (ND) and inelastic neutron scattering (INS) studies on phase transitions and vibrational spectra in powder samples of the superionic conductors CsHSO4 (CHS) and CsHSeO4 (CHSe) at the pulsed reactor IBR-2 are reported. Three different phases (III, II, I) in CHS and only two phases (II, I) in the partly deuterated crystal CsD0.7H0.3SO4 (CDS) are observed in the temperature range 123-420 K. The structural transformation of the ferroelastic type from monoclinic phase II to a possibly tetragonal phase I in CHS (CDS) at the superionic phase transition temperature Tc=414 K (412 K for CDS) is rather sharp compared with those in CHSe where the structural transformation starts far below Tc=400 K. Considerable changes in the INS spectra at the superionic phase are observed in all crystals: a librational mode of SO4 (or SeO4) groups in CHS (CHSe) becomes rather broad and the gamma (OH) proton bending mode softens and broadens. To explain the experimental results the authors propose a theoretical model for ferroelastic transformation at Tc induced by the Slater order-disorder-type superionic phase transition of protons on hydrogen bonds.