Neutron scattering studies of vibrational spectra and structural transformations in superionic conductors CsHSO4 and CsHSeO4

The results of neutron diffraction (ND) and inelastic neutron scattering (INS) studies on phase transitions and vibrational spectra in powder samples of superionic conductors CsHSO4 (CHS) and CsHSO4 (CHSe) at pulsed reactor IBR-2 are reported. Three different phases (III,II,I) in CHS and only two phases (II,I) in partly deuterated crystal CsDsub(0.7)Hsub(0.3)SO4(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 Tsub(c) = 414 K (412 K for CDS) is rather sharp as compared to those in CHSe where the structural transformation starts far below Tsub(c) = 400 K. Considerable changes in 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 γ(OH)-proton bending mode softens and broadens. To explain the experimental results, a theoretcal model for ferroelastic transformation at Tsub(c) induced by the Slater order-disorder type superionic phase transition of protons on hydrogen bonds is proposed