Raman scattering, DSC, XRD and ferroelastic domain studies at superionic phase transitions in [(NH4)1−xRbx]3H(SO4)2 for x > 0.8

Abstract Ferroelastic domains, Raman scattering, XRD and DSC studies of [(NH 4 ) 0.14 Rb 0.86 ] 3 H(SO 4 ) 2 and [(NH 4 ) 0.1 Rb 0.9 ] 3 H(SO 4 ) 2 mixed crystals are presented. Two endothermic peaks were found for both crystals in DSC heating runs at T S and T ′ S . A difference T ′ S − T S decreases with increasing Rb content. It was confirmed from X-ray diffraction studies that at T S the crystal structure changes from C 2/ c to R 3 ¯ m . At T ′ S the crystals undergo a successive structural transition to the phase which could be characterized by three dimensional system of virtual hydrogen bonds and a new type of virtual dimers. Our provisional XRD studies showed that the crystal structure at T > T ′ S may belongs to orthorhombic system. In Raman studies it was found that at T ′ S the Raman spectra of [(NH 4 ) 0.1 Rb 0.9 ] 3 H(SO 4 ) 2 undergo conspicuous changes. All our studies revealed that the crystals become polycrystalline at T ′ S , and temperature lowering results in a huge thermal hysteresis. It is presumed that structural relaxation exists in the crystals.