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

Ferroelastic domains, Raman scattering, XRD and DSC studies of [(NH4)0.14Rb0.86]3H(SO4)2 and [(NH4)0.1Rb0.9]3H(SO4)2 mixed crystals are presented. Two endothermic peaks were found for both crystals in DSC heating runs at TS and T′S. A difference T′S−TS decreases with increasing Rb content. It was confirmed from X-ray diffraction studies that at TS the crystal structure changes from C2/c to R3 ̅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 [(NH4)0.1Rb0.9]3H(SO4)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.