Ferroelastic domains, Raman scattering, XRD and DSC studies of [(NH4)(0.14)Rb-0.86](3)H(SO4)(2) and [(NH4)(0.1)Rb-0.9](3)H(SO4)(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) - 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 R3m. 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)(0.1) Rb-0.9](3)H(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. (c) 2007 Elsevier B.V. All rights reserved.