NH 4 ) 1Àx Rb x ] 3 H(SO 4 ) 2 XRD Raman scattering Phase transitions Ferroelastic domains a b s t r a c t Detailed temperature XRD and Raman studies were performed at successive low temperature phase transitions to determine the role of cation ions at low temperature phase transitions of [(NH 4 ) 1Àx Rb x ] 3 H(SO 4 ) 2 mixed crystals. Mixed crystals with low concentration of Rb (x = 0; 0.03; 0.05) were chosen for the studies. Temperature XRD studies of [(NH 4 ) 0.97 Rb 0.03 ] 3 H(SO 4 ) 2 revealed a conspicuous jump of lattice parameters at the phase transition from P2/n to P 1 at T 4 % 125 K and insignificant changes at T 2 % 260 K and T 3 % 140 K. The crystal structure of [(NH 4 ) 0.97 Rb 0.03 ] 3 H(SO 4 ) 2 for all low temperature phases was also determined. The results of temperature changes of Raman spectra showed conspicuous changes of the frequencies related to internal vibrations of the constituents of the SO 4 AH. . .SO 4 dimers at temperature T 4 . Significant changes were also observed for the external vibration related to NH 4 þ groups. It was shown that all low temperature phases exhibit ferroelastic properties. The ferroelastic domain structure undergoes a change at T 4 . The results of our studies indicate a fundamental role of NH 4 þ groups at low temperature phase transitions. A small amount of Rb added to the (NH 4 ) 3 H(SO 4 ) 2 leads to disappearance of these phases.