Low temperature phase transitions of [(NH4)1−xRbx]3H(SO4)2 studied by XRD and Raman spectroscopy

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 [(NH4)1−xRbx]3H(SO4)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 [(NH4)0.97Rb0.03]3H(SO4)2 revealed a conspicuous jump of lattice parameters at the phase transition from P2/n   to P1¯ at T4 ≈ 125 K and insignificant changes at T2 ≈ 260 K and T3 ≈ 140 K. The crystal structure of [(NH4)0.97Rb0.03]3H(SO4)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 SO4H…SO4 dimers at temperature T4. Significant changes were also observed for the external vibration related to NH4+ groups. It was shown that all low temperature phases exhibit ferroelastic properties. The ferroelastic domain structure undergoes a change at T4. The results of our studies indicate a fundamental role of NH4+ groups at low temperature phase transitions. A small amount of Rb added to the (NH4)3H(SO4)2 leads to disappearance of these phases.