Phase transitions in A4Li(HSO4)3(SO4); A = Rb, K: Single crystal X-ray diffraction studies

The crystal structure of ferroelastic Rb4Li(HSO4)3(SO4) has been determined at two temperatures, which indicates a structural phase transition, tetragonal P43 with a = 7,629(1) Å, c = 29,497(2) Å at 293 K and monoclinic P21 with a = 7,583(3) Å, b = 29,23(19) Å, c = 7,536(5) Å, b = 90,14(1)at 90 K. The crystal structure of K4Li(HSO4)3(SO4)4 has also been determined at two temperatures, tetragonal P41 with a = 7,405(1) Å, c = 28,712(6) Å at 293 K and tetragonal P41 with a = 7,371(5) Å, c = 28,552(5) Å at 100 K. The overall coordination features in both the structures have been analysed in terms of bond valence sum calculations.