The crystal structure of ferroelastic Rb4Li(HSO4)3(SO4) has been determined at two temperatures, which indicates a structural phase transition, tetragonal P43 witha = 7.629(1) Å,c = 29.497(2) Å at 293 K and monoclinic P21 witha = 7.583(3) Å,b = 29.230(19) Å,c = 7.536(5) Å,β = 90.14(1)° at 90 K. The crystal structure of K4Li(HSO4)3(SO4)4 has also been determined at two temperatures, tetragonalP41 witha = 7.405(1) Å,c = 28.712(6) Å at 293 K and tetragonalP41 witha = 7.371(5) Å,c = 28.522(5) Å at 100 K. The overall coordination features in both the structures have been analysed in terms of bond valence sum calculations.
