Phase transitions in $A_4Li(HSO_4)_3(SO_4)$; A = Rb, K: Single crystal X-ray diffraction studies

Abstract. The crystal structure of ferroelastic $Rb_4Li(HSO_4)_3(SO_4)$ has been determined at two temperatures, which indicates a structural phase transition, tetragonal $P4_3$ with $a = 7 \cdot 629(1) \AA $, $c = 29 \cdot 497(2) \AA $ at 293 K and monoclinic $P2_1$ with $a =7 \cdot 583(3) \AA $, $b = 29 \cdot 230(19) \AA $, $c = 7 \cdot 536(5) \AA $, $\beta = 90 \cdot 14(1)°$ at 90 K. The crystal structure of $K_4Li(HSO_4)_3(SO_4)_4$ has also been determined at two temperatures, tetragonal $P4_1$ with $a = 7 \cdot 405(1) \AA $, $c = 28 \cdot 712(6) \AA $ at 293 K and tetragonal $P4_1$ with $a = 7 \cdot 371(5) \AA $, $c = 28 \cdot 522(5) \AA $ at 100 K. The overall coordination features in both the structures have been analysed in terms of bond valence sum calculations