X-Ray Study of the Phase Transitions in K3D(SO4)2 and K3H(SO4)2

Structural phase transition of K 3 D(SO 4 ) 2 was studied by X-ray scattering experiments, and new reflections were observed below T c =85 K. The space groups are A 2/ a ( Z =4) above T c and P 2 1 / a ( Z =4) below T c . The postulated structure of the low temperature phase has an antipolar character. Temperature dependence of lattice parameters was also investigated, whose anomaly below T c was marginally small. The deviation from the curve calculated by the Debye approximation was utilized to estimate the nature of the order parameter. Similar experiments in K 3 H(SO 4 ) 2 have not shown any evidence of the phase transition.