X‐ray studies of the low‐temperature phase of K 2 SO 4

The low‐temperature phase transition of potassium sulphate (K 2 SO 4 ) has been investigated by X‐ray diffraction measurements on polycrystalline material. From the temperature dependence of the lattice parameters it was confirmed that K 2 SO 4 crystals undergo a structural phase transition of second‐order type at 56 K. Above the transition point (β‐phase) the K 2 SO 4 crystal belongs to the Pnam (D   2h 16 ) orthorhombic space group with lattice parameters a = 0.74780 nm, b = 1.00707 nm, and c = 0.57701 nm at 293 K. The crystal symmetry of the low‐temperature phase (below 56 K, γ‐phase) is probably monoclinic with space group P2 1 /n11 (C   2h 5 ). The low‐temperature phase transformation is thought to be of displacive type.