New structural phase sequence with incommensurate phases in A2BX4 halides with β-K2SO4 structure

Cs2ZnBr4, Cs2CdBr4, and Cs2HgBr4 are at room temperature isomorphous with β-K2SO4 (Pnma, Z = 4). X-ray diffraction and 81Br NQR measurements show that this structure is stable down to T = 5 K for Cs2ZnBr4. In Cs2CdBr4 and Cs2HgBr4 a new phase sequence is observed: Pnma (Z = 4) ↔ incommensurate phase ↔ P21/n (Z=4) ↔ unknown low temperature phase. The incommensurate phase is modulated in the x-direction with q = δa*, the lock-in transition into the P21/n phase occurs at the centre of the Pnma Brillouin zone.