Low-temperature phases in Cs₂CdBr₄ and Cs₂HgBr₄

Abstract

With respect to their phase transitions, Cs2CdBr4 and Cs2HgBr4 are the first known representatives of a new group having the flK2SO4-type structure. Upon coolmg they show the following phase sequence. Phase I: Pnma, Z -4. Cs, Cd /Hg and two Br are in special positions on the mirror plane. Second-order phase transition at Tc~. Phase II: satellites with q ~0.15a*. First-order phase transition at To2. 'Lock-in' phase III: P2~/n, Z--4 . Second-order phase transition at T~3. Phase IV: P1, Z 4 , no systematic absences in the X-ray patterns, doubling of the Br NQR frequencies. CsEHgBr4 shows a further first-order transition at T¢4--85 K with doubling of the Br NQR frequencies. Phase V: P1, Z = 8, doubling of the b lattice constant. The values of the transition temperatures Tcl to Tc3 are 252, 237 and 158 K for CszCdBr4 and 245,232 and 167 K for Cs2HgBr4. The phase transformations are explained by a rotation wave through the BX4 tetrahedra. In Pnma the wave has the appearance of a strong thermal motion. In the incommensurate phase the mode softens with a wavelength of about 7a and 'locks-in' with k = 0 (F point of the Brillouin zone) in P2~/n. At 200 K the tetrahedra are turned around the a axis by 7 °. In P1 there is also a slight distortion of the structure.