PHASE TRANSITION IN THE Cs₂XBr₄ (X = Hg, Cs). C. B. Pinheiro and N. L. Spezial, Departamento de Física, ICEx - UFMG, Belo Horizonte - Brazil

Among the compounds classified in the A₂BX₄ family, several members have been considered as candidates to present incommensurate phase. Two of them, Cs₂CdBr₄ and Cs₂HgBr₄, are special ones: their lock-in phase occurs at q=0, without the appearance of a superstructure. Their room temperature (RT) structure is isomorphous to [[beta]]-K₂SO₄. Decreasing temperature, an incommensurate (INC) phase appears as evidenced by the presence of satellite reflections.

Differential scanning calorimetry (DSC) and single crystal X-ray diffraction experiments have shown that both compounds present high temperature phase transitions at T₁820K and at T₂720K. Their complete phase transition diagram can be represented as follows:

[[beta]]-K₂SO₄ (Twinned)

? ? Pnma Incom. P2₁/n P

T₁ T₂ T₃ T₄ T₅

HT₂₂ HT₁₁ RT INC LT₁₁ LT₂₂

with Cs₂HgBr₄ presenting an extra low-temperature phase LT₃.

The disordered hexagonal phase evidenced in several members of the A₂BX₄ family (so called [[alpha]]-K₂SO₄ phase) has not yet been observed in Cs₂XBr₄. X-ray diffraction measurements are in progress in order to investigate the high temperature phases of such compounds.