An X-Ray, Raman and IR Study of α-CsReO4, the High-Temperature Modification of Cesium Perrhenate

Abstract

Cesium perrhenate undergoes a reversible phase transition from the orthorhombic room-temperature phase (β-CsReO4, space group Pnma) to a tetragonal high-temperature phase (α-CsReO4). The space group of α-CsReO4 was unambiguously confirmed by high-temperature X-ray diffraction to be I41/amd in contrast to the earlier literature, in which space group I41/a (and hence, a scheelite type structure) was assumed for α-CsReO4. From a data set obtained at 468 K the crystal structure of α-CsReO4 (a = 5.9607(4), c = 14.446(1)Å, Z = 4) was refined to R = 0.032, Rww = 0.029 using anisotropic displacement factors.In the vibrational spectra not all of the predicted com ponents for β-CsReO4 could be confirmed by the experiments, but for α-CsReO4 very good agreement was obtained between the predicted and observed bands. The fact that v2 at ~ 300 cm–1 is not observed in the IR spectra of α-CsReO4 confirms the space group I41/amd, since, under I41/a symmetry, this band is allowed.The structure of α-CsReO4 consists of isolated ReO4 tetrahedra which are linked together by cesium atoms. The relationships between the scheelite, α-CsReO4 and β-CsReO4 structural types are discussed.