The dehydration of SrTeO3(H2O) - a topotactic reaction for preparation of the new metastable strontium oxotellurate(IV) phase ε-SrTeO3

Abstract

Microcrystalline single-phase strontium oxotellurate(IV) monohydrate, SrTeO₃(H₂O), was obtained by microwave-assisted hydrothermal synthesis under alkaline conditions at 180 °C for 30 min. A temperature of 220 °C and longer reaction times led to single crystal growth of this material. The crystal structure of SrTeO₃(H₂O) was determined from single crystal X-ray diffraction data: P2₁/c, Z = 4, a = 7.7669(5), b = 7.1739(4), c = 8.3311(5) Å, β = 107.210(1)°, V = 443.42(5) ų, 1403 structure factors, 63 parameters, R[F²>2σ(F²)] = 0.0208, wR(F² all) = 0.0516, S = 1.031. SrTeO₃(H₂O) is isotypic with the homologous BaTeO₃(H₂O) and is characterised by a layered assembly parallel to (100) of edge-sharing [SrO₆(H₂O)] polyhedra capped on each side of the layer by trigonal-prismatic [TeO₃] units. The cohesion of the structure is accomplished by moderate O–H⋯O hydrogen bonding interactions between donor water molecules and acceptor O atoms of adjacent layers. In a topochemical reaction, SrTeO₃(H₂O) condensates above 150 °C to the metastable phase ε-SrTeO₃ and transforms upon further heating to δ-SrTeO₃. The crystal structure of ε-SrTeO₃, the fifth known polymorph of this composition, was determined from combined electron microscopy and laboratory X-ray powder diffraction studies: P2₁/c, Z = 4, a = 6.7759(1), b = 7.2188(1), c = 8.6773(2) Å, β = 126.4980(7)°, V = 341.20(18) ų, R_Fobs = 0.0166, R_Bobs = 0.0318, Rwp = 0.0733, Goof = 1.38. The structure of ε-SrTeO₃ shows the same basic set-up as SrTeO₃(H₂O), but the layered arrangement of the hydrous phase transforms into a framework structure after elimination of water. The structural studies of SrTeO₃(H₂O) and ε-SrTeO₃ are complemented by thermal analysis and vibrational spectroscopic measurements.