A new β-CdTeO₃ polymorph with a structure related to α-CdTeO₃

A new β-CdTeO₃ polymorph was obtained by hydrothermal synthesis and its structure was solved ab initio from powder X-ray diffraction data. It appears that the structure of β-CdTeO₃ (Pnma, Z = 16, a = 7.45850(3) Å, b = 14.52185(6) Å, c = 11.04584(5) Å) is closely related to that of α-CdTeO₃ (P2₁/c, Z = 8, a = 7.790(1) Å, b = 11.253(2) Å, c = 7.418(1) Å, β = 113.5(1)°) previously reported. The 3D framework of β-CdTeO₃ is built of both [CdO₆] distorted octahedra and [CdO₇] mono-capped trigonal prisms and three different tellurium polyhedra: trigonal pyramids [Te^IVO₃E] and trigonal bipyramids [Te^IVO₄E] and [Te^IVO₃₊₁E] (E denotes the lone pair of Te^IV). The electronic structure calculations based on density functional theory methods show that at the ground state α-CdTeO₃ is slightly more stable than β-CdTeO₃ with an energy difference of 4.64 kJ mol^-1. The band structures confirm the results of optical UV-Vis spectroscopy measurements: both polymorphs are wide bandgap semiconductors with E_g = 3.55 eV for β-CdTeO₃ and E_g = 3.91 eV for α-CdTeO₃. The DOS calculations for both polymorphs enable one to understand that the presence of the [Te^IVO₄E] polyhedra in β-CdTeO₃ (absent in α-CdTeO₃) lowers its bandgap. Above 540 °C β-CdTeO₃ transforms into α-CdTeO₃ in a first order phase transition.