The Phase Transition of TlTe: Crystal Structure

Though already known since the year 1912, reliable structural data of the semimetallic phase TlTe down and below the reported phase-transition temperature of 170 K were still lacking. To fill this gap, we performed low-temperature X-ray diffraction investigations on single crystals of TlTe down to 135 K with a conventional laboratory radiation source (MoKALPHA) and additionally at the D3 synchrotron beamline of HASLYLAB at DESY (LAMBDA=40 pm). The compound TlTe crystallizes at ambient temperature tetragonal in space group I4/mcm and the lattice parameters a=1295.3(1) pm and c=617.3(1) pm. The crystal structure reveals univalent Tl+ cations and a polytelluric counterpart with linear equidistant Te chains in the [001] direction at distances of 308.63(3) pm. One-half of the chains is unbranched; the other one consists of linear [Te3] units stacked crossshaped one upon the other. At 172 K one-half of the branched chains transforms by a Peierls distortion into a linear chain with alternating distances of 285.5(1) and 330.2(1) pm. By the transformation the unit cell volume is doubled and the space group changed into P42/nmc (a=1822.9(1) pm and c=615.7(1) pm at 157 K). Since the other half of the branched chains and the other unbranched chains remain equidistant, it is to be expected that the compound keeps up its semimetallic behavior. This finding is in accordance with experimental resistivity data in the literature.