Layered anion ordering in oxide chloride Ca2TlO3Cl

Abstract

Single crystals and polycrystals of the novel compound Ca₂TlO₃Cl can be obtained by gas-phase and solid-state reactions, respectively. The title compound's crystal structure is hexagonal (a = 3.8806(2) Å, c = 18.6168(7) Å, P6₃/mmc) as measured by single crystal X-ray diffraction. A Rietveld refinement on powder X-ray diffraction data confirms this crystal structure. The atomic arrangement can be described as a cation ordered anti-structure of ScAl₃C₃, with a homoleptically, five-fold oxygen coordinated Tl³⁺ that consitutes corner-sharing trigonal bipyramids. Between these Tl–O sheets, a Cl-layer is found, making the whole structure quasi-2D. Thermal expansion coefficients in the range 15–290 K, as estimated from X-ray powder diffraction data, reveal that the relative length of the crystallographic c-axis (α₃₃) is more temperature dependent than that of the a-axis (α₁₁) lengths. The brown color of the polycrystalline title compound was investigated by spectroscopic reflectometry measurements. These data suggest an optical band gap of either 1.4 eV (indirect) or about 2.4–2.7 eV (direct), depending on interpretation model. However, density funciton theory calculations deliver a band structure that indicates that indirect band gaps are most likely, but with a size of about 2.7 eV. This discrepancy is discussed as a result of further local excitation phenomena, like charge-transfers and crystal defects. Photo-electrochemical investigations suggest that the title compound has an n-type character and is electrochemically activated by solar light.