Layered anion ordering in oxide chloride Ca₂TlO₃Cl

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.