Crystal structure and optical properties of the new 8O polytype of Ca2Ta2O7

Single crystals of the new 80 modification of Ca2Ta2O7 were grown by optical floating zone melting and their structure was solved by X-ray diffraction. 8O Ca2Ta2O7 crystallizes in space group C222(1) with a = 7.3690(2) angstrom, b = 12.7296(3) angstrom, c = 48.263(1) angstrom. The structure is related to the weberite structure type and can be described as a stacking of 8 basic building units per unit cell. Each building unit consists of one Ta3Ca and one Ca3Ta layer with a cubic close packed arrangement. Optical spectroscopy in the UV-Vis and IR region reveals a high transparency in a wide energy range from 0.2-3.5 eV. Dielectric measurements yield a high epsilon' value of 60 at room temperature.