Growth, crystal structure, thermal properties and optical anisotropy of Tl4CdI6 single crystals *

We report on the growth and initial thermal and optical characterisation of a single-crystalline ternary halide, Tl4CdI6. The crystal is described by the centrosymmetric tetragonal class P4/mnc at the room temperature. The parameters of linear thermal expansion are studied in the temperature region 330-500 K. The crystal is optically positive and reveals very high optical anisotropy (the birefringence ~ 0.13 at 633 nm). Its optical pseudo-gap value corresponds to wide- gap semiconductors. A broad maximum at 400-410 K found in the temperature dependence of IR radiation-induced second harmonic generation may be indication of a polymorphic phase transformation.