Chromium iodate: the structure and origin of optical second harmonic generation

Abstract

Iodate crystal materials are some of the important candidates for mid-infrared nonlinear optical (NLO) crystals. Almost all the structures and NLO properties of monometallic iodates have been studied, except for a few difficult to synthesize crystalline phases. In this work, crystalline Cr(IO₃)₃ was synthesized and its optical properties were studied. Cr(IO₃)₃ has a strong powder second-harmonic generation (SHG) effect (1.3 × AgGaS₂) and a large birefringence (0.24) at 2100 nm. Theoretical calculations indicate that the 3d orbital of Cr³⁺ is strongly involved in the SHG process, and the contribution of the metal cation centered unit CrO₆ to the SHG response is much greater than that of the non-metal cation centered unit IO₃. This study provides a new case among the few NLO materials in which the SHG response is dominated by metal cation centered groups.