New materials for infrared non-linear optics. Syntheses, structural characterisations, second harmonic generation and optical transparency of M(IO3)3 metallic iodates

Abstract

Two new phases α-In(IO₃)₃ and β-In(IO₃)₃ have been synthesised by evaporation of concentrated nitric acid solution containing metallic salts and sources of iodate. Single crystal structures reveal an acentric three-dimensional network for α-In(IO₃)₃ and a centrosymmetric two-dimensional network for β-In(IO₃)₃. Temperature-dependent X-ray powder diffraction experiments shows the transformation of α-In(IO₃)₃ to β-In(IO₃)₃ at 365 °C. The phase transition is not reversible. α-In(IO₃)₃ is the kinetically favoured phase whereas β-In(IO₃)₃ is the thermodynamically stable phase. α-In(IO₃)₃ is isomorphic to Cr(IO₃)₃, Fe(IO₃)₃ and Ga(IO₃)₃ and crystallises in space group P6₃ (no. 173) with a = 9.541(1) Å and c = 5.266(1) Å. Structural relationships among the anhydrous non-centrosymmetric metallic iodates M(IO₃)₃ (M = Cr, Fe, In, Ga), M(IO₃)₂ (M = Mg, Mn, Co, Ni, Zn) and α-LiIO₃ are described. Structural analogies between these metallic iodates allow us to prepare solid solutions as In_1−xFe_x(IO₃)₃ and In_1−xCr_x(IO₃)₃. Luminescence of the In_1−xCr_x(IO₃)₃ phases has been observed. DSC analyses of β-In(IO₃)₃ and Ga(IO₃)₃ show that they decompose at 535 °C and 525 °C respectively. Fe(IO₃)₃, Ga(IO₃)₃ and α-In(IO₃)₃ generate second harmonics and have high non-linear coefficients and high optical damage thresholds in powder form. Furthermore, they do not absorb up to 12 µm. Consequently, they possess a large domain of transparency from visible light to the beginning of the far-infrared, including three atmospheric transparency windows. They are particularly interesting for potential applications in quadratic non-linear optics in atmospheric transparency bands II and III.