Ba5CdGa6Se15, a congruently-melting infrared nonlinear optical material with strong SHG response

Abstract

The quaternary selenide Ba₅CdGa₆Se₁₅ was prepared by reaction of BaSe and CdGa₂Se₄ at 1273 K and forms as light yellow crystals. It adopts a noncentrosymmetric structure (orthorhombic, space group Ama2, Z = 4, a = 24.2458(8) Å, b = 19.1582(7) Å, c = 6.6028(2) Å) featuring a three-dimensional anionic framework built up of corner-sharing tetrahedra centred by Cd and Ga atoms, with the intervening voids filled by Ba cations. The Cd atoms are partially disordered with Ga atoms within two out of five possible sites; the site preference can be understood in terms of optimizing bond valence sums. Band structure calculations predict that Ba₅CdGa₆Se₁₅ should be a semiconductor in which substitution of Cd for Ga atoms reduces the electron count in a hypothetical all-Ga compound “Ba₅Ga₇Se₁₅” so that the Fermi level falls within a band gap. Ba₅CdGa₆Se₁₅ is a potential new infrared nonlinear optical material with several attractive properties. First, the large band gap of 2.60(2) eV, as determined from the UV-vis-NIR optical absorption spectrum, implies a high laser damage threshold. Second, strong second harmonic generation signals were observed from nonlinear optical measurements on powder samples, with intensities comparable to that of the benchmark material AgGaS₂; type-I phase-matching behaviour was confirmed using 2090 nm as the fundamental laser wavelength. Third, this compound melts congruently at a relative low temperature of 866 °C, which could facilitate growth of large crystals eventually required for applications.