Synthesis, structure and theoretical studies of a new ternary non-centrosymmetric β-LaGaS3

New ternary β-LaGaS3 has been synthesized from the stoichiometric mixture of elements by a conventional solid-state reaction at 1100 °C and annealed at 820 °C. This compound represents a new structure type that crystallizes in a non-centrosymmetric orthorhombic space group Pna21 (No.33) with a=10.405(1) Å, b=21.984(2) Å, c=6.0565(5) Å, and Z=12, and features the wavy GaS4 tetrahedron chains that are separated by La3+ cations. Detailed structural differences between the title compound and its isomer, monoclinic α-LaGaS3, are discussed. With the aid of WIEN2k package, the absorption spectra and electronic structures as well as the refractive indexes, absorption coefficients and reflectivities of two types of LaGaS3 have been calculated. The calculated band gap and the absorption edge of β-LaGaS3 agree well with the experimental measurements. And a weak NLO response of β-LaGaS3 has been detected.