Polymorphism and second harmonic generation in a novel diamond-like semiconductor: Li2MnSnS4

High-temperature, solid-state synthesis in the Li2MnSnS4 system led to the discovery of two new polymorphic compounds that were analyzed using single crystal X-ray diffraction. The α-polymorph crystallizes in Pna21 with the lithium cobalt (II) silicate, Li2CoSiO4, structure type, where Z=4, R1=0.0349 and wR2=0.0514 for all data. The β-polymorph possesses the wurtz-kesterite structure type, crystallizing in Pn with Z=2, R1=0.0423, and wR2=0.0901 for all data. Rietveld refinement of synchrotron X-ray powder diffraction was utilized to quantify the phase fractions of the polymorphs in the reaction products. The α/β-Li2MnSnS4 mixture exhibits an absorption edge of ∼2.6–3.0eV, a wide region of optical transparency in the mid- to far-IR, and moderate SHG activity over the fundamental range of 1.1–2.1μm. Calculations using density functional theory indicate that the ground state energies and electronic structures for α- and β-Li2MnSnS4, as well as the hypothetical polymorph, γ-Li2MnSnS4 with the wurtz-stannite structure type, are highly similar. Two polymorphs, α- and β-Li2MnSnS4, have been discovered using single crystal X-ray diffraction. Rietveld refinement of synchrotron X-ray powder diffraction data indicates the presence of both polymorphs in the samples that were analyzed. [Display omitted] •Li2MnSnS4 exists as two polymorphs crystallizing in the Pna21 and Pn space groups.•The α- and β-Li2MnSnS4 mixture exhibits a moderate SHG response over a broad range.•The α- and β-Li2MnSnS4 mixture exhibits an optical absorption edge of ∼2.6–3.0eV.•Synchrotron powder diffraction data are necessary to distinguish α- and β-Li2MnSnS4.•Electronic structure calculations show similar total energies for α- and β-Li2MnSnS4.