Polymorphism in Novel Li2-II-IV-S4 Diamond-Like Semiconductors

Diamond-like semiconductors (DLSs) have structures that are derived from either the cubic or hexagonal form of diamond. The I2-II-IV-VI4 diamond-like semiconductors are particularly interesting systems for their tunable nature and technological applications in photovoltaic solar cells, spintronics, and non-linear optics, specifically second harmonic generation. Polymorphism, which may affect important physicochemical properties in these materials, has been commonly reported in binary and ternary systems, while investigations of polymorphism in quaternary DLSs have been less prevalent. Polymorphs have been observed crystallizing in the stannite (I-42m) and wurtzstannite (Pmn21) structure types, which differ in the closest-packed arrangement of the anions, cubic versus hexagonal respectively. Polymorphism may also be observed in quaternary DLSs that maintain the same anion packing, but differ only in the cation ordering arrangements within the tetahedral holes. In the hexagonally derived quaternary DLSs, the different cation ordering gives rise to at least three different structure types, wurtzstannite (Pmn21), wurtzkesterite (Pn), and cobalt (II) lithium silicate (Pna21). In this work, high-temperature solid-state synthesis in a Li2-II-IV-VI4 system lead to the discovery of two new polymorphic compounds, crystallizing in the Pna21 and Pn space groups. The two polymorphs were analyzed using single crystal X-ray diffraction, synchrotron X-ray powder diffraction, and optical diffuse reflectance UV/Vis/NIR spectroscop