Function of Tetrahedral ZnS3O Building Blocks in the Formation of SrZn2S2O: A Phase Matchable Polar Oxysulfide with a Large Second Harmonic Generation Response

Single crystals of a new zinc oxysulfide SrZn2S2O were grown in a eutectic KF–KCl flux, and the structure was determined by single-crystal X-ray diffraction. SrZn2S2O crystallizes in the noncentrosymmetric polar space group Pmn21 with lattice parameters of a = 3.87440(10) A, b = 9.9847(3) A, and c = 6.0916(2) A. In the crystal structure, close-packed corrugated double layers of ZnS3O tetrahedra, which are derived from the wurtzite structure, are vertically separated by Sr2+ ions. In addition, the O/S anion ordered arrangement in each close-packed layer yields two distinct orientations of the Zn-centered tetrahedra. The crystals of SrZn2S2O are colorless and transparent, and the oxysulfide has a band gap of 3.86 eV, based on UV–vis–NIR diffuse reflectance measurements. Thermogravimetric measurements showed that SrZn2S2O is stable up to 650 °C in O2 gas atmosphere. First-principle calculations indicate that the valence band maximum is mainly composed of O-2p and S-3p states, whereas the conduction band mini...