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) Å, b = 9.9847(3) Å, and c = 6.0916(2) Å. 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 minimum is derived from Zn-4s, Zn-4p, and Sr-4d states. The calculated band dispersion reveals a direct band gap corresponding to a transition between S-3p and Zn-4s energy levels. Second harmonic generation (SHG) measurements determined that SrZn2S2O is phase matchable with twice the SHG intensity of potassium dihydrogen phosphate (KDP) in contrast to CaZnSO with similar ZnS3O building units, which exhibits non-phase matching behavior.