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 SrZn 2 S 2 O were grown in a eutectic KF–KCl flux, and the structure was determined by single-crystal X-ray diffraction. SrZn 2 S 2 O crystallizes in the noncentrosymmetric polar space group Pmn 2 1 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 ZnS 3 O tetrahedra, which are derived from the wurtzite structure, are vertically separated by Sr 2+ 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 SrZn 2 S 2 O 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 SrZn 2 S 2 O is stable up to 650 °C in O 2 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 SrZn 2 S 2 O is phase matchable with twice the SHG intensity of potassium dihydrogen phosphate (KDP) in contrast to CaZnSO with similar ZnS 3 O building units, which exhibits non-phase matching behavior.