Phase Transitions and Second-Harmonic Generation in Sodium Monothiophosphate

Oxythiophosphate compounds, which contain anions comprised of oxidized phosphorus bound to oxygen and sulfur, are scarce and, in general, poorly characterized. Although α-Na3PO3S has been known for over 60 years, little has been revealed about this compound. Here we present thermal analysis and second-harmonic-generation (SHG) efficiency of α-Na3PO3S. Details about its transformation to the high-temperature β phase are discussed. Under vacuum, α-Na3PO3S undergoes one endothermic event at 553 °C upon heating and two exothermic events at 504 and 467 °C upon cooling. β-Na3PO3S can be trapped upon quenching Na3PO3S from high temperature. We have observed that the β phase converts back to the α phase at room temperature. Interestingly, relative humidity has been determined to catalyze this phase transformation. α-Na3PO3S oxidizes at 315 °C in air to produce Na4P2O7 and Na2SO4. Upon exposure to 1064-nm incident radiation, noncentrosymmetric α-Na3PO3S produces an SHG efficiency 200 times that of α-quartz and is nonphase-matchable (type 1).