Differentiating Electrical and Optoelectrical Properties of Oxysulfides La2Ta2MS2O8 (M=Zr, Ti) via Application of Pressure

The oxysulfides La2Ta2MS2O8 (M = Zr, Ti) form a new family of narrow-band-gap semiconductors that have a lot of potential for photocatalysis and optoelectronics. However, many of their fundamental properties are still unknown. Herein, we report their crystal structure evolution, electrical resistances, band gaps, and photocurrents under compression from near ambient pressure up to ∼50 GPa. Our results show that the orthorhombic phases of La2Ta2ZrS2O8 and La2Ta2TiS2O8 transform into two new high-pressure phases at ∼23 and 25 GPa, respectively. Although their band gaps show similar trends in variations with the pressure, the resistance of La2Ta2ZrS2O8 is ∼2–3 orders of magnitudes higher than that of La2Ta2TiS2O8. In addition, the former exhibits a sharp decrease with pressure after the phase transition, while the latter presents only a steady decrease with increasing pressure. This difference arises from their different responses in the carrier mobility under compression, which is related to their compressibility at high pressure (the bulk modulus of La2Ta2ZrS2O8 is 122.5 GPa and that of La2Ta2TiS2O8 is 132.6 GPa). Both the electrical conductivity and defect formation at high pressure can affect the photoelectric properties, making the La2Ta2TiS2O8 compound possess photocurrents superior to the La2Ta2ZrS2O8 one. The findings from this work show that the La2Ta2TiS2O8 compound is a better candidate than La2Ta2ZrS2O8 for developing applications in photocatalysis, photovoltaics, photoelectric devices, etc.