Revisiting two thiophosphate compounds constituting d0 transition metal HfP2S6 and d10 transition metal α-Ag4P2S6 as multifunctional materials for combining second harmonic generation response and photocurrent response

Two acentric thiophosphate compounds, HfP2S6 and alpha-Ag4P2S6, are revisited and studied as infrared nonlinear optical materials. HfP2S6 and alpha-Ag4P2S6 were structurally characterized without any property measurements. In this work, HfP2S6 and alpha-Ag4P2S6 were synthesized via high temperature salt flux reactions. Low-temperature polymorph acentric alpha-Ag4P2S6 was purified and grown as mm-sized crystals with the aid of AgBr flux. The AgBr flux was revealed to play an important role in stabilizing the acentric alpha-Ag4P2S6. The acentric alpha-Ag4P2S6 transforms to centrosymmetric beta-Ag4P2S6 at 850(5) K, which is revealed by differential scanning calorimetry (DSC) analysis and powder X-ray diffraction experiments. HfP2S6 and alpha-Ag4P2S6 are discovered by UV-vis spectrum measurements as indirect bandgap semiconductors with bandgaps of 2.2(1) eV and 2.5(1) eV, respectively, which is supported by DFT calculations and TB-LMTO-ASA calculations. The bonding pictures of alpha-Ag4P2S6 were studied by crystal orbital Hamilton population calculations (COHP) coupled with electron localization function (ELF) analysis. DFT calculations predict that HfP2S6 and alpha-Ag4P2S6 would exhibit different optical performances regardless of being constructed from identical [P2S6] motifs. HfP2S6 exhibits a low second harmonic generation (SHG) response, similar to 0.21 x AGS (for the sample of particle size of 25 mu m). alpha-Ag4P2S6 possesses moderate SHG response, similar to 0.61 x AGS (for the sample of particle size of 225 mu m) coupled with a high laser damage threshold (LDT) of similar to 3.2 x AGS. Characteristics of high ambient stability, moderate bandgap and SHG response, type-I phase-matching capability, and high LDT together with the easy growth of large crystals make alpha-Ag4P2S6 attractive for future infrared nonlinear optical applications. Photocurrent measurements found that alpha-Ag4P2S6 and beta-Ag4P2S6 have high photocurrent response, 165 nA cm(-2) and 135 nA cm(-2), respectively. alpha-Ag4P2S6 is a new multifunctional material of the ternary Ag-P-S system, which combines nonlinear optical (NLO) properties and photocurrent response.