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

Abstract

Two acentric thiophosphate compounds, HfP₂S₆ and α-Ag₄P₂S₆, are revisited and studied as infrared nonlinear optical materials. HfP₂S₆ and α-Ag₄P₂S₆ were structurally characterized without any property measurements. In this work, HfP₂S₆ and α-Ag₄P₂S₆ were synthesized via high temperature salt flux reactions. Low-temperature polymorph acentric α-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 α-Ag₄P₂S₆. The acentric α-Ag₄P₂S₆ transforms to centrosymmetric β-Ag₄P₂S₆ at 850(5) K, which is revealed by differential scanning calorimetry (DSC) analysis and powder X-ray diffraction experiments. HfP₂S₆ and α-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 α-Ag₄P₂S₆ were studied by crystal orbital Hamilton population calculations (COHP) coupled with electron localization function (ELF) analysis. DFT calculations predict that HfP₂S₆ and α-Ag₄P₂S₆ would exhibit different optical performances regardless of being constructed from identical [P₂S₆] motifs. HfP₂S₆ exhibits a low second harmonic generation (SHG) response, ∼0.21 × AGS (for the sample of particle size of 25 μm). α-Ag₄P₂S₆ possesses moderate SHG response, ∼0.61 × AGS (for the sample of particle size of 225 μm) coupled with a high laser damage threshold (LDT) of ∼3.2 × 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 α-Ag₄P₂S₆ attractive for future infrared nonlinear optical applications. Photocurrent measurements found that α-Ag₄P₂S₆ and β-Ag₄P₂S₆ have high photocurrent response, 165 nA cm⁻² and 135 nA cm⁻², respectively. α-Ag₄P₂S₆ is a new multifunctional material of the ternary Ag–P–S system, which combines nonlinear optical (NLO) properties and photocurrent response.