A₂P₂S₆ (A = Ba and Pb): a good platform to study the polymorph effect and lone pair effect to form an acentric structure

Three ternary thiophosphates α-Ba₂P₂S₆, β-Ba₂P₂S₆, and Pb₂P₂S₆ were synthesized via a high temperature salt flux method or an I₂ transport reaction. β-Ba₂P₂S₆ and Pb₂P₂S₆ were previously structurally characterized without investigating their optical properties. α-Ba₂P₂S₆ was discovered for the first time, and it is isostructural to Pb₂P₂S₆ and crystallizes in the acentric space group Pn (no. 7). β-Ba₂P₂S₆ crystallizes in the centrosymmetric space group P2₁/n (no. 14). There is a high structural similarity between α-Ba₂P₂S₆, β-Ba₂P₂S₆, and Pb₂P₂S₆ including close unit cell parameters and identical [P₂S₆] motifs. The structural relationships between α-Ba₂P₂S₆ and β-Ba₂P₂S₆, and β-Ba₂P₂S₆ and Pb₂P₂S₆ were elucidated by single crystal X-ray diffraction, differential scanning calorimetry (DSC), electronic structure calculations, and nonlinear optical property measurements. There are no phase transitions detected between α-Ba₂P₂S₆ and β-Ba₂P₂S₆. From centrosymmetric β-Ba₂P₂S₆ to acentric Pb₂P₂S₆, the chemical characteristics of Pb, such as stereoactive lone pairs, play a key role in the structural difference. Pb₂P₂S₆ is uncovered as a type-I phase-matching material with a moderate second harmonic generation (SHG) response of 1.4 × AgGaS₂ and a high laser damage threshold (LDT) of 2.5 × AgGaS₂. α-Ba₂P₂S₆ is not a type-I phase-matching material with a moderate second harmonic generation response (1.7 × AgGaS₂, a sample of 225 μm particle size) and a high laser damage threshold (5.5 × AgGaS₂).