A 2 P 2 S 6 (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-Ba 2 P 2 S 6 , β-Ba 2 P 2 S 6 , and Pb 2 P 2 S 6 were synthesized via a high temperature salt flux method or an I 2 transport reaction. β-Ba 2 P 2 S 6 and Pb 2 P 2 S 6 were previously structurally characterized without investigating their optical properties. α-Ba 2 P 2 S 6 was discovered for the first time, and it is isostructural to Pb 2 P 2 S 6 and crystallizes in the acentric space group Pn (no. 7). β-Ba 2 P 2 S 6 crystallizes in the centrosymmetric space group P 2 1 / n (no. 14). There is a high structural similarity between α-Ba 2 P 2 S 6 , β-Ba 2 P 2 S 6 , and Pb 2 P 2 S 6 including close unit cell parameters and identical [P 2 S 6 ] motifs. The structural relationships between α-Ba 2 P 2 S 6 and β-Ba 2 P 2 S 6 , and β-Ba 2 P 2 S 6 and Pb 2 P 2 S 6 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-Ba 2 P 2 S 6 and β-Ba 2 P 2 S 6 . From centrosymmetric β-Ba 2 P 2 S 6 to acentric Pb 2 P 2 S 6 , the chemical characteristics of Pb, such as stereoactive lone pairs, play a key role in the structural difference. Pb 2 P 2 S 6 is uncovered as a type-I phase-matching material with a moderate second harmonic generation (SHG) response of 1.4 × AgGaS 2 and a high laser damage threshold (LDT) of 2.5 × AgGaS 2 . α-Ba 2 P 2 S 6 is not a type-I phase-matching material with a moderate second harmonic generation response (1.7 × AgGaS 2 , a sample of 225 μm particle size) and a high laser damage threshold (5.5 × AgGaS 2 ).