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A noncentrosymmetric (NCS) polar compound, Pb 3 SeO 5, has been hydrothermally synthesized and structurally characterized by single crystal X-ray diffraction. Pb 3 SeO 5 exhibits a two-dimensional crystal structure consisting of layers of α-PbO-like “slabs” that are linked through SeO 3 polyhedra. Structurally, it is the bridging SeO 3 polyhedra between the α-PbO-like slabs that results in the NCS and polar nature of Pb 3 SeO 5 . Powder second-harmonic generation (SHG) measurements using 1064 nm radiation indicates that Pb 3 SeO 5 exhibits a strong SHG efficiency of ∼300 × α-SiO 2 . Additional SHG measurements indicate the material is type-I phase-matchable. Converse piezoelectric measurements revealed a d 33 value of ∼81 pm/V, and a pyroelectric coefficient of −42 μC/(m 2 K) at 65 °C was also determined. Using first principle density functional theory (DFT) calculations, we demonstrated that polarization reversal in Pb 3 SeO 5 is not energetically favorable—the material is polar but not ferroelectric. Our calculations also indicate that both Pb 2+ and Se 4+ cations exhibit a stereoactive lone-pair. In addition, differential scanning calorimetry measurements revealed an irreversible phase transition at ∼440 °C. Finally, infrared, UV−vis and thermogravimetric measurements were also performed. Crystal data: Pb 3 SeO 5, orthorhombic, space group Cmc 2 1 (no. 36), a = 10.5211(13) Å, b = 10.7151(13) Å, c = 5.7452(7) Å, V = 647.68(14) Å 3, and Z = 2.
Kim et al. (Mon,) studied this question.