Highly Polarizable Hg 2+ Induced a Strong Second Harmonic Generation Signal and Large Birefringence in LiHgPO 4 .

A new ultraviolet nonlinear optical material, namely LiHgPO ₄ , was synthesized using the mild hydrothermal method. The structure of LiHgPO ₄ features novel double layers constructed by severely distorted HgO ₆ octahedra and PO ₄ tetrahedra, which are further interconnected by LiO ₄ tetrahedra to form a three-dimensional structure. LiHgPO ₄ exhibits a very strong second harmonic generation (SHG) response of 11.0 times that of KH ₂ PO ₄ and a relatively large birefringence (0.068 at 1064 nm) among the reported SHG-active phosphates. Theoretical calculations revealed that the introduction of the Hg ²⁺ in a highly distorted HgO ₆ octahedral geometry is crucial to produce the strong SHG response and large birefringence.