Highly Polarizable Hg²⁺ Induced a Strong Second Harmonic Generation Signal and Large Birefringence in LiHgPO₄

A new ultraviolet nonlinear optical material, namely LiHgPO₄, was synthesized using the mild hydrothermal method. The structure of LiHgPO₄ features novel double layers constructed by severely distorted HgO₆ octahedra and PO₄ tetrahedra, which are further interconnected by LiO₄ tetrahedra to form a three-dimensional structure. LiHgPO₄ exhibits a very strong second harmonic generation (SHG) response of 11.0 times that of KH₂PO₄ and a relatively large birefringence (0.068 at 1064 nm) among the reported SHG-active phosphates. Theoretical calculations revealed that the introduction of the Hg²⁺ in a highly distorted HgO₆ octahedral geometry is crucial to produce the strong SHG response and large birefringence.