β-CsHgI, a compound with rare [HgI] dimers and large optical anisotropy

Hg-based compounds show abundant structural diversity and distinguished properties. Herein, a new phase transition compound CsHgI was reported. The high-temperature phase β-CsHgI with rare [HgI] dimers was synthesized by the flux method at 573 K, and it shows a reversible phase transition at a low temperature of ∼100 K to form the low-temperature phase α-CsHgI. The two phases crystallize in the same P2/c space group, with different crystal structures. β-CsHgI is composed of rare [HgI] dimers and [CsI] polyhedral units, while α-CsHgI is composed of [HgI] and [CsI] units. The experimental band gap of β-CsHgI was found to be 2.58 eV. Owing to the presence of [HgI] pseudo-layers, β-CsHgI exhibits large optical anisotropy with a calculated birefringence of 0.132@1064 nm. Meanwhile, β-CsHgI is a congruent compound and the congruent point is ∼481 K. Theoretical calculations indicate that the rare [HgI] dimer is a nonlinear active unit, which can be used as a new fundamental building block for the design of advanced nonlinear optical materials. Moreover, a CsI-HgI pseudo-binary diagram was drawn. The results enrich the structural diversity of Hg-based halides and give some insights into the development of new functional materials based on rare [HgI] dimers.