β -CsHg 2 I 5 , a compound with rare [Hg 2 I 5 ] dimers and large optical anisotropy.

Hg-based compounds show abundant structural diversity and distinguished properties. Herein, a new phase transition compound CsHg 2 I 5 was reported. The high-temperature phase β -CsHg 2 I 5 with rare [Hg 2 I 5 ] 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 -CsHg 2 I 5 . The two phases crystallize in the same P 2 1 / c space group, with different crystal structures. β -CsHg 2 I 5 is composed of rare [Hg 2 I 5 ] dimers and [CsI 11 ] polyhedral units, while α -CsHg 2 I 5 is composed of [Hg 4 I 11 ] and [CsI 10 ] units. The experimental band gap of β -CsHg 2 I 5 was found to be 2.58 eV. Owing to the presence of [Hg 2 I 5 ] ∞ pseudo-layers, β -CsHg 2 I 5 exhibits large optical anisotropy with a calculated birefringence of 0.132@1064 nm. Meanwhile, β -CsHg 2 I 5 is a congruent compound and the congruent point is ∼481 K. Theoretical calculations indicate that the rare [Hg 2 I 5 ] 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 2 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 [Hg 2 I 5 ] dimers.