β-CsHg₂I₅, a compound with rare [Hg₂I₅] dimers and large optical anisotropy

Hg-based compounds show abundant structural diversity and distinguished properties. Herein, a new phase transition compound CsHg₂I₅ was reported. The high-temperature phase β-CsHg₂I₅ with rare [Hg₂I₅] 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-CsHg₂I₅. The two phases crystallize in the same P2₁/c space group, with different crystal structures. β-CsHg₂I₅ is composed of rare [Hg₂I₅] dimers and [CsI₁₁] polyhedral units, while α-CsHg₂I₅ is composed of [Hg₄I₁₁] and [CsI₁₀] units. The experimental band gap of β-CsHg₂I₅ was found to be 2.58 eV. Owing to the presence of [Hg₂I₅]_∞ pseudo-layers, β-CsHg₂I₅ exhibits large optical anisotropy with a calculated birefringence of 0.132@1064 nm. Meanwhile, β-CsHg₂I₅ is a congruent compound and the congruent point is ∼481 K. Theoretical calculations indicate that the rare [Hg₂I₅] 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-HgI₂ 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 [Hg₂I₅] dimers.