Crystal-to-crystal transformation of a new selenite compound CaNi₂(SeO₃)₃·2H₂O induced by dehydration

A new selenite compound CaNi₂(SeO₃)₃·2H₂O (1) was obtained by a conventional hydrothermal method, featuring a distorted zemannite-type structure with a unique honeycomb-like hexagonal framework. The crystal-to-crystal transformation from 1 to a partially dehydrated form of CaNi₂(SeO₃)₃·1.5H₂O (2) with slightly shrunk hexagonal framework and further to an anhydrous CaNi₂(SeO₃)₃ (3) with a square framework can be achieved by step-by-step dehydration under heating. The UV-vis-NIR absorption spectra indicate that all 1–3 to possess a wide band gap due to spin allowed d–d transition at Ni²⁺ ion centers. Further, magnetic measurements show a similar magnetic behavior with an antiferromagnetic transition in 1 and 2, while a different magnetic behavior with two magnetic transitions is observed in 3.