BaMoO Polymorphs with Tunable Symmetries and Properties.

Polymorphism is a well-known but important phenomenon in the field of solid-state chemistry. Crystalline materials can form various polymorphs and present drastically varied physical and chemical properties. Herein, systematic exploration of the BaO-MoO binary system leads to the discovery of a new barium molybdate, α-BaMoO. The temperature-dependent phase transition from α-BaMoO to β-BaMoO is confirmed. The tunable linear and nonlinear optical properties induced by the phase transition are confirmed by both experimental and theoretical approaches. Also, β-BaMoO is identified as a nonlinear-optical crystal for the first time. The origin of linear- and nonlinear-optical properties of BaMoO polymorphs is confirmed by the additional theoretical means. This work indicates that a small change in the structure can induce tunable symmetries and thereby widely divergent optical properties.