Neutral X-Site ABX₃-Type Perovskites

ABX₃-type perovskite materials have garnered significant and sustained attention over the years owing to their diverse functional properties and chemical versatility. This class encompasses inorganic oxides, organic-inorganic hybrids, and emerging metal-free variants. However, it is noteworthy that nearly all perovskite materials developed thus far are constituted exclusively of ionic components occupying the A-, B-, and X-sites. Here, we introduce a new class of three-dimensional ABX₃ perovskites with a neutral molecule as the X-site, exemplified by (PF₆)Na(1,4-dioxane)₃ and (ClO₄)Ag(1,4-dioxane)₃. Neutral 1,4-dioxane molecules coordinate with monovalent B-site metal cations to form corner-sharing B(1,4-dioxane)₆ octahedra, generating positively charged three-dimensional frameworks that encapsulate spherical A-site counter-anions. Specifically, (PF₆)Na(1,4-dioxane)₃ undergoes a 4/mmmF2/m ferroelastic transition at 290 K. Meanwhile, (ClO₄)Ag(1,4-dioxane)₃ exhibits successive m $\overline{3}$ mF4/mmm and 4/mmmF2/m ferroelastic transitions at 302 and 292 K, accompanied by clearly observable stripe-like ferroelastic domains and switchable birefringence. Notably, constructing ABX₃ perovskites with a neutral X-site enables stable 3D frameworks within the Goldschmidt tolerance criteria and greatly expands chemical flexibility, allowing precise modulating of structural and functional properties. The introduction of this fundamentally new perovskite subclass opens up vast possibilities for achieving exotic properties and functionalities, further enriching this highly promising material platform.