New polymorphism for Ba${}_3$Zn${}_2$(BO${}_3$)${}_3$F with two polymorphs exhibiting anomalous phase transition

A new polymorph of Ba${}_3$Zn${}_2$(BO${}_3$)${}_3$F, α-Ba${}_3$Zn${}_2$(BO${}_3$)${}_3$F, was synthesized and its structure was determined by single crystal X-ray diffraction. The two polymorphous phases of Ba${}_3$Zn${}_2$(BO${}_3$)${}_3$F are referred to as α- and β‐Ba${}_3$Zn${}_2$(BO${}_3$)${}_3$F according to their synthesis temperature from low to high. Both structures exhibit a similar three-dimensional (3D) framework, while the two-dimensional (2D) ${}_{\infty }^2$[Zn${}_2$B${}_2$O${}_7$] layers in their structures show different configurations and feature diverse topological structures. Interestingly, an anomalous phase transition, that is, the high-temperature β-phase having a higher density, was observed and further confirmed by detailed structural analyses, which shows that the unusual density evolution mainly originates from the different configurations of ${}_{\infty }^2$[Zn${}_2$B${}_2$O${}_7$] layers. Moreover, the phase transformation was characterized by powder X-ray diffraction (PXRD) and differential scanning calorimetry (DSC) analyses. (© 2021 Wiley‐VCH GmbH)