CaZn(HPO3)2 and Ba2Zn(HPO3)3: novel alkaline-earth zincophosphites with diversified anionic frameworks

Abstract

Two novel alkaline-earth zincophosphites, namely CaZn(HPO₃)₂ and Ba₂Zn(HPO₃)₃, were successfully synthesized under hydrothermal conditions. CaZn(HPO₃)₂ exhibits a three-dimensional (3D) anionic framework with a 6-connected uni-nodal pcu α-Po primitive cubic topology, constructed by unique Zn₂O₈ dimers and HPO₃ pseudo-tetrahedra, while Ba₂Zn(HPO₃)₃ displays one-dimensional (1D) [Zn(HPO₃)₃]⁴⁻ anionic chains. It is worth mentioning that CaZn(HPO₃)₂ represents the first example of an alkaline-earth zincophosphite compound with a 3D framework structure. Our research also revealed the importance of both alkaline earth cation sizes and Zn/P ratios in anionic open-framework formation. The crystal structures of both compounds were further verified by energy dispersive spectroscopy, IR spectroscopy and Raman spectroscopy. Optical diffuse reflectance spectra, coupled with Tauc's fitting, revealed direct bandgaps with energy values of 4.33 and 4.48 eV for CaZn(HPO₃)₂ and Ba₂Zn(HPO₃)₃, respectively, which differ from the prediction of theoretical calculations. Density of states calculations were conducted to reveal the origin of the bandgaps and bond interactions. Both compounds exhibited moderate birefringence values. This work may have implications for the design and synthesis of novel metal phosphites with desired properties.