Sr5(CO3)2(BO3)2: A new family member of isostructural mixed borate and carbonate Ba4M(BO3)2(CO3)2 (M = Ba, Sr) with isolated BO3 and CO3 groups

Abstract

A new mixed borate and carbonate Sr5(CO3)2(BO3)2, which is isostructural with Ba4M(BO3)2(CO3)2 (M = Ba, Sr), has been synthesized by hydrothermal method. It crystallizes in Pnma (No. 61) space group with the lattice parameters a = 7.387(4), b = 16.556(9) and c = 8.971(5) Å (Z = 4). This structure features a three-dimensional (3D) network including SrO8 polyhedra, isolated BO3 and CO3 triangles. UV-Vis-NIR diffuse reflectance spectrum for Sr5(CO3)2(BO3)2 indicates that it has a deep ultra-violet cut-off edge of 190 nm. In addition, crystal structural comparison among mixed borates and carbonates shows that the interaction between boron-oxide groups and CO3 groups leads to different arrangements of anionic groups, which can be classified into three types according to the connection of boron-oxide groups. An analysis of theoretical calculations based on density functional theory was also performed to understand the relationships between the structures and properties.