Ba7(BO3)3GeO4X (X = Cl, Br): borogermanate halides with rigid GeO4 tetrahedra and flexible XBa6 octahedra

Abstract

The first borogermanate halides, Ba₇(BO₃)₃GeO₄X (X = Cl, Br), have been synthesized using a high temperature solution method. Ba₇(BO₃)₃GeO₄X (X = Cl, Br) are isostructural and crystallize in the orthorhombic space group Pbam (no. 55). They feature a Ba-based 3D framework, while isolated GeO₄ tetrahedra and BO₃ triangles fill in the space created by the framework. Significantly, Ba₇(BO₃)₃GeO₄X (X = Cl, Br) possess a similar formula to the borosilicate halides, Ba₇(BO₃)₃SiO₄X (X = Cl, Br), but they are not isostructural. An interpretation for the structural transformation between them is demonstrated here, considering rigid GeO₄ tetrahedra and flexible XBa₆ (X = Cl, Br) octahedra. IR spectroscopy, UV-Vis-NIR diffuse reflectance spectroscopy and first-principles calculations have been carried out on the title compounds.