Na3B4O7X (X = Cl, Br): two new borate halides with a 1D Na-X (X = Cl, Br) chain formed by the face-sharing XNa6 octahedra

Abstract

Two new sodium borate halides, namely, Na₃B₄O₇X (X = Cl, Br), have been successfully synthesized through high-temperature solid state reactions. Single-crystal X-ray diffraction analysis reveals that both of them crystallize in the same space group P6₅22 of the hexagonal system. In their crystal structures, the B₄O₉ groups link with each other by oxygen-sharing to form a three dimensional (3D) framework containing channels viewing along the [001] direction, in which the 1D helical [Na₃X] chain formed by the face-sharing XNa₆ octahedra resides. To the best of our knowledge, the 1D helical [Na₃X] chain has not been found in other alkali metal borate halides. From another perspective, the two compounds can also be regarded as NaX salt inclusion borates. In addition, infrared (IR) spectroscopy and bond valence sum (BVS) calculations are used to verify the validity of the structures. Differential scanning calorimetry (DSC) and powder X-ray diffraction (XRD) results prove that both of them are incongruently melting compounds. The electronic structures have been calculated by the first-principle method based on density-functional theory.