The structure of braitschite, a calcium rare earth borate

Abstract

The crystal structure of braitschite, Ca_6.15Na_0.85RE_2.08[B₆O₇(OH)₃(O,OH)₃]₄(H₂O), is reported here in space group P6/m with unit-cell parameters a = 12.1506(6), c = 7.3678(4) Å, and V = 942.03(8) ų. Data were collected from a single crystal using a MoKα source and a CCD detector, solved by direct methods, and refined to an R factor of 2.81%. The mineral structure consists of hexaborate fundamental building blocks that polymerize along [001] and are bound by Ca²⁺ and REE³⁺ counterions. The framework forms hexagonal channels, which are occupied alternately by Ca²⁺ cations and water molecules. In an investigation of its thermal stability, braitschite maintains its crystallinity to a temperature of 400 °C, after which it undergoes decomposition. Using Rietveld refinements against powder X-ray diffraction data, we were able to track the loss of water molecules in channels and hydroxyl groups in the covalent B-O network with increasing temperature.