In situ X-ray Scattering of the Crystallisation of Basic Magnesium Chlorides using a Laboratory Instrument

Abstract

We demonstrate a method for in situ monitoring of the crystallisation of basic magnesium chlorides using a laboratory-based SAXS (small angle X-ray scattering)/ WAXS (wide angle X-ray scattering) instrument. By simultaneous acquisition of SAXS/WAXS, time-resolved particle size and phase evolution information was obtained from room temperature to 120 °C. The WAXS data were analysed using two-phase Rietveld refinements, to produce crystallisation curves. From Avrami-type kinetic analysis two competing mechanistic processes were proposed for the formation of Mg₃Cl(OH)₅ ⋅ 4 H₂O with a nucleation-type mechanism extending further into the reaction with increased temperature. When comparing SAXS and WAXS, an offset between the consumption of MgO and the reduction of the sphere contribution to the SAXS scattering is observed. This is rationalised by the formation of an amorphous Mg(OH)₂ layer on the MgO particle surface. Although laboratory-based SAXS/WAXS instruments have limitations compared to synchrotron-based sources, we have demonstrated how they can provide new insights into the formation of materials.