Sodium mixed metal sulphates have attracted considerable attention, both in terms of mineralogy and more recently due to interest in Na ion containing materials for battery applications. The phase, Na 2 Mg 2 (SO 4 ) 3 , has been previously reported to undergo a phase change to langbeinite at high temperatures, which is interesting given that usually the langbeinite structure is only adopted when large alkali metal ions, e.g. K, Cs, are present. Nevertheless the room temperature structure of this phase has remained elusive, and so in this work, we report a detailed structural study of this system. We show that room temperature Na 2 Mg 2 (SO 4 ) 3 can only be prepared by quenching from high temperature, with slow cooling leading to phase separation to give the previously unreported systems, Na 2 Mg(SO 4 ) 2 and Na 2 Mg 3 (SO 4 ) 4 . We report the structures of quenched Na 2 Mg 2 (SO 4 ) 3 (monoclinic, P2 1 ), as well as Na 2 Mg(SO 4 ) 2 (triclinic, P1̅) and Na 2 Mg 3 (SO 4 ) 4 (orthorhombic, Pbca), detailing their complex structural features. Furthermore, we report a study of the thermal evolution of quenched Na 2 Mg 2 (SO 4 ) 3 with temperature through variable temperature XRD and Raman studies, which shows a complex series of phase transitions, highlighting why this phase has proven so elusive to characterise previously, and illustrating the need for detailed characterisation of such sulphate systems.
- Single crystal
- Sodium magnesium sulphate