Understanding the complex structural features and phase changes in Na2Mg2(SO4)3: A combined single crystal and variable temperature powder diffraction and Raman spectroscopy study

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, Na2Mg2(SO4)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 Na2Mg2(SO4)3 can only be prepared by quenching from high temperature, with slow cooling leading to phase separation to give the previously unreported systems, Na2Mg(SO4)2 and Na2Mg3(SO4)4. We report the structures of quenched Na2Mg2(SO4)3 (monoclinic, ), as well as Na2Mg(SO4)2 (triclinic, ̅) and Na2Mg3(SO4)4 (orthorhombic, , detailing their complex structural features. Furthermore, we report a study of the thermal evolution of quenched Na2Mg2(SO4)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.We would like to thank the Bolashak International Scholarship Fund, Kazakhstan (PhD scholarship for Ivan Trussov) for funding. The work has been supported by the Spanish Ministerio Economía y Competitividad and Feder funds through grant MAT2016-77769R as well.Peer reviewe