Phase Transitions\nin a Vanthoffite-Type Compound,\nNa<sub>6</sub>Zn(SO<sub>4</sub>)<sub>4</sub>: Insights from <i>In Situ</i> PXRD and Raman Spectroscopy

The study of phase transitions in\nminerals or synthetic\ncompounds\nanalogous to mineral structures is of fundamental interest in different\nscientific disciplines, with applications ranging from understanding\nthe Earth’s geological history to advancing materials science\nand technology. They provide valuable data and insight into developing\nnew functional materials with desired properties. In this context,\nwe have investigated the structural phase transitions of Na<sub>6</sub>Zn(SO<sub>4</sub>)<sub>4</sub>, a synthetic compound analogous to\nthe Vanthoffite mineral Na<sub>6</sub>Mg(SO<sub>4</sub>)<sub>4</sub>, which occurs abundantly in nature as oceanic salt deposits. The\nroom temperature crystal structure is refined from powder X-ray diffraction\n(PXRD) data, and it belongs to a monoclinic system, space group <i>P</i>2<sub>1</sub>/<i>c</i>, with <i>Z</i> = 2. Differential scanning calorimetry analysis suggests the possibility\nof multiple structural phase transitions between 338 and 400 °C.\nThese phase transitions are substantiated by <i>in situ</i> powder X-ray diffraction and Raman spectroscopy. PXRD data with\ntemperature reveal structural phase transitions with a change in crystal\nsymmetry accompanied by the appearance/disappearance of diffraction\npeaks. Further, the dynamics of the SO<sub>4</sub> tetrahedra units\nare probed using variable temperature Raman spectroscopy to understand\nthe mechanism of structural phase transitions. These phase transitions\nare driven by the anomalies of the molecular vibration of sulfate\ntetrahedra at higher temperatures as revealed from Raman data. Additionally,\nionic conductivity measurements also depict these structural phase\ntransitions with a change in slope with an increase in temperature.