Phase Transitions in a Vanthoffite-Type Compound, Na₆Zn(SO₄)₄: Insights from <i>In Situ</i> PXRD and Raman Spectroscopy

The study of phase transitions in minerals or synthetic compounds analogous to mineral structures is of fundamental interest in different scientific disciplines, with applications ranging from understanding the Earth's geological history to advancing materials science and technology. They provide valuable data and insight into developing new functional materials with desired properties. In this context, we have investigated the structural phase transitions of Na₆Zn(SO₄)₄, a synthetic compound analogous to the Vanthoffite mineral Na₆Mg(SO₄)₄, which occurs abundantly in nature as oceanic salt deposits. The room temperature crystal structure is refined from powder X-ray diffraction (PXRD) data, and it belongs to a monoclinic system, space group <i>P</i>2₁/<i>c</i>, with <i>Z</i> = 2. Differential scanning calorimetry analysis suggests the possibility of multiple structural phase transitions between 338 and 400 °C. These phase transitions are substantiated by <i>in situ</i> powder X-ray diffraction and Raman spectroscopy. PXRD data with temperature reveal structural phase transitions with a change in crystal symmetry accompanied by the appearance/disappearance of diffraction peaks. Further, the dynamics of the SO₄ tetrahedra units are probed using variable temperature Raman spectroscopy to understand the mechanism of structural phase transitions. These phase transitions are driven by the anomalies of the molecular vibration of sulfate tetrahedra at higher temperatures as revealed from Raman data. Additionally, ionic conductivity measurements also depict these structural phase transitions with a change in slope with an increase in temperature.