ORIGINAL PAPER Phase transition in Na 2 SO 4 : all five polymorphic transformations in DSC Santosh M. Bobade & P. Gopalan & A. R. Kulkarni Received: 16 May 2008 / Accepted: 6 August 2008 / Published online: 26 August 2008 # Springer-Verlag 2008 Abstract In this investigation, we have characterized the Na 2 SO 4 using differential scanning calorimetry (DSC). It is well known that Na 2 SO 4 exhibits five polymorphs. How- ever, the experimental evidence for all five polymorphs has never been observed in DSC. In this investigation, the five polymorphic transformations have been observed in heating cycle. Furthermore, the transition I–II is observed, which is not reversible. In addition, phase transition temperature for V→IV and IV→III transition appears to be much closed and may get unnoticed. Keywords Electrolyses . Thermal analyses . Na conductors Introduction The Na 2 SO 4 has extensively been studied as an alternative electrolyte material and has been found useful in SO x sensor. The electrical conductivity of Na 2 SO 4 approaches to 10 -5 S/cm at 300°C, and if this is improved, it can really be useful for the various electrochemical devices. Various polymorphic transformation schemes in Na 2 SO 4 have been suggested by Kareck [1]. It is known that the Na 2 SO 4 exhibits five polymorph, out of them Na 2 SO 4 -V, III, and I are well established. Recently, the existence of phase II has also been confirmed [2]. The room temperature polymorph is the Na 2 SO 4 -V [1, 3], which crystallizes in the orthorhombic structure (space group Fddd). The Na 2 SO 4 -III also exhibits orthorhombic crystal structure [4] (space group Cmcm), and the high temperature stable phase above 250°C shows hexagonal structure [5] (space group P 6/3 mmc). The various investigations have been reported on high-temperature crystal structure of the Na 2 SO 4 . Hofer et al. [6, 7] have provided a structural model that also explains the conduction mechanism in the Na 2 SO 4 . The electrical behavior of the Na 2 SO 4 is of great interest due to its low transition enthalpy and low transition temperature among known alkali sulfates. Being a low transition enthalpy and a transition temperature, it will be easy to stabilize the high-temperature phase. It has been observed that Y 2 (SO 4 ) 3 dopant stabilizes the high-temperature hexagonal structure at room temperature [6, 7]. In several investigations, the Na 2 SO 4 -III, a meta- stable phase, has been observed. The structural transformation in Na 2 SO 4 is still a subject of debate. Phase III is usually observed in differential scanning calorimetry (DSC) and electrical measurement. Furthermore, it exists in a narrow temperature range, and sometimes, it goes unnoticed during conductivity measure- ment. Phases II and IV are not generally observed. In this investigation, DSC of a polycrystalline Na 2 SO 4 has been studied in the heating, cooling, and reheating cycles, and the observations are reported. In this study, we have investigated DSC analysis for polycrystalline Na 2 SO 4 . Experimental The Na 2 SO 4 of 99.99% purity was procured from Aldrich Chem. The DSC has been carried out in a temperature range between 25°C and 300°C with a heating rate of 10°C per minute. The DSC was carried out in the heating, cooling, and reheating cycles. The DSC data was fitted to obtain the enthalpy of each transition and transition Ionics (2009) 15:353–355 DOI 10.1007/s11581-008-0272-6 S. M. Bobade (*) : P. Gopalan : A. R. Kulkarni Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India e-mail: santoshbobade@gmail.com