Minerals to Functional Materials: Comprehensive Evaluation of Vanthoffites and Related Materials towards Futuristic Multifunctional Applications
The thesis provides a systematic investigation of materials derived from different mineral species such as vanthoffite and related minerals structures. Search of new multifunctional materials continues to be a challenge and in this context, these naturally abundant mineral species offer a rich source. The phase modification in mineral structure leads to a variety of multifunctional materials like fast ion conductors, magnetic and multiferroic materials. Chapter 1 provides a brief outline of mineral sources and their plausible conversion to materials with desired properties. Chapter 2 discusses the dehydration pathway of tetra-hydrate/di-hydrate/anhydrous phase in Na6Co(SO4)4 characterized via in situ variable temperature single-crystal X-ray diffraction analysis. In addition other cognate techniques such as thermal analysis, variable temperature in situ powder X-ray diffraction and variable temperature in situ Raman spectroscopy have been used to relate the structural features with properties, in particular ionic conductivity. The material becomes a superionic conductor (σ =1.1× 10-2 S/cm) at 570 °C. The magnetic behaviour of vanthoffite series, Na6M(SO4)4 (M= Mn, Co, Ni, Cu), have been investigated in chapter 3. It is observed that among these materials, Na6Mn(SO4)4 shows long-range antiferromagnetic (AFM) ordering below 3 K as investigated by detailed neutron diffraction study and verified using theoretical analysis. Interestingly, Na6Cu(SO4)4 was synthesized serendipitously by the solid-state method. Chapter 4 presents the structural phase transition and ionic conductivity of the end member of the vanthoffite series, Na6Zn(SO4)4. It is observed based on differential scanning calorimetric measurements that in all anhydrous analogues, the structural transition temperature depicts an inverse behaviour with increasing atomic number of the transition element. In addition, Na6Zn(SO4)4 shows an unusual multiple meta-stable phases with temperature as confirmed by variable temperature in situ powder X-ray diffraction and variable temperature in situ Raman spectroscopic studies. The studies are extended beyond Na+ ion rich oxo-sulfate in chapter 5 which explores the mineral puninite, Na2Cu3O(SO4)3 as a possible source as an electrolyte for battery applications. The features associated with DSC plots and conductivity measurements indicate a possible phase transition in this volcanic mineral which needs to be analysed in the future. Overall, the present work clearly establishes that the rich source of sodium-containing minerals can be exploited to produce materials with properties of choice and this fact opens up the possibility of exploring structure-property correlation in many other Na ion containing mineral species.