Murunskite - a new class of functional material

The subject of the present work is discovery and in-depth characterization of a new class of functional materials. Tuning of the bond polarity and orbital occupation with a goal of establishing balance between localization and delocalization of electrons -ie. between ionicity and metallicity - follows the research paradigm of functional materials discovery with the ambition to introduce quantum effects by design. The main compound investigated in the present dissertation is murunskite, K2FeCu3S4. Large single crystals of murunskite were prepared for the first time ever. Structural characterization confirmed phase purity of high-quality single crystals which were subject to extensive physical and chemical characterization.As-synthesized murunskite is a quasi 2D semiconductor with a band gap of Eg = 0.9 eV, calculated by Density Functional Theory (DFT) calculations and confirmed by optical spectroscopy. Valence and conduction bands are copper and iron-dominated, respectively. Iron was found to be in the mixed oxidation state, with Fe2+ as a dominant species, where iron is reduced on the account of opening sulfur orbitals. Magnetically, murunskite is a complex antiferromagnet (AF) with a well defined transition at 97K while the 2D AF correlations appear below 150K. The magnetic moments order in two sub-lattices with two related incommensurate k-vectors. Mossbauer spectroscopy showed two separate nuclear iron sites at the room temperature allowing us to fully explain the magnetic structure of murunskite.Following the successful synthesis of the parent compound single crystals, further development of the material was achieved in three directions: production of stable murunskite compounds with varying Cu-Fe ratio; replacement of the ligand sulfur for selenium/tellurium; and hole/electron doping on both potassium and iron positions.Single crystals in a solid solution series K2FexCu4-xS4 were obtained in the range x = 1-2.6 showing gradual decrease in electrical resistivity with increasing iron content. In this thesis High-resolution Transmission Electron Microscopy (HRTEM) measurement show the presence of the iron-rich lines. This first in-depth characterization of quaternary chalcogenide structure can explain the previously reported spin-glass behavior for such compounds.The high compositional flexibility of the murunskite family is demonstrated by ligand substitution, and doping synthesis on potassium or iron positions. Electrical resistivity decreases by seven orders of magnitude in sulfur to tellurium exchange with significantly weaker antiferromagnetic 2D correlations. Complete isovalent substitution of K with Rb and Cs is presented showing the possibility to continuously adjust the chalcogenide layers distance. Interestingly, only low level doping is possible with Ba showing electronic resilience of the murunskite system. On the other hand, Co and Mn doped single crystals were grown both for sulfur and tellurium murunskite at different doping levels. Most notable is gradual transition from anti- to ferro-magnetic order by cobalt doping with close to metal-insulator transition (MIT) resistivity behavior. The presented work on the murunskite family of compounds demonstrates that it is possible to tune bond polarity and orbital occupation, i.e. balance between localization and delocalization of electrons. Further investigations may allow fully tuning from insulating to semiconductor and metal and perhaps even superconductivity.