Project 1: Preparation of new series of solid solutions on the basis of pnictides in order to enlarge representatives of the 3D Topological Dirac Semimetals (3D TDS). Thermodynamic study of binary pnictides by the electromotive force measurements (EMF) method

In the transition from insulators to metals, there exists an intermediate state, where the conduction and valence band touch only at discrete points, leading to a narrow or zero bandgap. Graphene is a well-known example for such materials. Narrow or zero gap semiconductors where two (or more) bands get strongly coupled near a level-crossing are called Dirac materials. Prominent examples of Dirac matter are graphene, topological insulators, Dirac semimetals, Weyl semimetals, various high-temperature superconductors etc.

Three-dimensional (3D) topological Dirac semimetals (TDSs) are considered to be a newly proposed state of quantum matter that have attracted immense attention in physics and materials science. Unique electronic structure makes 3D TDSs prominent base materials for unusually high bulk carrier mobility, high-temperature linear quantum magnetoresistance, quantum spin Hall effect and giant diamagnetism. It also helps to realize various applications of graphene in 3D materials. Thus, 3D TDS materials, sometimes noted as the topological bulk Dirac semimetal (BDS) possessing strong spin-orbit coupling and exhibiting many exotic quantum phenomena constitute one of the most active topics in modern materials science. Despite their predicted existence, experimental studies on the BDS phases have been lacking as it has been difficult to realize this phase in real materials, especially in stoichiometric single crystalline non-metastable system with high mobility.

Recently, it was proposed that some binary (Cd3As2, Zn3As2) and ternary pnictides (CaAuAs and its isostructural family, e.g., BaAgAs) are 3D TDSs . Systematical study of the electronic structure of Cd3As2 by the angle-resolved photoemission spectroscopy (ARPES) confirms that Fermi velocity of the 3D Dirac fermions in Cd3As2 is about 3 times higher than that of in the topological surface states of Bi2Se3, 1.5 times higher than in graphene. Observed large Fermi velocity of the 3D Dirac band explains unusually high mobility of Cd3As2 and creates opportunity to the unusual magneto-electrical and quantum Hall transport properties under high-magnetic field of this compound. The distinct electronic structure of this compound gives rise to very promising applied properties, which are important due to potential application in new generation of electronic devices. Cd3As2 exhibits very high mobility charge carrier and large thermoelectric power (S). At 350 K temperature, the value of ZT of this compound has been found to be large ~ 0.16, which is comparable to the well-known thermoelectric material Bi2Te3 .

Thus, a new class of inorganic functional materials – three-dimensional (3D) topological Dirac semimetals based on metal pnictides due to their extraordinary physical properties have extensive

and rational application capabilities, ranging from electronics, spintronics and quantum calculations to medicine and security systems. In this regard, the substantial expansion of the nomenclature of such substances and their modified phases on basis of a complex of information relating to phase diagrams and thermodynamic characteristics of relevant systems is a topical issue.

Considering above-stated facts, proposed research activity is dedicated to the design of new series of Dirac semimetal phases.

In this regard, the project addresses a number of specific issues given below:

Obtaining new solid solution series by replacement of atoms in already known binary and ternary pnictide molecules (Cd3As2, Zn3As2, CaAuAs, BaAgAs etc.) by new similar atoms. For instance, in the Cd3As2 Dirac semimetal, displacement of Cd atoms by Zn, Hg, Sn etc. bivalent elements, as well as displacement of As atoms by P and Sb can help to prepare new materials possessing superior and controllable functional properties while maintaining crystal structure of Cd3As2.

Thermodynamic data are key in the understanding and design of chemical processes, as well as, in enhancement of the synthesis and crystal growth technologies of chemical materials. Since fundamental thermodynamic properties of pnictides are not studied well, it is intended to calculate their thermodynamic properties by the electromotive force (EMF) measurements.