FIRST-PRINCIPLES STUDY OF ELECTRONIC, THERMAL, AND ANOMALOUS TRANSPORT IN TERNARY COMPOUNDS

Abstract

The fast-growing energy consumption in every realm of our life points to the need for clean and sustainable energy resources. For this thermoelectric (TE) materials (harnessing the waste energy) are rated the most exemplary kind owing to the worldwide presence of thermal energy in the form of waste heat and have been used in a wide range of applications starting from watchbands to space science and technology. TE devices play an important role in harnessing waste heat and producing electricity. The hunt for finding superior TE materials is still underway due to their inefficient performance. This thesis focuses on finding new materials, operational for a broad range of temperatures, with the help of Density Functional Theory (DFT) based tools. DFT helps to determine the properties of materials and their electronic structure and has played a pivotal role in theoretical condensed matter physics. In the present thesis, the structural, electronic, and thermal transport properties of some intriguing classes of compounds have also been explored. Attempts have been made to play around with their transport properties through doping and some exotic properties such as anomalous Hall conductivity (AHC) and spin Hall conductivity (SHC) for spintronics applications, have been investigated. Chapter 1 deals with the background of TE materials and discusses various kinds of TE effects. Chapter 2 deals with the mathematical equations and computational techniques used to calculate the properties of the studied materials. Basically, this chapter discusses the many-body problem and different approaches, as well as approximations to solve the related equations. In this thesis, all calculations have been carried out using the pseudopotential method implemented in the DFT-based software VASP. In Chapter 3, transport properties of two ternary centrosymmetric XAgP compounds (X= Sr and Ba) crystallized in the hexagonal structure with a space group (No. 194) have been thoroughly studied using the linearized Boltzmann transport equation with a singlemode relaxation time approach. Both the compounds are predicted to be stable (confirmed by phonon dispersion) with a narrow band gap ∼0.10 eV, which is within the confines for superior TE performance. Further, the mechanical strength of the materials is a prerequisite for practical applications, therefore the mechanical stability has been affirmed by calculating the elastic constants. The Zintl-concept based sheets of anionic AgP in these two compounds lead to lattice anharmonicity with surprisingly low lattice thermal conductivity (κl). Literature shows that degenerate bands are favorable for TE transport properties. Interestingly, the Valence band maximum (VBM) in the two studied compounds is twofold degenerate which hints towards superior electronic properties for p-type doping. The electrical conductivity (hence the power factor, PF) and electronic thermal conductivity are scaled by electronic relaxation time. The relaxation time has been derived from the deformation potential theory. At 300 K, the power factor of p-type SrAgP (40 μ W cm−1 K−2) is comparable to that of excellent TE performing Heusler compound FeNbSb at 1100 K (39.8 μ W cm−1 K−2); in comparison the power factor of p-type BaAgP at 300 K is 10 μ W cm−1 K−2. Notably, at 500 K, BaAgP can be used as both p-type and n-type thermoelectric legs, the zT values being 0.19 and 0.15, respectively. SrAgP has an excellent zT (=0.6) at 500 K. As the theoretical results, resorting to several approximations, usually give underestimated values, hence these results may be regarded to reliably project the potential of SrAgP and BaAgP for the TE applications. In Chapter 4, the dynamical stability, and structural and TE properties of experimentally grown hexagonal pnictide, CaAgP (space group no. 189) have been explored via DFT. Earlier studies show that CaAgP has a ring-torus Fermi surface and that a linear dispersion at the Fermi energy makes it a promising candidate for line-node Dirac semimetal. However, another study based on hybrid density functional theory and ARPES measurements shows that the bulk band gap along with large bandwidth surface states renders CaAgP as topologically trivial. In the present study, the calculated results show that simple PBE-GGA functional yields a topological band structure; however, with the hybrid Heyd-Scuseria- Ernzerhof (HSE) functional, a semiconducting nature has been found, in agreement with experimental observation. The right choice of exchange-correlation functional is thus essential and therefore the detailed analysis of the electronic structure and electronic transport properties has been carried out using the HSE06 functional. The estimated bandgap for CaAgP using HSE is ∼ 0.15 eV which is appealing for applications as TE devices. It is interesting to note that the estimated κl of CaAgP is 1.38 Wm−1K−1 at room temperature which is lower than that for commonly known thermoelectrics, e.g., PbSe (2.64 Wm−1K−1) and PbTe (2.30Wm−1K−1). Three appropriate temperatures have been picked out, i.e., 500, 700, and 900 K, with regard to the TE performance for the estimation of transport coefficients. For each temperature, the highest PF has been obtained for hole doping (for 0.0015 e/uc at 500 K; for 0.0025 e/uc at 700 K; and for 0.005 e/uc at 900 K, here e/uc represents the electron per unit cell). The corresponding doping quantity is so small that it can be easily achieved experimentally. The maximum value of zT is obtained as 0.73 (for the p-type doping), corresponding to 900 K. So overall, the p-doped CaAgP has been predicted to be a better performer with regard to the thermoelectrics. These zT values are quite encouraging, since, generally, a considerable decline in κl is observed with doping and hence the κl for the doped CaAgP is expected to be lower than the value for the pristine compound which has been employed in the calculation of zT for the doped material. Hence these zT values are underestimated ones and CaAgP is expected to perform even better experimentally. In the 5th chapter, static, dynamical, thermodynamical, and mechanical stability of a half-metallic ferromagnetic half-Heusler compound c-CoFeSn have been investigated along with its structural and magnetic properties. It crystallizes in Ni2In-type hexagonal symmetry with space group no. 194. Based on the inferences from structural stability, lattice dynamics, and magnetic analysis, a cubic polymorph of hexagonal CoFeSn has been proposed. The DFT calculations indicates a robust 3D half-metallic ferromagnetic compound, CoFeSn (P¯43m) with a Tc ∼ 693 K, calculated via the Heisenberg magnetic exchange interactions under mean-field approximation, and a magnetic moment of 3 μB. In addition, Wannier interpolation suggests anomalous Hall conductivity (AHC) and spin Hall conductivity (SHC) in cubic CoFeSn: the largest SHC at the Fermi level being ≈ 47 (h/2πe) S/cm. These theoretical results show that spin-orbit interaction at the Fermi level brings on finite Berry flux that gives rise to an intrinsic AHC ∼ 122 S/cm at room temperature. Finally, the last chapter addresses the conclusions of the thesis in addition to a quick overview of prospective planning.