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The present study unveils the topological insulating nature of Th-based oxy-chalcogenides and their transport properties which are less explored. A systematic analysis of electronic, topological, mechanical, dynamical and thermoelectric properties of ThOCh (Ch: S, Se and Te) in bulk and monolayer is presented. The effect of spin-orbit coupling is found to be appreciable in ThOTe compared to ThOS and ThOSe, causing a strong topological nature in bulk ThOTe. The detailed analysis of electronic structure, Z2 topological invariant and conducting surface states support the strong topological nature in bulk ThOTe. From thermoelectric studies, ThOS and ThOSe are found to be good thermoelectric candidates with heavy carrier doping (around 1020 cm-3). To explore further, we have applied hydrostatic strain on bulk ThOCh and found that all the compounds are dynamically stable and show topological metallic behavior. The appearance of highly linearized Dirac points in the same energy range at different high symmetry points in the BZ indicate the presence of nodal line in ThOS and ThOSe without spin-orbit coupling and with the inclusion of spin-orbit coupling, the nodal line is found to be disappear. This variation in bands with and without spin-orbit coupling might indicate the topological nature in monolayer ThOCh. The thermoelectric calculations for monolayer shows an enhancement in electrical conductivity scaled by relaxation time by an order of ten compared to bulk and the carrier independent thermoelectric properties in monolayer might fetch good thermoelectric device applications. Overall, the present study explores yet another series of potential candidates for topological and thermoelectric properties in both bulk and layer forms.

The present density functional calculations propose the compounds CaSrX (X: Si, Ge, Sn, Pb) as strong topological insulators, with appreciable thermoelectric properties. Emergence of Dirac semi-metallic states has been observed in CaSrX (X: Si, Ge, Sn, Pb), which is induced by uni-axial strain along 'b' axis. CaSrSi and CaSrGe evolved as normal semiconductors with uni-axial strain. The trivial and non-trivial topological phases are evaluated by band inversion and Z 2 topological invariants. A comprehensive analysis of thermopower, electrical conductivity scaled by relaxation time at these Dirac semi-metallic states exposes the highly oscillating behaviour, which gives insight to quantum oscillations driven by uni-axial strain. Further the thermoelectric properties at strong topological insulating states and normal insulating states have been summarized, which reveals the potential thermoelectric properties of these materials.

We present a highly versatile system ReS2, which transforms from a semiconductor to a two dimensional metal under uni-axial compressive strain along ‘a’ direction in both bulk and monolayer. The 2D nature is realised from highly flat Fermi surfaces and anisotropic transport properties. Moreover the layer independent electronic structure properties are revisited and thermoelectric properties of ReS2 in bulk, monolayer and bilayer forms reveal the competing thermoelectric (TE) coefficients in each form. The in-plane power-factor shows an enhancement over ‘c’-axis value as a function of strain, which is almost two orders of magnitude. In addition, strain induced tunable in-plane anisotropy of almost one order has been observed in both bulk and monolayer ReS2 (around 20%), which further open up the possibility of TE application as nanowires. Our analysis unveils a wide range of application for ReS2 in the field of thermoelectrics as bulk and thin films for a large temperature range. The magnitude of TE coefficients are comparable with other well established transition metal dichalocogenides.

In this work, we report the structural, magnetic, electrical and thermal transport properties of the Heusler-type alloy Ru2NbAl. From the detailed analysis of magnetization data, we infer the presence of superparamagnetically interacting clusters with a Pauli paramagnetic background, while short-range ferromagnetic interaction is developed among the clusters below 5 K. The presence of this ferromagnetic interaction is confirmed through heat capacity measurements. The relatively small value of electronic contribution to specific heat gamma (similar to 2.7 mJ mo1(-1) K-2 ) as well as the linear nature of temperature dependence of the Seebeck coefficient indicate a semimetallic ground state with a pseudogap, which is also supported by our electronic structure calculations. The activated nature of resistivity is reflected in the observed negative temperature coefficient and has its origin in the charge carrier localization due to antisite defects, inferred from magnetic measurements as well as structural analysis. Although the absolute value of thermoelectric figure of merit is rather low (ZT = 5.2 x 10(-3) ) in Ru2NbAl, it is the largest among all the reported nondoped full Heusler alloys.

We report the structural, vibrational and electrical transport properties up to 16 GPa of the 1T-TiTe2, a prominent layered 2D system, which is predicted to show a series of topologically trivial - nontrivial transitions under hydrostatic compression. We clearly show signatures of two iso-structural transition at 2 GPa and 4 GPa obtained from the minima in c/a ratio concomitant with the phonon linewidth anomalies of Eg and A1g modes at around the same pressures, providing strong indication of unusual electron-phonon coupling associated to these transitions. Resistivity presents nonlinear behavior over similar pressure ranges providing a strong indication of the electronic origin of these pressure driven isostructural transitions. Our data thus provide clear evidences of topological changes at A and L point of the Brillouin zone predicted to be present in the compressed 1T-TiTe2. Between 4 GPa and 8 GPa, the c/a ratio shows a plateau suggesting a transformation from an anisotropic 2D layer to a quasi 3D crystal network. First principles calculations suggest that the 2D to quasi 3D evolution without any structural phase transitions is mainly due to the increased interlayer Te-Te interactions (bridging) via the charge density overlap. In addition to the pressure dependent isostructural phase transitions, our data also evidences the occurrence of a first order structural phase transition from the trigonal (P-3m1) phase at higher pressures. We estimate the start of this structural phase transition to be 8 GPa and the symmetric of the new high-pressure phase to be monoclinic (C2/m).

We report a detailed first principles density functional calculations to understand the electronic structure and transport properties of Zn-based pnictides ZnXPn 2 (X: Si, Ge, and Sn; Pn: P and As) and ZnSiSb 2 . The electronic properties calculated using Tran-Blaha modified Becke-Johnson func- tional reveals the semi-conducting nature, and the resulting band gaps are in good agreement with experimental and other theoretical reports. We find a mixture of heavy and light bands in the band structure which is an advantage for good thermoelectric (TE) properties. The calculated transport properties unveils the favour p-type conduction in ZnXP 2 (X: Si, Ge, and Sn) and n-type conduc- tion in ZnGeP 2 and ZnSiAs 2 . Comparison of transport properties of Zn-based pnictides with the prototype chalcopyrite thermoelectric materials implies that the thermopower values of the investi- gated compounds to be higher when compared with the prototype chalcopyrite thermoelectric materials, together with the comparable values for electrical conductivity scaled by relaxation time. In addition to this, Zn-based pnictides are found to possess higher thermopower than well known traditional TE materials at room temperature and above which motivates further research in these compounds.

First-principles calculations were carried out to study the structural, mechanical, dynamical and transport properties of zintl phase materials CaLiPn (Pn=As, Sb and Bi). We have used two different approaches to solve the system based on density functional theory. The plane wave pseudopotential approach has been used to study the structural and dynamical properties whereas, full potential linear augment plane wave method is used to examine the electronic structure, mechanical and thermoelectric properties. The calculated ground-state properties agree quite well with experimental values. The computed electronic structure shows the investigated compounds to be direct band gap semiconductors. Further, we have calculated the thermoelectric properties of all the investigated compounds for both the carriers at various temperatures. We found a high thermopower for both the carriers, especially n-type doping to be more favourable, which enabled us to predict that CaLiPn might have promising applications as a good thermoelectric material. Further, the phonon dispersion curves of the investigated compounds showed flat phonon modes and we also find lower optical and acoustic modes to cut each other at the lower frequency range, which further indicate the investigated compounds to possess reasonably low thermal conductivity. We have also analysed the low value of the thermal conductivity through the empirical relations and discussions are presented here.