Predicted thermoelectric properties of olivine-type Fe2GeCh4 (Ch = S, Se and Te)

We present here the thermoelectric properties of olivine-type Fe2GeCh4 (Ch = S, Se and Te) using the linear augmented plane wave method based on first principles density functional calculations. The calculated transport properties using the semi-local Boltzmann transport equation reveal very high thermopower for both S and Se-based compounds compared to their Te counterparts. The main reason for this high thermopower is the quasi-flat nature of the bands at the valence and conduction band edges. The calculated thermopower of Fe2GeCh4 is in good agreement with the experimental reports at room temperature, with the carrier concentration around 1018-1019cm-3. All the investigated systems show an anisotropic nature in their electrical conductivity, resulting in a value less than the order of 102 along the a-axis compared to the b- and c-axes. Among the studied compounds, Fe2GeS4 and Fe2GeSe4 emerge as promising candidates with good thermoelectric performance.Comment: 21 pages, 9 figures, Published in Journa

Electronic structure and Fermi surface topology of binary and ternary compounds

To explore the material properties, electronic structure calculations are very much useful and can be obtained from the well known density functional theory(DFT) calculations. In the present thesis, we have focussed on the Fermi surface calculations and try to link the same with other physical properties. In addition, we have also explored the pressure effect on properties of the system. For the present study, we have selected different types of compounds which are Ni-based Heusler compounds, Nb-based A-15 compounds, Sn-based binary compounds and few magnetic compounds(one Zr-based Heusler compound and other Mn-based compounds). Electronic structure, mechanical, vibrational properties of Ni-based Heusler compounds, Ni2XAl (X=Ti, Zr, Hf, V, Nb, and Ta), Ni2NbGa and Ni2NbSn, are presented both at ambient and under compression. Among the mentioned compounds, Ni2NbAl, Ni2NbGa and Ni2NbSn are experimentally reported as superconductors at ambient and our calculated superconducting transition temperature (Tc) and electron-phonon coupling constant (λep) values are in good agreement with the experiments. In addition, we have predicted superconducting nature in Ni2VAl with electron-phonon coupling constant (λep) around 0.68, which leads to superconducting transition temperature (Tc) around ∼4 K (by using coulomb pseudopotential μ∗ = 0.13), which is a relatively high transition temperature for Ni based Heusler alloys and are compared with other Ni2NbY (Y = Al, Ga and Sn) compounds. From the calculated Fermi surfaces, flat Fermi sheets are observed along X

Pseudo-half-metalicity in the double perovskite Sr2_2CrReO6_6 from density-functional calculations

The electronic structure of the spintronic material Sr$_2$CrReO$_6$ is studied by means of full-potential linear muffin-tin orbital method. Scalar relativistic calculations predict Sr$_2$CrReO$_6$ to be half-metallic with a magnetic moment of 1 $\mu_B$. When spin-orbit coupling is included, the half-metallic gap closes into a pseudo-gap, and an unquenched rhenium orbital moment appears, resulting in a significant increase of the total magnetic moment to 1.28 $\mu_B$. This moment is significantly larger than the experimental moment of 0.9 $\mu_B$. A possible explanation of this discrepancy is that the anti-site disorder in Sr$_2$CrReO$_6$ is significantly larger than hitherto assumed.Comment: 3 Pages, 1 figure, 1 Tabl

Fermi Surface Studies of Co-Based Heusler Alloys: Ab-Initio Study

The electronic, Fermi surface (FS) and magnetic properties of ferromagnetic Heusler alloys Co2XY (X = Cr, Mn, Fe; Y=Al, Ga) have been investigated by means of first principles calculation. Out of these compounds, Co2CrAl is found to be perfectly half-metallic (HM) at ambient. Under pressure HM to nearly HM (NHM) transition is observed around 75 GPa for Co 2CrAl and NHM to HM transition is observed around 40 GPa and 18 GPa for Co2CrGa and Co2MnAl, respectively, while no transition is observed for other compounds under study and is also analyzed from the FS studies. The states at the Fermi level in the majority spin are strongly hybridized Co-d and X-d like states. The majority band FS topology change is observed under pressure for the compounds where we observe a transition, while the minority band FS remain unaltered under pressure for all compounds except in Co2FeGa, where we observed an electron sheet at X point instead of hole pocket at Γ poin

Ferromagnetically correlated clusters in semi-metallic Ru2NbAl Heusler alloy

In this work, we report the structural, magnetic and 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 (~2.7 mJ/mol-K2), as well as the linear nature of temperature dependence of Seebeck coefficient indicate a semi-metallic ground state with a pseudo-gap that 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*10-3) in Ru2NbAl, it is the largest among all the reported non-doped full Heusler alloys.Comment: 25 pages, 14 figure

Phase Stability and Thermoelectric Properties of the Mineral FeS2: An Ab Initio Study

First principles calculations were carried out to study the phase stability and thermoelectric properties of the naturally occurring marcasite phase of FeS$_2$ at ambient condition as well as under pressure. Two distinct density functional approaches has been used to investigate the above mentioned properties. The plane wave pseudopotential approach was used to study the phase stability and structural, elastic, and vibrational properties. The full potential linear augment plane wave method has been used to study the electronic structure and thermoelectric properties. From the total energy calculations, it is clearly seen that marcasite FeS$_2$ is stable at ambient conditions, and it undergoes a first order phase transition to pyrite FeS$_2$ at around 3.7 GPa with a volume collapse of about 3$\%$. The calculated ground state properties such as lattice parameters, bond lengths and bulk modulus of marcasite FeS$_2$ agree quite well with the experiment. Apart from the above studies, phonon dispersion curves unambiguously indicate that marcasite phase is stable under ambient conditions. Further, we do not observe any phonon softening across the marcasite to pyrite transition and the possible reason driving the transition is also analyzed in the present study, which has not been attempted earlier. In addition, we have also calculated the electronic structure and thermoelectric properties of the both marcasite and pyrite FeS$_2$. We find a high thermopower for both the phases, especially with p-type doping, which enables us to predict that FeS$_2$ might find promising applications as good thermoelectric materials.Comment: 10 Figure