Integrated computational materials discovery of silver doped tin sulfide as a thermoelectric material.

Accelerating the discovery of new materials is crucial for realizing the vision of need-driven materials development. In the present study we employ an integrated computational and experimental approach to search for new thermoelectric materials. High-throughput first principles calculations of thermoelectric transport coefficients are used to screen sulfide compounds conforming to the boundary conditions of abundant and innocuous components. A further computational screening step of substitutional defects is introduced, whereby SnS doped with monovalent cations is identified as having favorable transport properties. By silver doping of SnS under S-rich conditions an electric conductivity more than an order of magnitude higher than reported previously is realized. The obtained thermoelectric power-factor at room temperature is comparable to the state of the art for thermoelectric materials based on earth abundant, non-toxic elements. The high-throughput screening of extrinsic defects solves a long standing bottleneck in search of new thermoelectric materials. We show how the intrinsic carrier concentration in the low-temperature phase of SnSe is two orders of magnitude higher than in SnS. We furthermore find that the carrier concentration in SnSe can still be further optimized by silver doping.

High-Throughput Screening of All-d-Metal Heusler Alloys for Magnetocaloric Applications.

Due to their versatile composition and customizable properties, A2BC Heusler alloys have found applications in magnetic refrigeration, magnetic shape memory effects, permanent magnets, and spintronic devices. The discovery of all-d-metal Heusler alloys with improved mechanical properties compared to those containing main group elements presents an opportunity to engineer Heusler alloys for energy-related applications. Using high-throughput density-functional theory calculations, we screened magnetic all-d-metal Heusler compounds and identified 686 (meta)stable compounds. Our detailed analysis revealed that the inverse Heusler structure is preferred when the electronegativity difference between the A and B/C atoms is small, contrary to conventional Heusler alloys. Additionally, our calculations of Pugh ratios and Cauchy pressures demonstrated that ductile and metallic bonding are widespread in all-d-metal Heuslers, supporting their enhanced mechanical behavior. We identified 49 compounds with a double-well energy surface based on Bain path calculations and magnetic ground states, indicating their potential as candidates for magnetocaloric and shape memory applications. Furthermore, by calculating the free energies, we propose that 11 compounds exhibit structural phase transitions and suggest isostructural substitutions to enhance the magnetocaloric effect.

High throughput density functional investigations of the stability, electronic structure and thermoelectric properties of binary silicides.

The structural stabilities of binary Mg-X (X = Si, Ge, Sn) and 4d transition metal silicides Mo-Si and Ru-Si are investigated. The convex hulls of stable alloys are in overall good agreement with the known experimental phase diagrams. It is shown how the Si-rich Ru-Si structures have band gaps at the Fermi-level and how the Ru(2)Si(3) structure is stabilized compared to the corresponding Fe(2)Si(3) structure. We discuss the band structure of Ru(2)Si(3) and show how the anisotropic band masses lead to favorable calculated thermoelectric properties.

Multistep approach to microscopic models for frustrated quantum magnets: the case of the natural mineral azurite.

The natural mineral azurite Cu(3)(CO(3))(2)(OH)(2) is a frustrated magnet displaying unusual and controversially discussed magnetic behavior. Motivated by the lack of a unified description for this system, we perform a theoretical study based on density functional theory as well as state-of-the-art numerical many-body calculations. We propose an effective generalized spin-1/2 diamond chain model which provides a consistent description of experiments: low-temperature magnetization, inelastic neutron scattering, nuclear magnetic resonance measurements, magnetic susceptibility as well as new specific heat measurements. With this study we demonstrate that the balanced combination of first principles with powerful many-body methods successfully describes the behavior of this frustrated material.

Stability predictions of magnetic M2AX compounds.

Based on high throughput density functional theory calculations, we evaluated systematically the stability of 580 M2AX compounds. The thermodynamic, mechanical, and dynamical stability and the magnetic structure are calculated. We found 20 compounds fulfilling all three stability criteria, confirming Cr2AlC, Cr2GeC, Cr2GaC, Cr2GaN, and Mn2 GaC, which have been synthesized. The stability trends with respect to the M- and A-elements are discussed by analyzing the formation energies, indicating that Cr and Mn containing M2AX compounds are more stable than Fe, Co, or Ni containing compounds. Further insights on the stability are obtained by detailed analysis of the crystal orbital Hamilton population (COHP).

Pressure-driven phase transitions in TiOCl and the family (Ca, Sr, Ba)Fe2As2.

Motivated by recent experimental measurements on pressure-driven phase transitions in Mott insulators as well as the new iron pnictide superconductors, we show that first principles Car-Parrinello molecular dynamics calculations are a powerful method to describe the microscopic origin of such transitions. We present results for (i) the pressure-induced insulator to metal phase transition in the prototypical Mott insulator TiOCl as well as (ii) the pressure-induced structural and magnetic phase transitions in the family of correlated metals AFe(2)As(2) (A = Ca, Sr, Ba). Comparison of our predictions with existing experimental results yields very good agreement.

Revision of model parameters for kappa-type charge transfer salts: an ab initio study.

Intense experimental and theoretical studies have demonstrated that the anisotropic triangular lattice as realized in the kappa-(BEDT-TTF)2X family of organic charge transfer salts yields a complex phase diagram with magnetic, superconducting, Mott insulating, and even spin liquid phases. With extensive density functional theory calculations we refresh the link between many body theory and experiment by determining hopping parameters of the underlying Hubbard model. This leads us to revise the widely used semiempirical parameters in the direction of less frustrated, more anisotropic triangular lattices. The implications of these results on the systems' description are discussed.