Advanced modeling of materials with PAOFLOW 2.0:New features and software design

Recent research in materials science opens exciting perspectives to design novel quantum materials and devices, but it calls for quantitative predictions of properties which are not accessible in standard first principles packages. PAOFLOW, is a software tool that constructs tight-binding Hamiltonians from self consistent electronic wavefunctions by projecting onto a set of atomic orbitals. The electronic structure provides numerous materials properties that otherwise would have to be calculated via phenomenological models. In this paper, we describe recent re-design of the code as well as the new features and improvements in performance. In particular, we have implemented symmetry operations for unfolding equivalent k-points, which drastically reduces the runtime requirements of first principles calculations, and we have provided internal routines of projections onto atomic orbitals enabling generation of real space atomic orbitals. Moreover, we have included models for non-constant relaxation time in electronic transport calculations, doubling the real space dimensions of the Hamiltonian as well as the construction of Hamiltonians directly from analytical models. Importantly, PAOFLOW has been now converted into a Python package, and is streamlined for use directly within other Python codes. The new object oriented design treats PAOFLOW's computational routines as class methods, providing an API for explicit control of each calculation.</p

Origin of low thermal conductivity in In4Se3

In4Se3 is an attractive n-type thermoelectric material for mid-range waste heat recovery, owing to its low thermal conductivity (~ 0.9 W∙m- 1 K- 1 at 300 K). Here, we explore the relationship between the elastic properties, thermal conductivity and structure of In4Se3. The experimentally-determined average sound velocity (2010 m s-1), Young’s modulus (47 GPa), and Debye temperature (198 K) of In4Se3 are rather low, indicating considerable lattice softening. This behavior, which is consistent with low thermal conductivity, can be related to the complex bonding found in this material, in which strong covalent In-In and In-Se bonds coexist with weaker electrostatic interactions. Phonon dispersion calculations show that Einstein-like modes occur at ~ 30 cm-1. These Einstein-like modes can be ascribed to weakly bonded In+ cations located between strongly-bonded [(In3)5+(Se2-)3]- layers. The Grüneisen parameter for the soft-bonded In+ at the frequencies of the Einstein-like modes is large, indicating a high degree of bond anharmonicity and hence increased phonon scattering. The calculated thermal conductivity and elastic properties are in good agreement with experimental results

Jahn-Teller driven electronic instability in thermoelectric tetrahedrite

Tetrahedrite, Cu12Sb4S13, is an abundant mineral with excellent thermoelectric properties owing to its low thermal conductivity. The electronic and structural origin of the intriguing physical properties of tetrahedrite, including its metal-to-semiconductor transition, remains largely unknown. This work presents the first determination of the low-temperature structure of tetrahedrite that accounts for its unique properties. Contrary to prior conjectures, our results show that the trigonal-planar copper cations remain in planar coordination below the metal-to-semiconductor transition. The atomic displacement parameters of the trigonal-planar copper cations, which have been linked to low thermal conductivity, increase by 200% above the metal-to-semiconductor transition. The phase transition is consequence of the orbital degeneracy of the highest occupied 3d cluster orbitals of the copper clusters found inside the sodalite cages in the cubic phase. This study reveals that a Jahn-Teller electronic instability leads to the formation of “molecular-like” Cu57+ clusters and suppresses copper rattling vibrations due to the strengthening of direct copper-copper interactions. Our first-principles calculations demonstrate that the structural phase transition opens a small band gap in the electronic density of states and eliminates the unstable phonon modes. The present results provide insights on the interplay between phonon transport, electronic properties and crystal structure in mixed-valence compounds

Copper‐Rich Thermoelectric Sulfides: Size‐Mismatch Effect and Chemical Disorder in the [TS4]Cu6 Complexes of Cu26T2Ge6S32 (T=Cr, Mo, W) Colusites

International audienceHerein, we investigate the Mo and W substitution for Cr in synthetic colusite, Cu26Cr2Ge6S32. Primarily, we elucidate the origin of extremely low electrical resistivity which does not compromise the Seebeck coefficient and leads to outstanding power factors of 1.94 mW m−1 K−2 at 700 K in Cu26Cr2Ge6S32. We demonstrate that the abnormally long iono‐covalent T–S bonds competing with short metallic Cu–T interactions govern the electronic transport properties of the conductive “Cu26S32” framework. We address the key role of the cationic size‐mismatch at the core of the mixed tetrahedral–octahedral complex over the transport properties. Two essential effects are identified: 1) only the tetrahedra that are directly bonded to the [TS4]Cu6 complex are significantly distorted upon substitution and 2) the major contribution to the disorder is localized at the central position of the mixed tetrahedral–octahedral complex, and is maximized for x=1, i.e. for the highest cationic size‐variance, σ2

Toppling the Transport Properties with Cationic Overstoichiometry in Thermoelectric Colusite: [Cu 26 Cr 2 Ge 6 ] 1+δ S 32

International audienceThe excellent thermoelectric properties of colusite are known to be closely related to the nature of the cations at the core of the tetrahedral-octahedral complexes. Here, we demonstrate that cation overstoichiometry decreases the carrier concentration and also generates structural disorder, which modify the conduction mechanism in a way that resembles the effect of cation-size mismatch. This functionalization of the &quot;Cu26S32&quot; conductive network leads to a high figure of merit of 1.0 at 700 K. This study highlights the importance of the cationic arrangement and furthers our understanding on the fascinating transport properties in colusite

High Power Factors of Thermoelectric Colusites Cu26T2Ge6S32 (T = Cr, Mo, W) Toward Functionalization of the Conductive "Cu-S" Network

International audienceThe introduction of hexavalent T6+ cations in p-type thermoelectric colusites Cu26T2Ge6S32 (T = Cr, Mo, W) leads to the highest power factors among iono-covalent sulfides, ranging from 1.17 mW m(-1) K-2 at 700 K for W to a value of 1.94 mW m(-1) K-2 for Cr. In Cu26Cr2Ge6S32, ZT reaches values close to unity at 700 K. The improvement of the transport properties in these new sulfides is explained on the basis of electronic structure and transport calculations keeping in mind that the relaxation time is significantly influenced by the size and the electronegativity of the interstitial T cation. The rationale is based on the concept of a conductive "Cu-S" network, which in colusites corresponds to the more symmetric parent structure sphalerite. A detailed structural analysis of these colusites shows that the distortion of the conductive network is influenced by the presence in the structure of mixed octahedral-tetrahedral [TS4]Cu-6 complexes where the T cations are underbonded to sulfur and form metal-metal interactions with copper, Cu-T distances decreasing from 2.76 angstrom for W to 2.71 angstrom for Cr. The interactions between these complexes are responsible for the outstanding electronic transport properties. By contrast, the thermal conductivity is not significantly affected

High Power Factors of Thermoelectric Colusites Cu26T2Ge6S32 (T = Cr, Mo, W) Toward Functionalization of the Conductive "Cu-S" Network

International audienceThe introduction of hexavalent T6+ cations in p-type thermoelectric colusites Cu26T2Ge6S32 (T = Cr, Mo, W) leads to the highest power factors among iono-covalent sulfides, ranging from 1.17 mW m(-1) K-2 at 700 K for W to a value of 1.94 mW m(-1) K-2 for Cr. In Cu26Cr2Ge6S32, ZT reaches values close to unity at 700 K. The improvement of the transport properties in these new sulfides is explained on the basis of electronic structure and transport calculations keeping in mind that the relaxation time is significantly influenced by the size and the electronegativity of the interstitial T cation. The rationale is based on the concept of a conductive "Cu-S" network, which in colusites corresponds to the more symmetric parent structure sphalerite. A detailed structural analysis of these colusites shows that the distortion of the conductive network is influenced by the presence in the structure of mixed octahedral-tetrahedral [TS4]Cu-6 complexes where the T cations are underbonded to sulfur and form metal-metal interactions with copper, Cu-T distances decreasing from 2.76 angstrom for W to 2.71 angstrom for Cr. The interactions between these complexes are responsible for the outstanding electronic transport properties. By contrast, the thermal conductivity is not significantly affected

Toppling the Transport Properties with Cationic Overstoichiometry in Thermoelectric Colusite: [Cu26Cr2Ge6](1+delta)S-32

International audienceThe excellent thermoelectric properties of colusite are known to be closely related to the nature of the cations at the core of the tetrahedral-octahedral complexes. Here, we demonstrate that cation overstoichiometry decreases the carrier concentration and also generates structural disorder, which modify the conduction mechanism in a way that resembles the effect of cation-size mismatch. This functionalization of the &quot;Cu26S32&quot; conductive network leads to a high figure of merit of 1.0 at 700 K. This study highlights the importance of the cationic arrangement and furthers our understanding on the fascinating transport properties in colusite

Key Role of d(0) and d(10) Cations for the Design of Semiconducting Colusites: Large Thermoelectric ZT in Cu26Ti2Sb6S32 Compounds

International audienceCu-S-based materials with sphalerite-derivative structures are of interest for their complex cationic distribution, rich crystal structure chemistry, and potential in energy conversion and optoelectronic applications. In this study, a new member of colusite, Cu26Ti2Sb6S32, was designed by exploiting the key role of d(0) (T) and d(10) (M) cations in the sphalerite-derivative structure of Cu26T2M6S32 colusites. We succeeded to incorporate d(0) Ti4+ and d(10) Sb5+ into T and M sites, respectively, with a tetrahedral coordination rarely found for these two cations in solid-state chemistry. The synthesis produced the first semiconducting compound with the colusite structure. In addition, Cu26Ti2Sb6S32 exhibits a low lattice thermal conductivity. Partial substitution of Ge for Sb increased the hole carrier concentration, leading to an enhanced thermoelectric power factor and dimensionless figure of merit (ZT of 0.9 at 673 K). The electronic and phonon structures, responsible for the high thermoelectric performance, were elucidated by first-principles calculations

Key Role of d(0) and d(10) Cations for the Design of Semiconducting Colusites: Large Thermoelectric ZT in Cu26Ti2Sb6S32 Compounds

International audienceCu-S-based materials with sphalerite-derivative structures are of interest for their complex cationic distribution, rich crystal structure chemistry, and potential in energy conversion and optoelectronic applications. In this study, a new member of colusite, Cu26Ti2Sb6S32, was designed by exploiting the key role of d(0) (T) and d(10) (M) cations in the sphalerite-derivative structure of Cu26T2M6S32 colusites. We succeeded to incorporate d(0) Ti4+ and d(10) Sb5+ into T and M sites, respectively, with a tetrahedral coordination rarely found for these two cations in solid-state chemistry. The synthesis produced the first semiconducting compound with the colusite structure. In addition, Cu26Ti2Sb6S32 exhibits a low lattice thermal conductivity. Partial substitution of Ge for Sb increased the hole carrier concentration, leading to an enhanced thermoelectric power factor and dimensionless figure of merit (ZT of 0.9 at 673 K). The electronic and phonon structures, responsible for the high thermoelectric performance, were elucidated by first-principles calculations