Laurent Chaput

Distributions of phonon lifetimes in Brillouin zones

Lattice thermal conductivities of zincblende- and wurtzite-type compounds with 33 combinations of elements are calculated with the single-mode relaxation-time approximation and linearized phonon Boltzmann equation from first-principles anharmonic lattice dynamics calculations. In 9 zincblende-type compounds, distributions of phonon linewidths (inverse phonon lifetimes) are discussed in detail. The phonon linewidths vary non-smoothly with respect to wave vector, which is explained from the imaginary parts of the self energies. It is presented that detailed combination of phonon-phonon interaction strength and three phonon selection rule is critically important to determine phonon lifetime for these compounds. This indicates difficulty to predict phonon lifetime quantitatively without anharmonic force constants. However it is shown that joint density of states weighted by phonon numbers, which is calculated only from harmonic force constants, can be potentially used for a screening of the lattice thermal conductivity of materials.

Prediction of Low-Thermal-Conductivity Compounds with First-Principles Anharmonic Lattice-Dynamics Calculations and Bayesian Optimization.

Compounds of low lattice thermal conductivity (LTC) are essential for seeking thermoelectric materials with high conversion efficiency. Some strategies have been used to decrease LTC. However, such trials have yielded successes only within a limited exploration space. Here, we report the virtual screening of a library containing 54,779 compounds. Our strategy is to search the library through Bayesian optimization using for the initial data the LTC obtained from first-principles anharmonic lattice-dynamics calculations for a set of 101 compounds. We discovered 221 materials with very low LTC. Two of them even have an electronic band gap <1 eV, which makes them exceptional candidates for thermoelectric applications. In addition to those newly discovered thermoelectric materials, the present strategy is believed to be powerful for many other applications in which the chemistry of materials is required to be optimized.

Phonon-phonon interactions in transition metals

In this paper the phonon self-energy produced by anharmonicity is calculated using second-order many-body perturbation theory for all bcc, fcc, and hcp transition metals. The symmetry properties of the phonon interactions are used to obtain an expression for the self-energy as a sum over irreducible triplets, very similar to integration in the irreducible part of the Brillouin zone for one-particle properties. The results obtained for transition metals shows that the lifetime is on the order of

STUDY OF ELECTRON, PHONON AND CRYSTAL STABILITY VERSUS THERMOELECTRIC PROPERTIES IN Mg2X (X = Si, Sn) COMPOUNDS AND THEIR ALLOYS

{10}^{\ensuremath{-}10}

Ab initio phonon properties of half-Heusler NiTiSn, NiZrSn and NiHfSn

s. Moreover, the Peierls approximation for the imaginary part of the self-energy is shown to be reasonable for bcc and fcc metals. For hcp metals we show that the Raman-active mode decays into a pair of acoustic phonons, their wave vector being located on a surface defined by conservation laws.

STM and DFT Investigations of Isolated Porphyrin on a Silicon‐Based Semiconductor at Room Temperature

We present results of extensive theoretical and experimental investigations of Mg2Si and Mg2Sn and their Mg2Si1-xSnx alloys. Electronic and phonon properties of binary compounds were studied by ab initio calculations. Then, both compounds were synthesized by the solid-state reaction and electrical resistivity and thermopower was measured at high temperature (300–900 K). In both the compounds, the theoretical bandgaps (0.56 eV in Mg2Si and 0.16 eV in Mg2Sn) agree very well with the experimental values (0.6 eV in Mg2Si and 0.17 eV from activation law in Mg2Sn) upon applying the modified Becke–Johnson semilocal exchange potential and including spin–orbit coupling in the calculations. Calculated phonon spectra support crystal stability of both compounds. For Mg2Si, the contributions from Si and Mg are spread over all the spectrum (0–10 THz), whereas in the case of Mg2Sn, a gap opens around 4 THz with Sn and Mg contributions dominating in lower and higher energy range, respectively. The calculated heat capacity at low temperature (0–300 K) fairly agrees with available experimental data. The crystal structure of Mg2Si1-xSnx with x = 0, 0.25, 0.4, 0.75, 1 was studied by X-ray diffraction measurements. The alloy compositions exist in the ranges 0 < x < 0.4 and 0.6 < x < 1 and the obtained samples are almost single phased. Detailed crystal stability study with temperature revealed that all powder samples started to decompose into MgO, Si and Sn at ~ 630 K. For hot pressed bulk materials, the decomposition is much slower than in powder compounds but it still appears. Interestingly, thermoelectric properties measurements performed in Mg2Si1-xSnx show that both electrical resistivity and thermopower curves are repeatable during temperature cycles up to 770 K. On the other hand, temperature-dependent X-ray powder diffraction suggests that these compounds are not stable. Furthermore, electronic structure calculations of almost 40 impurities (s- and p-block, 3d and 4d transition metal elements) diluted in Mg2Si and Mg2Si0.75Sn0.25 were performed by the KKR-CPA method. Based on calculated impurity density of states the character of doping (n or p) is predicted, which, however, strongly depends on the substituted crystallographic site.

Ab initio based calculations of the thermal conductivity at the micron scale

A theoretical investigation of phonon properties from first-principles calculations is carried out for the half-Heusler compounds NiXSn, [Formula: see text], Zr and Hf. The crystal structures are optimised via ab initio calculations within the framework of density functional theory. The phonon properties are retrieved from harmonic and anharmonic interatomic force constants calculations using the finite size displacements method and many-body perturbation theory. A solution to the linearized phonon Boltzmann transport equation is then used to compute the ab initio thermal conductivities. For X   =   Ti, Zr and Hf, we found 15.4, 13.3 and 15.8 W m(-1) K(-1) at 300 K, respectively. Thanks to a spectral analysis of the velocities and lifetimes we were able appreciate the differences in the thermal conductivities between the three compounds under study. Our results provide insights to understand the behaviour of the thermal conductivity and therefore to improve the thermoelectric figure of merit for such materials.

First-principles phonon calculations of thermal expansion in Ti3SiC2, Ti3AlC2, and Ti3GeC2

Metalloporphyrins represent a class of flexible molecules with a nearly square planar core conformation and a two dimensional conjugated p-electron delocalization. Due to their interesting physicochemical properties, metalloporphyrins adsorbed on a surface can be used in many technological applications such as molecular electronics, light-harvesting arrays for solar energy generation, catalysts, sensors, etc. The fine determination of the conformation and arrangement of adsorbed molecules on a surface are key points, since they are strongly related to the physical and chemical properties of the final organic–inorganic interfaces. They are changed by the subtle balance of internal deformation and substrate–molecule interactions, leading to a conformational adaptation of the molecule on the substrate lattice. These features are even more relevant in semiconductors than in metals because the moleculesemiconductor interactions are usually greater than molecule– metal interactions. Although scanning tunneling microscopy (STM) is a remarkable tool to investigate individual adsorbed molecules on semiconductors, experimental STM images of metalloporphyrins were achieved only on metals and only an unique very recent article investigates theoretically the adsorption of a metalloporphyrin on a Si(111)-H surface. Herein, we report the first experimental investigation at room temperature of the adsorption of Cu-5,10,15,20-tetrakis(3,5-di-tert-butylphenyl) porphyrin (Cu-TBPP) as a model of metalloporphyrin on a passivated silicon based surface (Si(111)-B) using STM and by theoretical calculations in order to fully understand the conformational adaptation of the Cu-TBPP on a Si(111)-B surface.

Electronic structure and thermopower ofNi(Ti0.5Hf0.5)Snand related half-Heusler phases

Heat transport in bulk semiconductors is well understood, and during the last few years, it has been shown that it can be computed accurately from ab initio calculations. However, describing heat transport in micro- and nanodevices used in applications remains challenging. In this paper, we propose a method, based on the propagation of wave packets, for solving the phonon Boltzmann transport equation parametrized with ab initio calculations. It allows computing the thermal conductivity of micro- and nano-sized systems, without adjustable parameters, and for any materials. The accuracy and applicability of the method are demonstrated by computing the cross plane thermal conductivity of cubic and hexagonal silicon thin films as a function of their thickness.