Romain Viennois

Physical properties of thermoelectric zinc antimonide using first-principles calculations

We report first-principles calculations of the structural, electronic, elastic, and vibrational properties of the semiconducting orthorhombic ZnSb compound. We study also the intrinsic point defects in order to eventually improve the thermoelectric properties of this already very promising thermoelectric material. Concerning the electronic properties, in addition to the band structure, we show that the Zn (Sb) crystallographically equivalent atoms are not exactly equivalent from the electronic point of view. Lattice dynamics, elastic, and thermodynamic properties are found to be in good agreement with the experiments and they confirm the nonequivalency of the zinc and antimony atoms from the vibrational point of view. The calculated elastic properties show a relatively weak anisotropy and the hardest direction is the y direction. We observe the presence of low energy modes involving both Zn and Sb atoms at about 5-6 meV, similar to what has been found in Zn4Sb3, and we suggest that the interactions of these modes with acoustic phonons could explain the relatively low thermal conductivity of ZnSb. Zinc vacancies are the most stable defects, and this explains the intrinsic p-type conductivity of ZnSb.

Experimental determination of the phonon density of states in filled skutterudites: evidence for a localized mode of the filling atom

The generalized density of states of LaFe4Sb12 and CeFe4Sb12 has been determined by inelastic neutron scattering and its main features are found to be in agreement with recently published calculations (J. L. Feldman, D. L. Singh, C. Kendziora, D. Mandrus and B. C. Sales, Phys. Rev. B, 2003, 68, 094301). In both compounds a localized vibrational contribution appears superposed on the low-energy Debye response. The distinct inelastic response of La in LaFe4Sb12 is obtained by subtraction of the data for the Ce filled compound and it shows even more clearly the resolution limited peak at 7 meV, attributed to the localized mode of La-atoms.

Revisited phonon assignment and electro-mechanical properties of chromium disilicide

We report a complete study of the lattice dynamics, dielectric, elastic and piezoelectric properties of hexagonal semiconducting chromium disilicide (CrSi2). From a combined experimental and theoretical study, we have revisited the phonon mode assignments at the zone-center, so that the contradictions met in previous experimental studies between 250 and 300 cm−1 are now explained and understood. We found that the temperature dependence of the Raman frequencies is mainly due to an implicit volume contribution and related to the large Gruneisen parameter. This explains why CrSi2 has a moderate thermal conductivity although its Debye temperature is quite large. Optic and static dielectric constants have also been analyzed and discussed. The elastic constants of CrSi2 are large, but this compound is quite brittle. In addition, the relatively low Poisson coefficient associated to the large negative Cauchy pressure of CrSi2 indicate the angular nature of its bonding. The calculation of its piezoelectric coefficient shows a sizable value with a magnitude similar to that reported for α-quartz. This prediction requires, however, experimental confirmation.

Low-temperature anomalies of photoinduced second harmonic generation in skutterudites

Photoinduced second harmonic generation (PISHG) was found in skutterudite compounds of CeFe4Sb12 and Ce0.7Fe3.5Ni0.5 Sb12. Measurements versus temperature, pump–probe delaying time and external magnetic field were performed. The studied compounds belong to moderate heavy fermion compounds (HFC) in the ground state. The PISHG signals appear at 6.8 and 4.9 K for CeFe4Sb12 and Ce0.7Fe3.5Ni0.5Sb12, respectively. We suspect that these signals are due to anharmonic electron–phonon interactions creating a charge density non-centrosymmetry. The observed effects are caused either by a possible phase transition or by drastic changes in the electron structure of the HFC with decreasing temperature.

Origin of the highly anisotropic thermal expansion of the semiconducting ZnSb and relations with its thermoelectric applications

This article is devoted to the thermal expansion of ZnSb combining experiments (neutron and X-ray) and calculations based on density functional theory. Related properties are also studied such as: the zone-center (Raman and infrared) phonon modes, the dielectric (electronic and static) tensors, the phonon density-of-states, the specific heats and the isotropic atomic displacement parameters. Our experimental data show highly anisotropic thermal expansion with large values along the a-direction. Concomitantly, a large increase of the Zn–Zn intra-ring distances and of one of the intra-ring Zn–Sb distances is observed, while other interatomic distances do not significantly change. In agreement with our calculations, the thermal expansion has positive values along the three crystal directions except around 30 K where it has weak negative values along the b and c-directions. This anomalous expansion is more important along the c-direction and it is mainly due to phonon modes with frequencies up to 75 cm−1. These modes are located in the S–Y–Γ (resp. Γ–Z) q-point range along the b (resp. c) direction. Phonon modes located in the Γ–X and in the Y–Γ–Z q-point range with frequencies up to 175 cm−1 are responsible for the positive large thermal expansion at room temperature along the a-direction. The much reduced anisotropy of the thermal conductivity is related to the lower Debye temperatures along the b and c-directions and mainly to the small transverse sound velocity between these directions.

Observation of various magnetic-field–induced states in B20 cubic MnGe

We report on the magnetic properties of the B20 polycrystalline MnGe compound synthesized under high-temperature and high-pressure conditions. From isothermal ac susceptibility experiments, we found the presence of several different magnetic-field–induced phases presenting similarities with those in the isostructural compounds MnSi and FeGe but with much broader existence range in the domain. From anomalies found in the ac magnetic susceptibility, we found not only the presence of an A phase similar to the one observed in MnSi but also a second phase we call B, whose magnetic signature is very similar to that of the A phase. Our discovery indicates that MnGe is a promising compound in order to further explore the existence conditions of the phases containing skyrmions.

Effect of Doping on the Thermoelectric Properties of Thallium Tellurides Using First Principles Calculations

We present a study of the electronic properties of Tl5Te3, BiTl9Te6 and SbTl9Te6 compounds by means of density functional theory based calculations. The optimized lattice constants of the compounds are in good agreement with the experimental data. The band gap of BiTl9Te6 and SbTl9Te6 compounds are found to be equal to 0.589 eV and 0.538 eV, respectively and are in agreement with the available experimental data. To compare the thermoelectric properties of the different compounds we calculate their thermopower using Mott’s law and show, as expected experimentally, that the substituted tellurides have much better thermoelectric properties compared to the pure compound.