Xiao-Jia Chen

Synthesis and Raman spectroscopy of a layered SiS2 phase at high pressures

Dichalcogenides are known to exhibit layered solid phases, at ambient and high pressures, where 2D layers of chemically bonded formula units are held together by van der Waals forces. These materials are of great interest for solid-state sciences and technology, along with other 2D systems such as graphene and phosphorene. SiS2 is an archetypal model system of the most fundamental interest within this ensemble. Recently, high pressure (GPa) phases with Si in octahedral coordination by S have been theoretically predicted and also experimentally found to occur in this compound. At variance with stishovite in SiO2, which is a 3D network of SiO6 octahedra, the phases with octahedral coordination in SiS2 are 2D layered. Very importantly, this type of semiconducting material was theoretically predicted to exhibit continuous bandgap closing with pressure to a poor metallic state at tens of GPa. We synthesized layered SiS2 with octahedral coordination in a diamond anvil cell at 7.5-9 GPa, by laser heating together elemental S and Si at 1300-1700 K. Indeed, Raman spectroscopy up to 64.4 GPa is compatible with continuous bandgap closing in this material with the onset of either weak metallicity or of a narrow bandgap semiconductor state with a large density of defect-induced, intra-gap energy levels, at about 57 GPa. Importantly, our investigation adds up to the fundamental knowledge of layered dichalcogenides.

Origin of efficient thermoelectric performance in half-Heusler FeNb0.8Ti0.2Sb

A half-Heusler material FeNb0.8Ti0.2Sb has been identified as a promising thermoelectric material due to its excellent thermoelectric performance at high temperatures. The origin of the efficient thermoelectric performance is investigated through a series of low-temperature (2–400u2009K) measurements. The high data coherence of the low and high temperatures is observed. An optimal and nearly temperature-independent carrier concentration is identified, which is ideal for the power factor. The obtained single type of hole carrier is also beneficial to the large Seebeck coefficient. The electronic thermal conductivity is found to be comparable to the lattice thermal conductivity and becomes the dominant component above 200u2009K. These findings again indicate that electron scattering plays a key role in the electrical and thermal transport properties. The dimensionless figure of merit is thus mainly governed by the electronic properties. These effects obtained at low temperatures with the avoidance of possible thermal fluctuations together offer the physical origin for the excellent thermoelectric performance in this material.A half-Heusler material FeNb0.8Ti0.2Sb has been identified as a promising thermoelectric material due to its excellent thermoelectric performance at high temperatures. The origin of the efficient thermoelectric performance is investigated through a series of low-temperature (2–400u2009K) measurements. The high data coherence of the low and high temperatures is observed. An optimal and nearly temperature-independent carrier concentration is identified, which is ideal for the power factor. The obtained single type of hole carrier is also beneficial to the large Seebeck coefficient. The electronic thermal conductivity is found to be comparable to the lattice thermal conductivity and becomes the dominant component above 200u2009K. These findings again indicate that electron scattering plays a key role in the electrical and thermal transport properties. The dimensionless figure of merit is thus mainly governed by the electronic properties. These effects obtained at low temperatures with the avoidance of possible therm...

Phonon anharmonicity in thermoelectric palladium sulfide by Raman spectroscopy

Recent advances in the study of thermoelectric materials mainly focus on the developments or designs of methods to reduce thermal conductivities. The information of phonon scattering processes is the key to the understanding of the thermal transfer and transport of a material. Such information is essential for the understanding of the thermal conductivity of a material itself and for the further improvement to demand the requirements for technological applications. Recently, palladium sulfide has been examined as a potential thermoelectric material. However, the high thermal conductivity limits its thermoelectric performance and technological applications. Here, Raman scattering spectroscopy is used to investigate the thermal transport properties of this material over a wide range of temperatures. The nonlinear temperature-dependent frequencies and linewidths of the Raman modes illustrate the anharmonicity of phonon scattering for thermal transport in this material. Three-phonon scattering processes are found to account for the thermal transport in the temperature range of 10–620u2009K. The high-energy bands of the Bg mode related to the light atom (S) contribute most to the thermal transport properties. More phonon scattering processes including higher orders are seemingly needed to further reduce the thermal conductivity of this material.Recent advances in the study of thermoelectric materials mainly focus on the developments or designs of methods to reduce thermal conductivities. The information of phonon scattering processes is the key to the understanding of the thermal transfer and transport of a material. Such information is essential for the understanding of the thermal conductivity of a material itself and for the further improvement to demand the requirements for technological applications. Recently, palladium sulfide has been examined as a potential thermoelectric material. However, the high thermal conductivity limits its thermoelectric performance and technological applications. Here, Raman scattering spectroscopy is used to investigate the thermal transport properties of this material over a wide range of temperatures. The nonlinear temperature-dependent frequencies and linewidths of the Raman modes illustrate the anharmonicity of phonon scattering for thermal transport in this material. Three-phonon scattering processes are f...