Deviation from guest dominated glass like lattice dynamics in prototypical ternary Ba8Ni x Ge[Formula: see text] clathrates

Self-Doping for Synergistically Tuning the Electronic and Thermal Transport Coefficients in n-Type Half-Heuslers

Ternary intermetallic half-Heusler (HH) compounds (XYZ) with 18 valence electron count, namely, ZrCoSb, ZrNiSn, and ZrPdSn, have revealed promising thermoelectric properties. Exemplarily, it has been experimentally observed that a slight change in the content of Y site atoms (by ∼3-12.5% i.e., m = 0.03 and 0.125 in ZrY_1+mZ) leads to a drastic decrease in lattice thermal conductivity κ_L by more than 65-80% in many of these compounds. The present work aims at exploring the possibility of maximizing the electronic transport scenario after achieving the low κ_L limit in these compounds. By taking into account the full anharmonicity of the lattice dynamics, Boltzmann transport calculations are performed under the framework of density functional theory. Our results show that these excess atoms present in the vacant lattice site induce scattering either by acting as a rattling mode or by hybridizing with the acoustic modes of the host depending upon their mass and bonding chemistry, respectively. Furthermore, the introduction of these scattering centers may lead either to the formation of a defect midgap state in the electronic band structure (detrimental for electronic transport) or to light doping of the host compound. The latter is found to be particularly conducive for attaining synergy in both thermal and electronic transport.

Ultralow Lattice Thermal Conductivity at Room Temperature in Cu4 TiSe4

Ultralow thermal conductivity draws great attention in a variety of fields of applications such as thermoelectrics and thermal barrier coatings. Herein, the crystal structure and transport properties of Cu₄ TiSe₄ are reported. Cu₄ TiSe₄ is a unique example of a non-toxic and low-cost material that exhibits a lattice ultra-low thermal conductivity of 0.19 Wm^-1  K^-1 at room temperature. The main contribution to the unusually low thermal conductivity is connected with the atomic lattice and its dynamics. This ultralow value of lattice thermal conductivity (k_L ) can be attributed to the presence of the localized modes of Cu, which partially hybridize with the Se atoms, which in turn leads to avoidance of crossing of acoustic phonon modes that reach the zone boundary with a reduced frequency. Like a phonon glass electron crystal, Cu₄ TiSe₄ could also open a route to efficient thermoelectric materials, even, with chalcogenides of relatively high electrical resistivity and a large band gap, provided that their structures offer a sublattice with lightly bound cations.