Ultralowthermal conductivity draws great attention in avariety of fields of applications such as thermoelectrics and thermal barrier coatings.H erein, the crystal structure and transport properties of Cu 4 TiSe 4 are reported. Cu 4 TiSe 4 is au nique example of an on-toxic and low-cost material that exhibits al attice ultra-low thermal conductivity of 0.19 Wm À1 K À1 at room temperature.T he main contribution to the unusually low thermal conductivity is connected with the atomic lattice and its dynamics.T his ultralow value of lattice thermal conductivity (k L )c an be attributed to the presence of the localized modes of Cu, which partially hybridizew ith the Se atoms,w hich in turn leads to avoidance of crossing of acoustic phonon modes that reach the zone boundary with ar educed frequency.L ike ap honon glass electron crystal, Cu 4 TiSe 4 could also open ar oute to efficient thermoelectric materials,even, with chalcogenides of relatively high electrical resistivity and al arge band gap,p rovided that their structures offer as ublattice with lightly bound cations.

Defect engineering of thermoelectric (TE) materials enables the alteration of their crystal lattice by creating an atomic-scale disorder, which can facilitate a synergistic modulation of the electrical and phonon transport, leading to the enhancement of their TE properties. This work employs a compositional nonstoichiometry strategy for manipulation of Nivacancies and Ni-interstitials through Ni-deficient and Ni-excess compositions of (Zr, Hf)Ni 1±x Sn-based half-Heusler (HH) alloys to realize a stateof-the-art TE figure-of-merit (ZT) of ∼1.4 at 873 K in 4 atomic % Ni-excess HH composition, which corresponds to a remarkable TE conversion efficiency of ∼12%, estimated using the cumulative temperature dependence model. These alloys are synthesized employing arc-melting followed by spark plasma sintering and are characterized for their phase, morphology, structure, and composition along with electrical and thermal transport properties to examine the implication of Niexcess and Ni-deficiency on the TE properties of the synthesized Zr 0.6 Hf 0.4 NiSn HH alloy. A significant enhancement (∼30%) of ZT is observed in the low doping limit of Ni-excess HH compositions over their stoichiometric counterpart due to Ni-interstitials and in situ full-Heusler precipitation, which enable a strong phonon scattering for a drastic reduction in lattice thermal conductivity and lead to an enhancement of ZT. However, Ni-deficient HH compositions exhibit a deterioration in the TE properties owing to the vacancy-induced bipolarity. The defect-mediated optimization of electrical and thermal transport, thus, opens up promising avenues for boosting the TE performance of HH alloys.

Enhancing the thermoelectric performance of half Heusler through isovalent compositional tailoring.

The current work aims to explore the enhancement in thermoelectric performance of CoSb3-based materials via doping with less expensive rare-earth-free dopants. For this purpose, Te was chosen as a dopant on the Sb site in pre-optimized parent alloy Ni0.07Co0.93Sb3. The facile and fast synthesis route of arc melting and spark plasma sintering was utilized to prepare uniform and dense samples. The structural and transport properties were experimentally measured over a range of temperatures. As a result of suitable optimization of thermal and electronic transport properties, a state-of-the-art ZT value of ∼0.9 at 870 K was achieved for the composition Ni0.07Co0.93Sb2.94Te0.06. This acquired value of peak ZT is ∼50% enhanced from the parent compound, which is majorly attributed to the augmented power factor. In order to analyze this huge increment, first-principles-based density functional theory calculations were performed. Ni doping in CoSb3 is found to result in an n-type doped semimetal. The co-doping with Te at the Sb site was effective in opening a gap resulting in a heavily doped n-type semiconductor with a higher power factor than the Ni-doped one. Further, the theoretically calculated electronic transport properties were found in agreement with those observed experimentally for the synthesized samples.