Half-Heusler thermoelectrics first attracted interest in the late-1990s and are currently undergoing a renaissance. This has been driven by improved synthesis, processing and characterisation methods, leading to increases in the thermoelectric figure of merit and the observation of novel phenomena such as carrier filtering in nanocomposite samples. The difficulty in extracting good thermoelectric performance is at first glance surprising given the relative simplicity of the ideal crystal structure with only site occupancies and lattice parameter as crystallographic variables. However, the observed thermoelectric properties are found to depend sensitively on sample processing. Recent work has shown that prepared ingots can contain a range of inhomogeneities, including interstitials, nano- and micron sized Heusler inclusions and multiple half-Heusler phases. For this reason, the prepared materials are far more complex than initially appreciated and this may offer opportunities to enhance the thermoelectric figure of merit.

Thermoelectric figures of merit, ZT > 0.5, have been obtained in arc-melted TiNiSn-based ingots. This promising conversion efficiency is due to a low lattice thermal conductivity, which is attributed to excess nickel in the half-Heusler structure.

Quantitative X-ray powder diffraction analysis demonstrates that mixing Ti, Zr and Hf on the ionic site in the half-Heusler structure, which is a common strategy to lower the lattice thermal conductivity in this important class of thermoelectric materials, leads to multiphase behaviour. For example, nominal Ti 0.5 Zr 0.5 NiSn has a distribution of Ti 1Àx Zr x NiSn compositions between 0.24 # x # 0.70. Similar variations are observed for Zr 0.50 Hf 0.5 NiSn and Ti 0.5 Hf 0.5 NiSn. Electron microscopy and elemental mapping demonstrate that the main compositional variations occur over micrometre length scales. The thermoelectric power factors of the mixed phase samples are improved compared to the single phaseend-members (e.g. S 2 /r ¼ 1.8 mW m À1 K À2 for Ti 0.5 Zr 0.5 NiSn, compared to S 2 /r ¼ 1.5 mW m À1 K À2 for TiNiSn), demonstrating that the multiphase behaviour is not detrimental to electronic transport. Thermal conductivity measurements for Ti 0.5 Zr 0.5 NiSn 0.95 suggest that the dominant reduction comes from Ti/Zr mass and size difference phonon scattering with the multiphase behaviour a secondary effect. † Electronic supplementary information (ESI) available: Laboratory X-ray powder diffraction data for all samples. Rened phase distributions and thermoelectric properties of the Ti 1Àx Zr x NiSn 0.95 series. Low resolution image of the Ti 0.5 Zr 0.5 NiSn FIB lamella. SeeFig. 4 Temperature dependence of (a) the Seebeck coefficient (S), (b) the electrical resistivity (r) and (c) the power (S 2 /r) for the other studied XNiSn compositions (X ¼ Hf, Ti 0.5 Zr 0.5 , Ti 0.5 Hf 0.5 and Zr 0.5 Hf 0.5 ). 6112 | J. Mater. Chem. A, 2014, 2, 6107-6114 This journal is

The introduction of A-site vacancies in SrTiO 3 results in a glass-like thermal conductivity while Nb substituted samples maintain good electrical conductivity. This unexpected result brings SrTiO 3 one step closer to being a high-performing phonon-glass electron-crystal thermoelectric material.Thermoelectric waste heat recovery is widely expected to be an important component of a sustainable energy future. 1 However, affordable and relatively high-performance materials are lacking. Metal oxides are good candidates because of their abundance, low toxicity, and stability at high temperatures. 2 In addition, metal oxides are used in multilayer capacitors and the infrastructure for large scale device production exists. 3 SrTiO 3 is among the most promising n-type materials because it has the unusual combination (for an oxide) of good electrical conduction (s) and a high Seebeck coefficient (S), yielding power factors S 2 s # 3.5 mW m À1 K À2 in single crystals and epitaxial lms. 4,5 Unfortunately, the overall performance is compromised by a large thermal conductivity k ¼ 12-6 W m À1 K À1 for undoped SrTiO 3 (Fig. 1), 6 which reduces the thermoelectric gure of merit, ZT ¼ (S 2 s/k)T, where T is the absolute temperature. Outstanding thermoelectric materials generally have phononglass and electron-crystal (PGEC) properties, 7 meaning that the electronic transport is characteristic of a crystalline solid, while the k is low and resembles that of a glass. The main focus in the optimisation of SrTiO 3 has therefore been on the reduction of k, which consists of a large lattice contribution (k lat ) and a small electronic component (k el ¼ sLT; L is the Lorenz number). One approach to reduce k lat is to introduce point defects within the perovskite structure. This is for example used in Sr 1Àx La x TiO 3Àd (0 # x # 0.15) and SrTi 1Ày Nb y O 3Àd (0 # y # 0.2) and results in k lat z 3 W m À1 K À1 at 1073 K and a maximum ZT ¼ 0.35 at 1073 K. 5 However, to achieve ZT ¼ 1, a much more substantial reduction to k ¼ 1-2 W m À1 K À1 is needed. One drawback of La 3+ and Nb 4+ substitution is that the conicting requirements of charge carrier doping limit x, y # 0.2. However, even for Sr 1Àx Eu x Ti 0.8 Nb 0.2 O 3Àd with Sr 2+ /Eu 2+ mixtures (0 # x # 1), k was not reduced below $3 W m À1 K À1 for x ¼ 0.5 at 1000 K. 8 Recently, perovskites with A-site vacancies have started to generate attention. Promising S 2 s values and low k were reported in A-site and oxygen decient Sr 1Àx Pr 0.67x TiO 3Àd and Sr 1Àx Ti 0.8 Nb 0.2 O 3Àd , 9 while reductions in k were also observed in Ca 1Àx Nd 0.67x MnO 3Àd perovskites. 10 Here, we report a systematic investigation of the A-site decient Sr 1Àx La 0.67x , 0.33x Ti 1Ày Nb y O 3Àd perovskites. In order to separate the impact of A-site vacancies from oxygen defects we rst prepared the electrically insulating oxygen stoichiometric Sr 1Àx La 0.67x , 0.33x TiO 3 series and measured its thermal conductivity. ‡ Compositions with x ¼ 0, 0.4 and 0.8 were synthesised using standard solid state reactions...

Abstract. The crystal structure of the thermoelectric material tin selenide has been investigated with angle-dispersive synchrotron x-ray powder diffraction under hydrostatic pressure up to 27 GPa. With increasing pressure, a continuous evolution of the crystal structure from the GeS type to the higher-symmetry TlI type was observed, with a critical pressure of 10.5(3) GPa. The orthorhombic high-pressure modification, β -SnSe, is closely related to the pseudo-tetragonal high-temperature modification at ambient pressure. The similarity between the changes of the crystal structure at elevated temperatures and at high pressures suggests the possibility that strained thin films of SnSe may provide a route to overcoming the problem of the limited thermal stability of β-SnSe at high temperatures.