SnSe:K_x intermetallic thermoelectric polycrystals prepared by arc-melting

Thermoelectric materials hold the promise to help create a greener and eco-friendlier energy economy [1]. Thermoelectrics can convert temperature gradients into electrical voltage, or electric current to (negative) heat through the Seebeck or Peltier effect. Thermoelectrics are typically good semiconductors with large Seebeck coefficient (S), but they also need good electrical conductivity (σ), which requires a compromise by tuning the Fermi level with chemical doping. Furthermore, good thermoelectrics extract low thermal conductivity (κ), but since the Wiedemann–Franz law connects the electrical conductivity to the electronic thermal conductivity (κ_el), only the lattice thermal conductivity (κ_latt) may be improved independently [2]. Nanostructuring is often the chosen strategy to decrease κ_latt and preserve the electronic transport properties S and σ [3‐8]. In the realm of material science, these competing properties are evaluated in terms of the dimensionless thermoelectric figure of merit zT = T * S²σ/(κ_el + κ_latt).

Thermoelectrics offer versatile ways for applications, such as devices for waste heat to electricity conversion in thermoelectric generators, or Peltier coolers for reliable systems with no gases or moving parts [9, 10]. The full design from the synthesis of materials to the fabrication of devices must be assessed [11, 12]. For many applications such as powering sensors or for Peltier coolers, good room temperature thermoelectrics are needed [13].

There are several successful families of thermoelectrics that can provide both the required p- and n-type semiconductors for thermoelectric modules [14, 15]. Thermoelectrics are often made of environmentally toxic or rare elements. Finding cost-effective synthesis of environmentally friendlier nanostructured thermoelectrics has become a focus of recent research [16]. Nevertheless, some of the more successful types of thermoelectrics are based on tellurides [17], such as bismuth telluride (Bi₂Te₃) [4‐7, 18‐22], lead telluride (PbTe) [23‐25] and germanium telluride (GeTe) [26‐31]. Skutterudites are another attractive family of thermoelectrics, as they are high mobility semiconductors that chemically and structurally offer many ways of doping and property tuning [1, 32, 33]. They can be prepared by various methods in addition to conventional metallurgy, including high-pressure synthesis [34‐36], melt spinning [37, 38], microwave synthesis [39], etc. As alternatives, oxides, such as SrTiO₃ have been considered recently [40, 41].

There are multiple strategies to improve some of the thermoelectric properties in addition to nanostructuring, such as band engineering [42‐47], energy filtering [48‐50], the phonon glass electron crystal approach [51, 52] or high entropy design [53].

Tin selenide, SnSe, is a semiconductor with a quasi-2D laminar crystal structure resembling that of black phosphorous, also a potential thermoelectric [54]. In 2014, single crystals of SnSe were found to be the best thermoelectric up then with zT ~ 2.6 [55]. It has remarkable 3D charge but 2D phonon transport properties due to its crystal structure [56], anharmonicity, multiple valence band along with a continuous phase transition near maximum zT [57]. Polycrystalline SnSe can be prepared by several synthesis techniques [58], among them with co-doping and multi-nanoprecipitates [59], with aqueous synthesis [60], as well as arc-melting. SnSe films can also show good thermoelectric performance [61‐63]. Successful thermoelectric generators have been made based on SnSe very recently [64]. The effects of vacancy on the thermoelectric properties have been studied in detail [65‐68]. Applied pressure transforms SnSe into a semimetal [69]. SnSe can be doped with a wide variety of elements, including Ge [70‐72], Sb [73], or alloyed with SnTe [8] and SnS [74].

Arc-melting produces highly nanostructured SnSe efficiently [3] and is a versatile synthesis procedure for various dopings [70, 75]. This synthesis produces material with elemental ratios or doping different from the nominal amount, as either dopant, tin or selenide is easily lost to the arc [73, 76]. The polycrystalline n-type SnSe pellets produced by arc-melting can reach high zT up to 1.8 [77].

High-performance p-type SnSe polycrystal may be stabilized by alkali doping (with Li, Na and K) [78‐80], as well as Na-doped SnSe_1−xTe_x [81]. In alkali-doped SnSe, the location of the dopant is crucial. As stated by Wei et al., “…actual occupancy (i.e., substitutional, interstitial, or intercalated type) of these dopants is an important concern" [78]. Alkali doping may be beneficial in removing or preventing the formation of the high-thermal conductivity intergrain tin oxide layers and forming coherent nanoprecipitates [82]. In conventional synthesis techniques, the handling and introduction of highly flammable and air-sensitive alkali metals poses a technical nuisance.

Here we report the synthesis of SnSe:K_x intermetallic alloys, using potassium hydride, and their high resolution neutron powder diffraction (NPD) investigation directed to characterize the position of the K dopant, along with the thermoelectric properties. The advantage of using KH as starting material to introduce K has to do with its easy handling, avoiding issues with ambient oxidation, with the expectation that H₂ is released and lost during synthesis. A similar method, using KH, was employed recently to produce K-filled CoSb₃ skutterudite under high pressure [32]. The synthesis may also help avoiding the formation of tin oxide, which has been shown to be responsible for an undesired increase of effective thermal conductivity [83‐85].

SnSe:K_x intermetallic alloys were prepared in an Edmund Buhler MAM-1 mini-arc furnace. A mixture of stoichiometric amounts of Sn and Se powder metals, and KH as a source of K, was pelletized under N₂ atmosphere in a glove box; the pellets were arc-molten under Ar atmosphere in a water-cooled Cu crucible, leading to compact ingots, which were ground to powder for structural characterization, or cold-pressed into regular disk-shaped specimens for transport measurements. Cold-pressing SnSe is sufficient for decent thermoelectric performance [77, 86]; it provides samples with about 90% of the crystallographic density thanks to its malleability. On the contrary, more typical consolidation techniques, such as hot pressing [87] or spark plasma sintering [21, 85], would yield higher densities but tend to deteriorate the electrical and thermal conductivities, mainly by causing intergrain oxide growth [88].

Neutron powder diffraction (NPD) data were taken in the D2B diffractometer at the Institut Laue-Langevin, Grenoble, in the high-resolution configuration, with a neutron wavelength of λ = 1.594 Å. About 2 g of the sample were contained in a vanadium can and placed in the isothermal zone of a furnace with a vanadium resistor operating under vacuum (P_O2 ≈ 10^–6 Torr residual pressure). The measurements were carried out upon warming at 295, 523, 673, 773, 873 and 973 K. The NPD patterns were collected for 2 h in the high-flux mode. The diffraction patterns were analyzed by the Rietveld method with the FULLPROF program [89]. The line shape of the diffraction peaks was generated by a pseudo-Voigt function. The following parameters were refined: background points, zero shift, half width, pseudo-Voigt, scale factor and unit-cell parameters. Positional and occupancy factors and isotropic displacement factors were also refined. The coherent scattering lengths for Sn, K and Se were, respectively, 6.225, 3.670 and 7.970 fm. A preferred orientation correction was applied, considering platelets perpendicular to the [100] direction.

All the thermoelectric properties were measured along the pressing direction, where previous reports show a higher ZT [90‐92]. Seebeck coefficient and four-probe resistivity measurements were carried out in high vacuum (10^–6 mbar) in a commercial MMR instrument. Thermal conductivity was determined from the thermal diffusivity (α) using a Linseis LFA 1000 instrument by a laser-flash technique, as \(\kappa = \alpha ,C_{{\text{p}}} ,d\), where C_p = 252–263 J/kg K is the specific heat calculated using the Dulong–Petit equation, and d = 5.5–6.0 g/cm³ is the sample density (90–96% of the crystallographic value), for the various compositions between x = 0–0.1.

Scanning transmission electron microscopy (STEM) is carried out in a JEOL ARM 200 electron microscope operated at 200 kV. The microscope is equipped with a Gatan Quantum electron energy loss spectrometer (EELS). Transmission electron microscopy (TEM) image S7 is done in JEOL 3000F electron microscope operated at 300 kV.

Summarizing, we have demonstrated that K doping has important repercussions on the transport properties of SnSe, which may be articulated as a function of some of the structural properties unveiled from neutron diffraction data. We observed that below the Pnma-to-Cmcm structural phase transition, K ions cannot be located by diffraction methods, but the conspicuous expansion along the a axis suggests that they are present in the interlayer space, also confirmed by STEM-EELS spectroscopic images. The transition to the more symmetric Cmcm structure, where the coordination index increases from 3 to 5, facilitates the incorporation of large K⁺ ions into the Sn positions, where they can indeed be located from NPD data. This incorporation drives important consequences concerning the electronic transport, accounting for the comparatively much higher resistivity above 700 K, where the K migration from the interlayer space to the Sn positions is triggered by the phase transition. Indeed, the electronic resistivity of pristine SnSe reaches values inferior by two orders of magnitude to the doped specimen at 800 K, which explains the poor Figure of Merit reached by K-doped specimens. At the same time, the characteristic change of sign of the Seebeck coefficient is also altered by the partial occupancy of Sn sites by K atoms. Finally, the K doping results in an additional decrease of the thermal conductivity, probably as a consequence of the structural disordering induced by the substitutional incorporation of K.

Supplemental Information includes Supplemental Experimental Data on crystal structure, and calculations based on the single parabolic band model (SPB) of the predicted maximum ZT figure of merit, weighted mobility and electronic quality factor in 3 Figures and 1 Table can be found with this article online at [link inserted by JMS].