Order–Disorder Transition in Inorganic Clathrates Controls Electrical Transport Properties

Abstract

Inorganic clathrates have been extensively investigated owing to their unique and intriguing atomic structure as well as their potential as thermoelectric materials. The connection between the chemical ordering and the physical properties has, however, remained elusive. Here, this relation is uncovered through a combination of first-principles calculations, atomistic simulations, and experimental measurements of thermodynamic as well as electrical transport properties. This approach is, specifically, used to reveal the existence of an order−disorder transition in the quaternary clathrate series Ba 8 Al x Ga 16−x Ge 30 . The results, furthermore, demonstrate that this phenomenon is responsible for the discontinuity in the heat capacity that has been observed previously. Moreover, the unusual temperature dependence of both Seebeck coefficient and electrical conductivity can be fully explained by the alterations of the band structure brought about by the phase transformation. It is finally argued that the phenomenology described here is not limited to this particular material but should be present in a wide range of inorganic clathrates and could even be observed in other materials that exhibit chemical ordering on at least one sublattice.

First-Principles Study of Order–Disorder Transitions in Pseudobinary Clathrates

“…This result is, however, by no means exclusive. Since our previous experimental and computational studies of the Ba 8 Al x Ga y Ge 46– x – y system included differential scanning calorimetry (DSC) experiments on the pseudobinaries Ba 8 Ga 16 Ge 30 and Ba 8 Al 16 Ge 30 as well as Ba 8 Al 5 Ga 11 Ge 30 , we were able to conclude that both of these compounds undergo a transition at about 650 K, in agreement with the observations by May et al In the case of Ba 8 Al x Si 46– x , the data published by Condron et al include not only a small peak at ∼800 K in the heating curve from DSC measurements (Figure 4a in ref ) but also a distinctive shift in thermal conductivity (Figure 5c in ref ) at the same temperature. Crucially, corresponding features can be found in the results presented by Tsujii et al (Figure 8a,c in ref ).…”

“…In recent years, the importance of order–disorder transitions in functional materials has received increased attention. Such transformations have not only been observed in a wide variety of different materials, which either contain vacancies or exhibit mixed occupancies, but also been shown to profoundly influence physical properties, e.g., in pnictides, − oxides, chalcogenides, − half-Heuslers, , skutterudites, and vacancy-containing clathrates. , Moreover, it has been recently established that order–disorder transitions in the inorganic clathrate Ba 8 Al x Ga y Ge 46– x – y have a notable impact on the electrical transport properties . This group of materials has been extensively studied because of their comparatively good thermoelectric properties and their fascinating structural motifs, which can be described as an interwoven network of host and guest lattices.…”

“…Illustration of a representative type-I clathrate structure (a) as well as heat maps (b–e) showing the temperature and composition dependence of the contribution to heat capacity from the disorder for Ba 8 Ga x Ge 46– x (b), Ba 8 Al x Ge 46– x (c), Ba 8 Ga x Si 46– x (d), and Ba 8 Al x Si 46– x (e). Experimental transition temperatures that have been extracted from ref (purple diamonds in b), ref (blue square in b and c), refs , (purple diamond in d), ref (blue squares in d), ref (purple diamond in e), and ref (blue square in e) are also indicated. If no error bars for the compositions were stated in the original publications, we estimated them to be 0.5 atoms per unit cell, similar to other data from energy-dispersive X-ray spectroscopy.…”

“…Materials with complex crystal structures, such as type-I clathrates, feature a vast configurational space. Computationally, this principal challenge can be efficiently addressed using a combination of density functional theory (DFT) calculations, alloy CEs, and MC simulations. ,− In addition to providing evidence for the existence of order–disorder transitions, , these studies have, moreover, revealed the temperature and composition dependence of the chemical ordering − and established a strong correlation between site occupation factors (SOFs) and electronic transport properties. , Here, we build on these discoveries using a combination of CEs and Wang–Landau (WL) simulations to map the temperature and composition dependence of the transition for the four pseudobinary clathrates Ba 8 Ga x Ge 46– x , Ba 8 Al x Ge 46– x , Ba 8 Ga x Si 46– x , and Ba 8 Al x Si 46– x . We start by analyzing the contributions to the heat capacity from the disorder as a function of composition and compare the transition temperatures extracted from the calculations with data from the literature.…”

Efficient Calculation of the Lattice Thermal Conductivity by Atomistic Simulations with Ab Initio Accuracy

“…As noted above one of the distinct advantages of MD simulations is that they make no a priori assumption as to the heat conduction mechanism in the material and are therefore equally applicable to materials with high and low LTC. To demonstrate this explicitly, we now consider the inorganic clathrate Ba 8 Ga 16 Ge 30 , a material with a very low LTC that exhibits strong phonon scattering and presents a considerable challenge for BTE approaches both due the size of the unit cell, complex ordering [41,[60][61][62] and its very strong anharmonicity. [21,22,33] The latter requires one to account for phonon renormalization,for example, via selfconsistent phonon theory [21] or temperature-dependent (effective) IFCs.…”