Order–Disorder Transition in Inorganic Clathrates Controls Electrical Transport Properties

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 Ba8AlxGa16−xGe30. 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. ■ INTRODUCTION Since their discovery, inorganic clathrates have fascinated and intrigued the scientific community because of their complex crystal structure and interesting physical properties. Their usefulness for thermoelectric applications was recognized, when it was hypothesized, and later proven, that semiconducting materials with host−guest structures could be potential realizations of the “phonon glass−electron crystal” concept. The most relevant and well-studied members of this group consist of alkali or alkaline earth “guest” atoms that are trapped inside a “host” framework, made up of elements from groups 13 and 14 of the periodic table. In this study, we consider a type I clathrate, which is by far the most widely explored subclass. This collection of materials belongs to space group Pm3̅n (international tables of crystallography number 223) and is represented by the chemical formula A8BxC46−x, where A corresponds to the guest, while B and C are host species. The structure consists of a framework that incorporates two types of cages in the form of large pentagonal dodecahedra and smaller tetrakaidecahedra, which are centered at Wyckoff sites 6d and 2a, respectively (Figure 1a). Each of these cages contains a single guest atom, which is often assumed to be located at the center, while the host atoms are distributed between three sites (6c, 16i, and 24k). Although type I clathrates have long been believed to be thermally stable, a recent study has revealed that this is not the case. While this investigation clearly shows that Ba8Ga16Ge30 starts to decompose at elevated temperatures, this phenomenon cannot account for all the inconsistencies in the reported high-temperature (HT) properties. In particular, this explanation is not consistent with the fact that the transformations, which give rise to the peculiarities in the property measurements, in many instances appear to be reversible. Additionally, several recent studies have revealed the importance of taking structural transitions between ordered and disordered states into account when assessing the properties of various functional materials. The most compelling experimental evidence for the existence of a phase transformation presented so far is the sharp jump in the heat capacity at around 650 K observed by May et al. Several of their Ba8GaxGe46−x samples also display unexpected features in the electrical resistivity, Seebeck coefficient, and thermal diffusivity at around the same temperature. It is also interesting to note that this phenomenon does not appear to be unique for Ba8GaxGe46−x since similar results have been obtained for several other type I clathrates including Ba8AlxSi46−x, 16,17 Ba8BxAlySi46−x−y, 18 B a 8 − yEu yA l x S i 4 6 − x , 1 6 B a 8 − y S r yA l x S i 4 6 − x , 1 9 a n d Ba8GaxInyGe46−x−y. 20 Additionally, it has been reported that the binary clathrates Rb8Sn44□2 and Cs8Sn44□2 undergo Received: February 27, 2021 Revised: May 24, 2021 Published: June 3, 2021 Article pubs.acs. org/cm © 2021 The Authors. Published by American Chemical Society 4500 https://doi.org/10.1021/acs.chemmater. 1c00731 Chem. Mater. 2021, 33, 4500−4509 D ow nl oa de d vi a 94 . 2 54 . 1 10 . 4 5 on J un e 26 , 2 02 1 at 0 8: 08 :1 4 (U T C ). Se e ht tp s: //p ub s. ac s. or g/ sh ar in gg ui de lin es f or o pt io ns o n ho w to le gi tim at el y sh ar e pu bl is he d ar tic le s.