Strategic Optimization of the Electronic Transport Properties of Pseudo‐Ternary Clathrates

Abstract

Based on our previous studies of Ba 8 Ga 16 Ge 30 [14] and Ba 8 Al x Ga 16−x Ge30, [19] we know that the electronic transport properties are strongly influenced by chemical ordering, which is a direct consequence of the unique structural motifs of clathrates. They feature a rigid host framework that contains cages which enclose loosely bound guest atoms. Like most studies, our investigation pertains to structures that belong to the type-I subgroup, which possesses P 3 m n (International Tables of Crystallography number 223) symmetry and can be described by the chemical formula A 8 B x C 46−x . Here, A represents the guest atoms, which are found at Wyckoff sites 2a and 6d, while the host atoms, B and C, occupy 6c, 16i, and 24k sites (see Figure 1a. According to the Zintl concept, it is expected that the guest atoms are not directly bonded to the host framework, but rather donate their electrons to the latter so as to ensure that all atoms have a full octet in their valence shell. In Ba 8 Al x Ga 16−x Ge30 and Ba 8 Ga 16 Ge x Si 16−x , the 8 pairs of electrons provided by the Ba atoms (+2) are compensated by the relative deficiency (−1) of the 16 trivalent elements (Al, Ga). While both compounds should thus be expected to behave as intrinsic semiconductors, actual samples tend to deviate from the stoichiometric composition, which leads to either a lack (p-type) or surplus (n-type) of electrons.The Zintl concept alone is, however, not enough to explain the differences in the property measurements that are generally observed, even for samples that have the same nominal composition. [20][21][22][23][24][25][26][27][28][29][30][31][32][33] Detailed structural characterisations of different pseudo-binary clathrates have, moreover, revealed that different synthesis methods produce materials with remarkably different site occupation factors (SOFs). [16,34] In addition, our previous studies of the pseudo-binaries, Ba