Year: 2002
Type: article
Abstract: Type I inorganic clathrates constitute some of the most promising nonoptimized structures discovered in the search for improved thermoelectric materials (TMs).The extent of charge transfer (CT) from the guest metal atoms to the semiconductor framework and the detailed nature of the host-guest interactions are both supposed to affect significantly each one of the three material properties "Seebeck coefficient, thermal and electrical conductivity" determining the figure of merit of TMs.Based on the distinction between charge donation "the flowing of electrons from the valence band of the guest sub-lattice to the conduction band of the framework sub-lattice" and charge transfer, the spatial displacement of electrons from the region of the metal guests sub-lattice to the region of the semiconductor frame, it was in the past concluded that the guest atoms are charge donors, but not ionic in these TMs.By using an ab-initio density functional approach and an atomic space partitioning based on quantum mechanics rather than on purely geometrical considerations, we show that the guest atoms (Sr, Ba) are indeed mostly ionic, regardless of the kind of cage they occupy.The insensitivity of CT to the cage size is confirmed by Diffraction Anomalous Fine Structure (DAFS) results across the Sr K edge.The significant displacement of the guest atom from the center of the 24-atom cage is found to be driven by the formation of strong frame-guest interactions also in the larger cage, a mechanism which eventually leads to similar CTs in the two kind of cages.The interplay between CT and thermo-power is investigated by using a one-electron transport model for the Seebeck coefficient evaluation and a number of model clathrates with varied CT, as induced by compression or expansion of the lattice.
Authors: C. Gatti, L. Bertini, B. B. Iversen, N. P. Blake
Language: English
Subfield: Materials Chemistry
Field: Materials Science
Domain: Physical Sciences
Open Access status: bronze
OpenAlex ID: W2152505458