Guest–Framework Interaction in Type I Inorganic Clathrates with Promising Thermoelectric Properties: On the Ionic versus Neutral Nature of the Alkaline‐Earth Metal Guest A in A₈Ga₁₆Ge₃₀(A=Sr, Ba)

Abstract

Periodic density functional calculations using pseudopotentials and a local basis set were performed on the type I clathrates A(8)Ga(16)Ge(30) (A=Sr, Ba). Both are known to show promising thermoelectric properties. Ab initio wave functions were analyzed within the framework of the quantum theory of atoms in molecules. This enabled us to analyze both the charge transfer and bonding properties of the clathrate from a rigorous quantum mechanical viewpoint. The Ba and Sr centers were found to be largely ionic (charge: ca. +1.7 e) both in the smaller 20-atom and in the larger 24-atom cages, consistent with a Zintl-phase view of these type I clathrates. The assertion that the Sr atoms in the different cages have similar oxidation states is shown to be consistent with multiwavelength diffraction experiments on Sr(8)Ga(16)Ge(30); while the assertion of ionicity of the Sr center is supported by the observation that the adsorption edge lies close to that previously found in the Sr K-edge XANES spectra of Sr(OH)(2).8 H(2)O. As such, this work contradicts previous experimental and theoretical studies that claim that the guest atoms are neutral. We show that the discrepancy is related to the definitions used for electron transfer. Definitions based on electron displacement (rearrangement) in space, as in previous works, do not account for the variation in shape and volume of the atomic catchment regions upon change in the number and average locations of the particles in the system. Eventually, such definitions lead to underestimation of charge transfer. The large binding energy found in earlier work for Ba and Sr in these materials is found to be consistent with a simple picture of charge transfer from the guest to the frame. Preliminary investigations on a clathrate of perfect stoichiometry appear to rule out any important relationship between the observed increase in the thermoelectric figure of merit with increasing external pressure and host-guest charge transfer.

Semiconducting Clathrates Meet Gas Hydrates: Xe24[Sn136]

“…In contrast, in the semiconducting clathrates, the cages are typically occupied by alkaline, earth-alkaline, or halogen atoms and there is a strong ionic interaction between the guests and the tetrahedrally coordinated framework. [7] However, in the case of cubic clathrate-II framework (Figure 1), practically "guest-free" modifications have been synthetized, [8][9][10] in line with computational studies showing the clathrate-II framework to be the energetically the most favorable one within various experimentally observed and hypothetical clathrate frameworks. [11] Recent synthesis of alloyed, guest-free SiGe clathrates is a promising step in the direction of tunable band gap materials for optoelectronic applications.…”

Ab initiocomputational study on the lattice thermal conductivity of Zintl clathrates[Si19P4]Cl4andHärkönen1, Karttunen2 2016Phys. Rev. B12190View full textAdd to dashboardCite

“…The Na 4 4 , there is a charge transfer from the less electronegative framework atoms (Si-P) to the guest atoms (Cl). The bonding within the framework can be considered to be covalent, while the framework-guest interactions are of ionic nature [58]. The ab initio density functional calculations to optimize the crystal structures and to calculate the phonon eigenvalues and eigenvectors were carried out with the QUANTUM ESPRESSO program package (QE, version 5.0.3) [59].…”

Electronic and transport properties of polycrystalline Ba8Ga15Ge31 type I clathrate prepared by SPS method

“…Among this group of compounds a number of materials derived from Ba 8 Ga 16 Ge 30 composition with high maximum thermoelectric figure-of-merit ZT max values up to 1.35 [2][3][4] have been discovered. Chemical bonding between the guest and the host atoms for most of the intermetallic anionic clathrates is predominantly ionic with a complete electron transfer between the guest and the framework atom [5][6][7]. Therefore, the majority of intermetallic clathrates fulfill Zintl rule and can be regarded as Zintl compounds.…”