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Structural Aspects of Skutterudites
Published in Ctirad Uher, Thermoelectric Skutterudites, 2021
The full power of the Zintl concept is realized when considering filled skutterudites and aiming to achieve the desired semiconducting structure. A fully filled skutterudite RM4X12 with the transition metal M in the low spin d6 configuration, is expected to be a stable semiconducting structure when its valence electron count is 4×24 = 96 electrons per formula unit. Therefore, if one wishes to fill the structure fully with a trivalent rare earth element, such as La, one must remove three electrons from the pnicogen site X in order to keep the electron count at 96 and thus maintain a semiconducting structure. Among the first reports on this balancing act is the work of Tritt et al. (1996) with LaIr4Sb9Ge3, where three Ge atoms were substituted for Sb to compensate for three electrons donated by La. In the case of divalent alkaline earth Ba, one must remove two electrons from the pnicogen site X to achieve a semiconducting skutterudite, for instance by synthesizing BaCo4Sb10Sn2, where two tetravalent Sn atoms replace two pentavalent Sb atoms. Such precise valence electron counting enables the expansion of the family of semiconducting skutterudites to numerous new compositions, as demonstrated by Lou et al. (2015). The approach was supported by DFT calculations and verified by synthesizing some 63 new single-phase skutterudites, with a potential to expand the synthesis to hundreds of new structures when including phosphide- and arsenide-based skutterudites and not just antimonides. Although I doubt that many of these new precise count-synthesized skutterudites will turn out to be outstanding thermoelectric materials (the disruption of the pnicogen rings might lower lattice thermal conductivity but it will also dramatically degrade the carrier mobility, just as was the case of ternary skutterudites), expanding the spectrum of skutterudite compounds should open exciting possibilities for the study of new physical phenomena that will shed more light on these remarkable structures.
CF4-n (SO3) n (n = 1–4): a new series of organic superhalogens
Published in Molecular Physics, 2022
Jitendra Kumar Tripathi, Ambrish Kumar Srivastava
A class of atomic clusters having higher electronic affinities (EAs) than halogens is referred to as superhalogen as proposed by Gutsev and Boldyrev [1]. According to them, superhalogens consist of a central atom or core connected to highly electronegative ligands such as fluorine (F), chlorine (Cl), or oxygen (O) atoms following the octet rule. Typical examples include LiF2, BF4, PF6, BO2, etc. In such systems, there is an increase in the electronegativity of the central core because of the delocalisation of electrons over electronegative atoms and hence, an increase in the EA of the systems. Various kinds of superhalogens have been explored in the last four decades following different electron counting rules such as octet rule [2–10], Wade-Mingos rule [11–15] and Huckel 4n+2 rule [16–19]. However, only a few studies [18–21] explored the superhalogen properties of organic species. Due to high EAs, these clusters can be used as strong oxidising agents. The application of superhalogens in the design of superacids has been well studied [14–16,22–25]. The use of superhalogens in the design of new materials for hydrogen storage [26] and new electrolytes for Li-ion batteries [12,17,27] has been also reported. Besides this, superhalogens appear in the organic superconductors [28]. The most recent study [29] suggests that superhalogens can be used as building blocks of ionic liquids. Jena [30] and Skurski [31] have reviewed and discussed the recent developments in this field. Ever-increasing applications of superhalogens make them worthy of investigation even today.
Theory of chemical bonds in metalloenzymes XXIV electronic and spin structures of FeMoco and Fe-S clusters by classical and quantum computing
Published in Molecular Physics, 2020
Koichi Miyagawa, Mitsuo Shoji, Hiroshi Isobe, Shusuke Yamanaka, Takashi Kawakami, Mitsutaka Okumura, Kizashi Yamaguchi
Recently, large Fe-S clusters have accepted current interest in relation to interesting targets systems for future hybrid classical and quantum computations [45–57]. The size of UNO CAS determines the necessary memory size for diagonalisation of the CI matrix, which is closely related to the selection of type of computers employed. Our valence-electron counting procedure in the section 2.1 is extended to other F-S clusters and clusters of Fe-S clusters. The valence configurations of Fe(III)aFe(II)bSc (19) and Fe(III)a Fe(II)bSc Mo(III)d (20) clusters are generally given as follows. where carbon atom(s) in the FeMoco cluster are neglected in this modelling. The effective exchange interactions for the Fe-Fe and Mo-Fe pairs are generally negative in sign. Therefore, total spin configurations of 19 and 20 are low spin in accord with available experimental results. The highest spin (HS) configurations have been often employed for DFT computations to obtain trial orbitals for UNO CAS CI and UNO DMRG calculations [45,47,61]. On the other hand, the low-spin (LST) DFT solutions responsible for the ground and lower-excited states are also constructed by the spin flipping procedure as described previously [62].
C/N/O centred metal clusters: super valence bonding and magic structure with 26 valence electrons
Published in Molecular Physics, 2020
Jianling Tang, Cairong Zhang, Hongshan Chen
The physical and chemical properties of clusters differ from those of the bulk and exhibit strong dependence on size and composition. One of the most exciting developments in the field of clusters is that chosen cluster can mimic the chemical behaviours of a group of atoms in the periodic table. This idea offers the prospect of a new dimension of the periodic table formed by stable clusters called superatoms. And it offers the potential to create novel materials with tailored properties by using clusters as building units [1–4]. The electron counting rules are central to the understanding of superatoms, and they play an important part in designing these new species. The simplest electron counting rules are the octet rule and 18-electron rule; they correspond to the closed s2p6 and s2p6d10 electronic configurations of noble gas atoms. For simple metal clusters, the Jellium model is very successful in understanding their stabilities and properties [5–9]. This model assumes a uniform background of positive charge for the atomic nuclei and the core electrons, and the valence electrons from the individual atoms are treated nearly free and move in this potential. This leads to the Jellium shells of 1S21P61D102S21F142P6 … , and the shell closure gives the series of magic numbers 2, 8, 18, 20, 34, 40 … . (In this paper we use the uppercase S, P, D to denote the Jellium orbitals, and the lowercase s, p, d for atomic orbitals). The well-known example of Al13, with 39 valence electrons, needs one extra electron to close the 2P6 shells and behaves as a halogen atom [10–13]. While the Jellium model works very well for pure metal clusters, the scope of application of the Jellium model and modification of the theory to account for nonmetal doped metal clusters are still illusive.