Frustrated Packing in Simple Structures: Chemical Pressure Hindrance to Isolobal Bonds in the TiAl3 type and ZrAl2.6Sn0.4

While simple close-packed arrangements convey a sense of optimization, they can, in fact, host competition between different types of interactions. The TiAl₃ structure type, for example, represents one of a series of ordered TE₃ variants (T = transition metal, E = main group element) of the face-centered cubic structure, alongside the AuCu₃ and ZrAl₃ types. These structures differ in their T-T connectivity corresponding to the 18-n rule: electronic pseudogaps occur at electron concentrations of 18-n/T atom, where n is the number of electron pairs each T atom shares with other T atoms in T-T isolobal bonds. Facile stacking variations interrelate these structures, presumably setting the stage for an electronically precise series. However, the prototype of the TiAl₃ type itself violates the 18-n rule, with its count of 13 electrons/Ti atom calling for n = 5 rather than the 4 isolobal T-T bonds/T atom available in this type. Here, we investigate the factors underlying this deviation from the 18-n rule and their relation to the new TiAl₃-type compound ZrAl_3-xSn_x (x ∼ 0.4). First, the relative stabilities of the TiAl₃ and ZrAl₃ types are compared for TAl₃ compounds (T = Zr and Ti). While for T = Zr, the structure adhering to the 18-n rule is highly preferred, for T = Ti, the energy difference essentially vanishes. This trend is connected through DFT-Chemical Pressure (CP) analysis to a tension that emerges in TiAl₃ between the optimization of the T-T isolobal bonds and the space requirements of Al-Al contacts elsewhere. This picture elucidates the transition of ZrAl₃ from its own type to the TiAl₃-type upon partial Sn substitution in ZrAl_2.6Sn_0.4: the incorporation of Sn brings the electron count closer to that predicted for the TiAl₃ type, while electronegativity and CP direct the larger Sn atoms to the site that resists isolobal bond formation in TiAl₃.

Magnetically stabilized Fe8(μ4-S)6S8 clusters in Ba6Fe25S27

We have prepared Ba₆Fe₂₅S₂₇, and studied its magnetic properties and electronic structure. Using a combined DFT and molecular orbital based approach, we provide an interpretation of the bonding and stability within Ba₆M₂₅S₂₇ (M = Fe, Co, Ni) and related M9S8 phases.

Charge distributions in metallic alloys: a charge-excess functional theory approach

The distribution of local charge (DLC) excesses in metallic alloys, previously obtained as a result of the analysis of order N electronic structure calculations, is derived from a variational principle. A phenomenological charge-excess functional theory is obtained which is determined by three concentration dependent, material specific parameters that can be obtained from ab initio calculations. The theory requires modest computational efforts and reproduces with an excellent accuracy the DLC and the electrostatic energies of ordered, substitutionally disordered, or segregating metallic alloys and, hence, can be considered an efficient approach alternative to conventional electronic structure calculations. The substantial reduction of computing time opens new perspectives for the understanding of metallic systems and their mechanical properties.

Structural principles and amorphouslike thermal conductivity of Na-doped Si clathrates

The postulated low thermal conductivity and the possibility of altering the electronic conductivity of metal-doped clathrates with semiconducting host elements have stimulated great interest in exploring these compounds as promising thermoelectric materials. The electronic and thermal properties of the prototypical Na xSi (46) system are studied in detail here. It is shown that, despite the fact that the Na/Si clathrate is metallic, its thermal conductivity resembles that of an amorphous solid. A theoretical model is developed to rationalize the structural stability of the peculiar structural topology, and a general scheme for rational design of high efficiency thermoelectric materials is presented.