With a view to the design of hard magnets without rare earths we explore the possibility of large magnetocrystalline anisotropy energies in Heusler compounds that are unstable with respect to a tetragonal distortion. We consider the Heusler compounds Fe2YZ with Y = (Ni, Co, Pt), and Co2YZ with Y = (Ni, Fe, Pt) where, in both cases, Z = (Al, Ga, Ge, In, Sn). We find that for the Co2NiZ, Co2PtZ, and Fe2PtZ families the cubic phase is always, at T = 0, unstable with respect to a tetragonal distortion, while, in contrast, for the Fe2NiZ and Fe2CoZ families this is the case for only 2 compounds -Fe2CoGe and Fe2CoSn. For all compounds in which a tetragonal distortion occurs we calculate the MAE finding remarkably large values for the Pt containing Heuslers, but also large values for a number of the other compounds (e.g. Co2NiGa has an MAE of -2.11 MJ/m 3 ). The tendency to a tetragonal distortion we find to be strongly correlated with a high density of states at the Fermi level in the cubic phase. As a corollary to this fact we observe that upon doping compounds for which the cubic structure is stable such that the Fermi level enters a region of high DOS, a tetragonal distortion is induced and a correspondingly large value of the MAE is then observed.PACS numbers:
The energy spectra of three classes of polybenzenoid hydrocarbons with a large number N (N ≈ 10 3 ) of carbon atoms have been studied theoretically. It is shown that in the asymptotic case N f ∞ the energy gap (EG) ∆E(Nf∞) is different from zero if the electron correlation is taken into account; that is, the π systems calculated should possess semiconductor properties. The results for the ∆E(Nf∞) * 0 of the hydrocarbons are in qualitative agreement with the results calculated for the EG of three classes of 1-D ladder polymers, which can be considered as models of quasi-1-D graphite. With increasing width (L) of the polymers, the band gap ∆E(Lf∞) approaches a value different from zero. The problem of the existence of defect states of hydrocarbons with vacancies is briefly discussed.
We have performed first-principles calculations in order to understand the binding mechanism of Li atoms in disordered carbon materials that are used for negative electrodes of rechargeable lithium batteries. We used pyrene, anthracene, and phenanthrene molecules as parts of disordered carbon. We examined several binding sites for two Li atoms in these aromatics and found that they are bound with substantial negative binding energies. The most negative one was -142.8 kJ/mol for Li-containing pyrenes, -211.0 kJ/mol for anthracenes, and -146.2 kJ/mol for phenanthrenes at the B3LYP/6-31G*//HF/6-31G* level of calculation. Li atoms are bound to interstitial (ring-over) and edge sites. In addition to these binding mechanisms, we found that Li atoms could be bound, forming a Li dimer in anthracene and phenanthrene. Their binding energies are -200.5 and -146.2 kJ/mol, respectively, being larger in magnitude than Li 2 dissociation energy. These aromatics lose their planarity when they accommodate Li atoms. We found that larger distortion brings more strong interaction between the aromatics and Li atoms. The amount of energy required for the distortion increases in the order the interstitial, edge, and Li-dimerized sites. The highest occupied molecular orbital energy, which is closely related to the electrode potential during discharge process, decreases in that order. This energy lowering may be related to the origin of the hysteresis observed during the charge/discharge cycles.
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