We present a theoretical investigation on the structural and electronic properties of isolated nickel impurities in diamond. The atomic structures, symmetries, formation and transition energies, and hyperfine parameters of isolated interstitial and substitutional Ni were computed using ab initio total energy methods. Based on our results, we ultimately propose a consistent microscopic model which explains several experimentally identified nickel-related active centers in diamond.

We carried out a first-principles investigation on the microscopic properties of nickel-related defect centers in diamond. Several configurations, involving substitutional and interstitial nickel impurities, have been considered either in isolated configurations or forming complexes with other defects, such as vacancies and boron and nitrogen dopants. The results, in terms of spin, symmetry, and hyperfine fields, were compared with the available experimental data on electrically active centers in synthetic diamond. Several microscopic models, previously proposed to explain those data, have been confirmed by this investigation, while some models could be discarded. We also provided insights into the microscopic structure of several of those centers.

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Ferropericlase, (Mg,Fe)O, is a major lower mantle mineral, and studying its properties is a fundamental step toward understanding the Earth's interior. Here, we performed a first principles investigation on the properties of iron as an isolated impurity in magnesium oxide, which is the condition of ferropericlase that iron-iron interactions could be neglected. The calculations were carried using the all-electron full-potential linearized augmented plane wave method, within the density functional theory/generalized gradient approximation plus the on-site Hubbard correction.We present the electronic and magnetic properties, electric and magnetic hyperfine splitting of this impurity in high and low spin states, for several charge states at zero pressure, which were then extended to high pressures. For the impurity in the neutral charge state, our results indicated that there is a metastable intermediate spin state (S=1), in addition to the high (S=2) and low (S=0) spin states. Those results were discussed in the context of an intermediate spin state, experimentally identified in ferrosilicate perovskite. PACS numbers: 91.60.Pn, 75.30.Kz, 81.40.Rs electrons generate a t 2 + e pair of orbitals in the crystalline field, with the t 2 orbital below the e one. At low pressures, the exchange splitting prevails over the crystalline field one, favoring the high spin (HS) S = 2 state. With increasing pressure, the crystalline field splitting becomes more important, favoring the low spin (LS) S = 0 state beyond a certain transition pressure. Early experiments at room temperature [3] have suggested that this transition should be very sharp, occurring in a narrow pressure range. On the other hand, more recent theoretical [6, 9, 10] and experimental [7, 12, 13] investigations showed that it should be smoother, across wide pressure and temperature ranges. The current model for this transition describes ferropericlase as a solid solution with simultaneous concentrations of iron atoms in HS and LS states, which are determined by the thermodynamic conditions of the material [9, 10].The phenomenology of iron in mantle minerals is very rich and a proper investigation on its properties is a fundamental step to build compositional models for the Earth's interior.However, there are many open questions that still need to be addressed, in order to understand the implications of the spin transition for the mantle physical properties. For example, there are several conflicting results in the literature concerning the elasticity of ferropericlase across this transition [12][13][14][15]. Another important element concerns the radiative conductivity of the mineral across the spin transition [5,16], since such knowledge may help to build better temperature profiles of the Earth's lower mantle and core. There is also considerable uncertainty on the pressure range of that spin transition. While earlier experiments [1,[3][4][5] indicated that it occurred in the 30-40 GPa pressure range, at room temperature, more recent investigations suggested highe...

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