Spin states of iron impurities in magnesium oxide under pressure: A possible intermediate state

Abstract

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...

Prediction of Site Preference of Implanted Transition Metal Dopants in Rock-salt Oxides

“…Our calculations revealed that stable charge states for Fe in interstitial site are +3, +2 and 0, which is in good agreement with the experimental observation of stable charge states of Fe in MgO [1]. Substitutional defect formation of Fe in MgO show +3, +2, +1 and 0 as stable charge states of Fe, as has also been reported earlier [89]. As charge states of substitution doped Fe do not match with experimental observation, hence Fe atoms most likely occupy interstitial sites in MgO [1].…”

Elastic properties of ferropericlase at lower mantle conditions and its relevance to ULVZs

“…Intermediate spin states are only metastable and thus should not be overwhelmingly present (Larico et al, 2013). For low amounts of iron (<20%) the spin transition is concentration independent, occurring between 40-60 GPa at ambient temperatures and rising to 70-125 GPa for mid-mantle temperatures (2200-2400 K) (Lin et al, 2013).…”

Pressure driven spin transition in siderite and magnesiosiderite single crystals

“…We were able to describe the XRS spectra of siderite measured within the spin crossover regime by a superposition of HS and LS reference spectra. This indicates that only parts of the single crystal crossed the spin transition and coexistence of HS and LS iron can be assumed without occurrence of iron in an intermediate 3d electronic configuration as discussed in 36 , 61 .…”

“…Next, we investigate if spectra measured within the transition region can be modeled by a simple superposition of HS and LS spectra. In that case, rather than intermediate spin states, domains with sample in LS and HS states due to pressure gradient is assumed 13 , 36 , 60 , 61 . Hence, for siderite we fitted the XRS M 2,3 - and L 2,3 -edge spectra using a HS and LS reference.…”