Equation of state (EOS) and collapse of magnetism in iron-rich meteorites at high pressure by first-principles calculations

Abstract

We have calculated electronic and structure properties, equation of state (EOS) and collapse of magnetism of different meteoritic materials at high pressure using density functional theory (DFT). The fully optimization structures are obtained by minimization of the total energy and atomic forces of iron rich materials, which are in good agreement with the experiment. Phase transition of Fe 2 P (C22 type structure) barringerite into Fe 2 P (C23 type structure) allabogdanite is calculated to be 26 GPa. Collapse of magnetism (high spin-low spin) of some iron rich materials are described at various pressures. It has been observed that the optimized equilibrium volume has been changed due to transition of non-magnetic to magnetic material at high pressure, so EOS parameters have also been changed. EOS parameters such as bulk modulus (B o ) and derivative of bulk modulus with respect to pressure ( o B′ ) for magnetic and nonmagnetic configurations have been calculated by fitting E-V curves with Birch-Murnaghan equation, which confirm the experimental results. Page 2 of 15A c c e p t e d M a n u s c r i p t IntroductionHigh pressure study of materials has great importance for the understanding of phase transition mechanism, mechanical stability and the internal structure of Earth core and other planets. The Bulk modulus of material determines mechanical properties of the material and the amount of energy stored in solid material in the Earth's core. The equation of state (EOS) is an important parameter for characteristic of a material. One of the most abundant constituent of the Earth core is iron. The Earth's outer core also contains certain amount of light elements because the difference in the density between solid iron and the inner core may be as much as 5%.( McDonough et al., 1995;Anderson et al.,1994;Sohl et al., 1997). Phosphorous, carbon and sulpur are believed to be alloying light elements in iron-rich planetary cores, such as, those of the Earth and Mars (Li et al., 2003;Poirier., 1994).Determination of the physical properties of the magnetic and non-magnetic phase is important for the interpretation of existence of light elements in Earth and planetary cores. The study of density difference between pure iron and light element contained compounds such as Fe 2 P, Fe 3 P, Fe 3 S and Fe 3 C is very important for the estimation of light elements in Earth's core.Physical properties of meteoritic materials under core conditions can be significantly affected by the state of magnetization. High pressure magnetism in transition metal oxides had been investigated with in density function theory (DFT) (Cohen et al., 1997). Fe 2 P is a very common material which is found in two structures as meteoritic samples. Hexagonal Fe 2 P barringerite with space group m P 2 6 _ (C22 structure type) is found in ambient pressure and orthorhombic polymorph of Fe 2 P allabogdanite with space group Pnma (C23 type structure) are also known as stable or metastable at ambient conditions (Britvin et al., 2002). Page 3 of 15A c c e...

Role of zero-point effects in stabilizing the ground state structure of bulk Fe2P

“…Since experimental or theoretical data are not available for magnetic moments of C23 structure considered in the present work, we cannot make a comparison. However, the average magnetic moment per Fe atom ( 0.84 µ B ) is in reasonable agreement with the earlier DFT result (0.7 µ B ) [26].…”

“…As evident from the figure 1, no phase transformation occurs between C22 and C23 phase under compression up to 80 GPa. This is in disagreement with the computational results presented in the earlier study [25], where they predict C22 to C23 phase transformation under a pressure of 26 GPa. However, our results are in accordance with the experimental observations [12,13], where C22 to C23 structural transformation under pressure is observed only when the samples are heated to higher temperatures of ∼1400 K.…”

“…However, the average magnetic moment per Fe atom (0.84 b Reference [11]. µ B ) is in reasonable agreement with the earlier DFT result (0.7 µ B ) [25].…”

Mechanical Properties of Cementite

“…The variations in the predicted lattice parameters 44,86,88,[90][91][92][93]95,96) and elastic constants (Tables 6 and 7) using first-principles calculations were studied. It was found that the variations of the predicted lattice parameters among researchers is less than 1% and the variations of the elastic constants among the researchers is several %.…”

“…( 1), which expresses the relationship between P obtained from ∂E tot /∂V and V. This method of obtaining B 0 F has been attempted by many researchers as a method to obtain the bulk modulus of cementite using first-principles calculations. Table 6 44,[86][87][88][89][90][91][92][93][94] summarizes the reported values of B 0 F , B' 0 F , and V 0 , the EOS, and the range of pressure and volume values. Since cementite is FM at room temperature and becomes NM under high pressure, the values in Table 6 are shown separately according to the magnetic state, that is, either in the FM state or in the NM state.…”