Compressional behavior of MgCr2O4 spinel from first-principles simulation

Abstract

The compressional behavior of the MgCr 2 O 4 spinel has been investigated with the CASTEP code using density functional theory and planewave pseudopotential technique. We treated the exchange-correlation interaction by both the local density approximation (LDA) and generalized gradient approximation (GGA) with the Perdew-Burker-Ernzerhof functional. Our simulation was conducted for the pressure range of 0-19 GPa. We obtained the isothermal bulk modulus (K T ) of the MgCr 2 O 4 spinel as 181.46(48) GPa (GGA; low boundary) or 216.1(11) GPa (LDA; high boundary), with its first derivative (K' T ) as 4.41(6) or 4.5(1), respectively. The oxygen parameter u is not constant but negatively correlated with P, and decreases by about 0.5-0.6% for the investigated P range. The component polyhedra have different compressibilities, increasing in the order of (O 4 ) 1 <CrO 6 <(O 4 ) 2 <O 6 <MgO 4 . The Mg-O bond in the MgO 4 tetrahedron is much more compressible than the Cr-O bond in the CrO 6 octahedron.

A Metastable Fo-III Wedge in Cold Slabs Subducted to the Lower Part of the Mantle Transition Zone: A Hypothesis Based on First-Principles Simulations

“…The [MgO6] octahedra fall into two groups, [Mg(I)O6] and [Mg(II)O6] (with the [Mg(II)O6] larger and more-distorted than the [Mg(I)O6]) and share both corners and edges. As expected for the simulation results using the GGA technique [66,68], our optimized unit-cell parameters a, b, c, and V at 0 GPa and 0 K are slightly larger than those experimentally reported values (Hazen [54]; 1 atm and 296 K) by ~0.73%, 0.79%, 0.70% and 2.23%, respectively (Table 3). The Fo-II phase was experimentally determined as a triclinic phase (space group P1, Z = 4), with no crystallographic details refined from the single-crystal XRD data though (Finkelstein et al [37]; ~52.4 GPa).…”

“…Our optimized structure at 60 GPa (Figure 1c) shows that although the Si cations maintain as either fourfold-coordinated (50%) or sixfold-coordinated (50%), the Mg cations have divided into two groups, with one quarter of them in sevenfold coordination (forming distorted pentagonal bipyramid) and the rest in sixfold coordination, in good agreement with the simulated results of Finkelstein et al [37]. The optimized lattice parameters at 52.4 GPa and 0 K are compared in Table 3 with the experimentally obtained ones at the same pressure but 298 K. The agreement between these two sets of lattice parameters is good, with our calculated values slightly larger (a by 0.58%, b by 0.56%, c by 0.31%, and V by 1.32%), a common phenomenon for the GGA method [66,68].…”

“…Fo Fo-II Fo-III Fo-IV results using the GGA technique [66,68], our optimized unit-cell parameters a, b, c, and V at 0 GPa and 0 K are slightly larger than those experimentally reported values (Hazen [54]; 1 atm and 296 K) bỹ 0.73%, 0.79%, 0.70% and 2.23%, respectively (Table 3). Supplementary Tables S1-S8. Essentially identical to the experimental results [54], all the O atoms in the Fo phase form an expanded and distorted hexagonally close-packed array stacked along the a direction whereas the Si atoms locate in the isolated tetrahedra which share corners with the [MgO6] octahedra ( Figure 1a).…”

“…In specific, the K T from Finkelstein et al [37] is slightly larger than the present determination ( Figure 6a); their K T at 300 K (K T = 131.5(4) GPa, as obtained in this study by refitting their volume-pressure data with the 3rd BM-EoS) is~3.9% larger than ours at 0 K (K T = 126.6(1) GPa), on the basis of K T = 4. Two factors may have made their contributions to this phenomenon: (1) our theoretical calculation, done with the GGA method, is well known to lead to an underestimated K T [68,77]; (2) the experimental data, collected with the P medium of He which is unquestionably the best available P-transmitting medium but still develops some deviatoric stress at P >~40 GPa [78], are expected to generate slightly overestimated K T . If we consider the effect of T on the K T (0 K in the theoretical investigation versus 298 K in the compression experiments), the influences of these two factors become even more prominent.…”

Equation of State of a Natural Chromian Spinel at Ambient Temperature

“…They all have the cubic Fd3m structure, but show wide compositional variation [2]. Due to their compositional diversity, they have been used as key petrogenetic indicators of many geological processes [2][3][4] and played important roles in constraining pressure, temperature, oxygen fugacity, and other quantities [5][6][7].…”