Quantum Monte Carlo Calculations of Structural Properties of FeO Under Pressure

We determine the equation of state of stoichiometric FeO by employing the diffusion Monte Carlo method. The fermionic nodes are fixed by a single Slater determinant of spin-unrestricted orbitals. The calculated ambient-pressure properties (lattice constant, bulk modulus, and cohesive energy) agree very well with available experimental data. At approximately 65 GPa, the atomic lattice changes from the rocksalt B1 to the NiAs-type inverse B8 structure.

Figure 1

Finite-size errors of the twist averaged DMC energy at volumes $15.9{\AA }^3/\mathrm{FeO}$ (left) and $20.4{\AA }^3/\mathrm{FeO}$ (right). Shown are Ewald energies (red squares) and values corrected by Eq. (3) (blue circles). The finite-size errors increase with increasing electron density. The residual bias of the corrected data also grows, the rate is $\approx 0.015\mathrm{eV}{\AA }^{-3}/\mathrm{FeO}$. The energy in the infinite cell $E_{\infty }$ is extrapolated from the data shown. Statistical error bars are smaller than the symbol sizes (here, as well as in Fig. 2).

Figure 2

Total energies of the B1 (red squares) and the iB8 (blue circles) phases. Lines are fits with Murnaghan equation of state. Inset: Difference between Gibbs potentials of the two phases at $T=0\mathrm{K}$.

Figure 3

$P(V)$ curves of the B1 (red, low pressure) and the iB8 (blue, high pressure) phases. Lines are our DMC fits as in Fig. 2. Points are experimental data: filled circles [6] (B1, ${\mathrm{Fe}}_{0.98}\mathrm{O}$), empty circles [28] (B1, ${\mathrm{Fe}}_{0.94}\mathrm{O}$), empty squares [7] (iB8, ${\mathrm{Fe}}_{0.98}\mathrm{O}$). All B1 data [6, 28] were taken at room temperature, the iB8 data [7] at 900 K. B1 data sets are shown relative to the equilibrium volume to reduce the nonstoichiometry effects.