Abstract:
We determine the equation of state of stoichiometric FeO employing the diffusion Monte Carlo method. The fermionic nodes are fixed to those of a wave function having the form of a single Slater determinant. The calculated ambient pressure properties (lattice constant, bulk modulus and cohesive energy) agree very well with available experimental data. At approximately 65 GPa, the lattice structure is found to change from rocksalt type (B1) to NiAs based (inverse B8).
At ambient conditions, FeO crystallizes in B1 (NaCltype) structure. It is antiferromagnetically ordered at temperatures below 198 K and this ordering is accompanied by a small rhombohedral distortion denoted as rB1—the unit cell is stretched along the [111] body diagonal. In shock-wave studies it was observed that around 70 GPa the oxide transforms to a different structure [4], which was inferred as B2 (CsCl-type) in analogy with similar materials, but LDA calculations hinted that much larger pressure, around 500 GPa, would be needed to stabilize B2 against the B1 phase [5]. Besides that, no such structural transition was detected in static compression experiments [6], unless the material was significantly heated up [7]. X-ray diffraction performed along the high-temperature static compression revealed that the high-pressure structure is actually B8 (NiAs-type) [7]. There are two distinct ways of putting FeO on NiAs lattice, the so-called normal B8, where iron occupies Ni sites, and inverse B8 (iB8 for short), where iron sits on As sites. It is the latter configuration that comes from bandstructure theories as the more stable of the two [1, 2, 3]. Also, reinterpretation of the data of Ref. 7 suggests that the high pressure phase is a mixture of B8 and iB8 phases [1]. On the theoretical side, the introduction of the iB8 structure into the picture revealed a serious deficiency in the LDA (and GGA) as applied to FeO, since the iB8 phase is predicted more stable not only than B8 but also than B1 at all pressures, which contradicts experimental findings. It was demonstrated that inclusion of Coulomb U to better account for electron-electron correlations alleviates this problem [2, 3]. In this Letter, we calculate the equation of state of stoichiometric FeO using the fixed-node diffusion Monte Carlo method (DMC) [8], a many-body computational technique that accurately treats even strongly correlated systems. Based on the aforementioned studies, we con- fined ourselves to only two structures—B1 with the typeII antiferromagnetic (AFM) ordering (symmetry group R¯3m) and iB8 also in the AFM state (group P¯6m2).
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