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DOE OSTI2019

Materials Data on Fe3S by Materials Project

Fe3S is Cementite structured and crystallizes in the orthorhombic Pnma space group. The structure is three-dimensional. there are two inequivalent Fe sites. In the first Fe site, Fe is bonded in a 3-coordinate geometry to three equivalent S atoms. There are a spread of Fe–S bond distances ranging from 2.27–2.37 Å. In the second Fe site, Fe is bonded in a distorted bent 150 degrees geometry to two equivalent S atoms. There are one shorter (2.20 Å) and one longer (2.21 Å) Fe–S bond lengths. S is bonded in a 8-coordinate geometry to eight Fe atoms.

36 MATERIALS SCIENCE↗

DOE OSTI2018

New structures of Fe3S for rare-earth-free permanent magnets

We applied adaptive genetic algorithm (AGA) to search for low-energy crystal structures of Fe 3 S. A number of structures with energies lower than that of the experimentally reported Pnma and I-4 structures have been obtained from our AGA searches. These low-energy structures can be classified as layer-motif and column-motif structures. In the column-motif structures, Fe atoms self-assemble into rods with bcc type of underlying lattice, which are separated by the holes terminated by S atoms. In the layer-motif structures, the bulk Fe is broken into slabs of several layers passivated by S atoms. Magnetic properties calculations showed that the column-motif structures exhibit reasonably high uniaxial magnetic anisotropy. In addition, we examined the effect of Co doping to Fe 3 S and found magnetic anisotropy can be enhanced through Co doping.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Doping dependence of the magnitude of fluctuating spin moments in the normal state of the pnictide superconductor Sr ( F e 1 - x C o x ) 2 A s 2 inferred from photoemission spectroscopy

We report systematic temperature- and doping-dependent measurements of the Fe3s core-level photoemission spectra in the normal state of superconducting Sr(Fe 1- x Co x ) 2 As 2 . The analysis of the Fe3s spectrum provides an element-specific determination of the mean value of the magnitude of the Fe spin moment measured on the fast (10 -16 -10 -15 s) timescale of the photoemission process. The data reveal the ubiquitous presence in the normal state of Fe spin moments with magnitude fluctuating on short timescales. The data reveal a significant reduction of the magnitude of the effective Fe spin moment on going from the parent to the optimal doped compound. The doping dependence of the magnitude of the spin moment at higher doping level is less clear, being either constant, or even nonmonotonic, depending on temperature. This phenomenology indicates the importance of the interaction between spin and itinerant degrees of freedom in shaping the properties of the normal state. These findings reaffirm the complexity of the normal state of 122 Fe-pnictides, which are typically viewed as the least correlated of the high-temperature unconventional superconductors.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2023

The iron spin transition of deep nitrogen-bearing mineral Fe3N1.2 at high pressure

Abstract Nitrogen is an essential element for life, one of the most abundant volatiles in the atmosphere, and an important component in the Earth’s interior, where iron nitride is an essential host of deep nitrogen. Here, we investigate the pressure-induced electronic spin-pairing transition of iron in siderazot (Fe3N1.2) at pressures up to 45.8 GPa at room temperature, using diamond-anvil cell techniques coupled with synchrotron X-ray emission spectroscopy. The integrated intensity of the satellite emission peak (K′β) decreases upon compression but remains unchanged at pressures greater than 30.5 GPa. In other words, the high-spin to low-spin transition of iron in Fe3N1.2 starts immediately at very low pressures and completes at ~30.5 GPa. The iron spin transition completion pressures increase with the nitrogen concentration of hexagonal close-packed iron nitrides (i.e., Fe3N1.2, Fe7N3, and Fe2N). Moreover, the identity and concentration of light elements in binary iron-rich compounds such as Fe3N, Fe3C, Fe3P, Fe3S, Fe7C3, and Fe7N3, together with their crystal structure, could affect the iron spin transition pressures. The spin transition of iron-rich alloys could alter the bonding nature and the physical properties, including the thermal and electrical conductivity, thereby influencing the thermal state and evolution of planetary interiors.

Geochemistry & Geophysics↗