iron ii arsenide is the most common form of iron in nature. It consists of interlocked octahedra crystals and occurs as minerals such as magnetite, loellingite, goethite, and limonite. This compound is important in metallurgical processing, as it contaminates sulfidic metal ores with arsenic, which can be poisonous when released into the environment. In addition, it has potential as an alternative energy source due to its high thermal conductivity.

However, the synthesis of this compound requires high temperatures to avoid its oxidation. To this end, the synthesis of pure iron ii arsenide is a challenge because it has exhibited very little structural stability above 800 degC. Moreover, it is unclear whether the known FeAs2 phase exists above 800 degC or is replaced by another structure. In order to address these questions, we performed in situ powder X-ray diffraction experiments (PXRD) on samples of 99.5% Fe2As (commercial, Alfa Aesar) at the Materials Science Beamline X04SA of the Swiss Lightsource in Switzerland. This high-quality data allowed us to identify an unexpected new high-temperature phase and its transformations.

The in situ PXRD experiment showed that the formation of a new phase was accompanied by a large increase in the concentration of a-Fe. This a-Fe phase has the structure of a zincblende-like material with octahedra coordination and predominantly covalent bonding. This is the first time that a pure iron ii arsenide phase has been observed at such a high temperature, which makes it a promising candidate for industrial applications and could potentially facilitate the development of a direct reductive melting (DRM) process to remove As from double-refractory gold-arsenic concentrates.