Laser photochemical deposition of magnetite nanograins in a-Fe/C/O composite: High-pressure metal oxide polymorph surviving ambient conditions

UV laser-induced gas-phase photolysis of Fe(CO)5 results in deposition of Fe/C/O nanocomposite film which was analyzed by Raman, Fourier transform infrared, X-ray photoelectron and laser-induced breakdown spectroscopy and electron microscopy. The nanocomposite film was revealed as an amorphous phase composed of iron oxides (FeO and Fe2O3) and carbonaceous constituents possessing CO and CO bonds. This phase contains crystalline nanograins of ambient cubic and high-pressure orthorhombic magnetite. The survival of the high-pressure Fe3O4 nanograins at ambient conditions is tentatively ascribed to their Laplace pressure and/or to stabilizing effect of the amorphous Fe/C/O phase. The o-Fe3O4 nanograins represent a very rare example of high-pressure nanosized polymorph surviving at ambient conditions and not retrieving ambient-pressure-stable phase upon decompression.