Highly crystalline porous magnetite and vacancy-ordered maghemite microcrystals of rhombohedral habit

Abstract Porous crystals of magnetite and vacancy-ordered maghemite, rhombohedron-shaped, reaching 150 μm in size, and having remarkably large specific surface area (88.55 m 2 /g for magnetite, 40.14 m 2 /g for maghemite) were obtained via topotactical conversion, starting from hydrothermally grown siderite single crystals. The increase in density (from 3.9 g/cm 3 for siderite to 5.24 g/cm 3 for magnetite and 4.9 g/cm 3 for maghemite) caused quasi-ordered internal pores-grains pattern, with mesocrystalline appearance. The X-ray Line-Profile Fitting-based microstructure analysis gave 64±6 nm and 84±8 nm for the average inner grains size in magnetite and maghemite. Structure and phase content analysis indicated high purity and crystallinity. Magnetic measurements indicated saturation magnetization (92.1 emu/g for magnetite and 85.5 emu/g for maghemite) approaching the upper limits reported for the pure bulk oxides.