Synthesis and microstructural properties of α-Fe1−xGaxOOH solid solutions

Isostructural iron oxyhydroxide α-FeOOH (goethite) and gallium oxyhydroxide α-GaOOH, as well as their solid solutions α-Fe1−xGaxOOH in a whole concentration range (x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.30, 0.50, 0.80 or 1.00) were synthesized by precipitation in a weakly alkaline medium using an organic alkali tetramethylammonium hydroxide (TMAH) for pH adjustment. Lath-shaped α-Fe1−xGaxOOH particles common for goethite formed in alkaline media were observed in all samples containing more Fe than Ga, whereas fairly uniform ellipsoidal α-Fe1−xGaxOOH particles characteristic of α-GaOOH formed in alkaline media were observed in samples containing more Ga than Fe. An increased content of Ga3+ ions in α-Fe1−xGaxOOH led to a decrease in the unit-cell size (in line with the smaller radius of Ga3+ ions). A reduction in the hyperfine magnetic field and a collapse of the room temperature magnetic order in α-Fe1−xGaxOOH with x > 0.2 were found (due to the dilution of the magnetic lattice brought about by the replacement of the magnetic Fe3+ ions by the diamagnetic Ga3+ ions). A shift in the position of (Fe,Ga)-O-H bending bands and Fe-O and Fe-OH stretching (lattice vibration) bands towards higher wave numbers (due to increased strength of the O-H⋯O hydrogen bond), a decrease in intensity of all absorption bands in UV-Vis-NIR spectra and a shift of Fe3+ ligand field transition bands to lower energies (due to an increase in the ligand field splitting energy), and an increase in dehydroxilation temperature (due to the higher stability of the Ga-OH bond in comparison with the Fe-OH bond) with the increased Ga3+ content were also found.