Synthesis, structure and spectroscopy of Fe2+:MgAl2O4 transparent ceramics and glass-ceramics

ABSTRACT We report on a comparative study of a transparent Fe:MgAl2O4 (spinel) ceramics and a transparent nanophase Fe:MgAl2O4-based glass-ceramics. The 0.1 mol% Fe:MgAl2O4 ceramics was synthesized by hot pressing (at 1500 °C/50 MPa) of powders obtained by the sol-gel method using LiF as a sintering aid. The Fe:MgAl2O4 ceramic is a single-phase material (cubic structure, sp. gr. Fd3‾m, a = 8.083 A) with a mean grain size of ∼50 μm. The ceramic exhibits a broadband transparency of 0.2–6.0 μm and a high in-line transmission at ∼1 μm of 74.4%. The iron ions are presented in the ceramics in the single state of Fe2+ species in tetrahedral (Td) sites. A broad absorption band spanning from ∼1.2 to 3.7 μm assigned to the 5E → 5T2 (5D) transition of Fe2+ ions in Td sites is observed, corresponding to a ground-state absorption cross section of 0.28×10-18 cm2 at 1.90 μm. The glass-ceramics were prepared by secondary two-stage heat-treatments of the magnesium aluminosilicate glass nucleated by titanium oxide and doped with 0.1 mol% FeO. Transparent Fe:MgAl2O4-based glass-ceramics obtained at the temperature of the second stage of 800 – 1000 °C were multi-phase materials containing two crystalline nanophases, i.e., spinel (mean size: 3.7 – 7.4 nm) and magnesium aluminotitanate solid solution (mean size: 6.4 - 20.6 nm), as well as residual silica-rich glass. Glass-ceramics obtained at the temperature of the second stage of 1050 °C were transparent and based on Fe-doped sapphirine. For glass-ceramics, absorption has a much more complex character as it is caused by interplay of iron and titanium ions in different valence states, coordination sites and locations. The iron ions enter the spinel nanocrystals but unlike the ceramic, in the form of both VIFe2+ and IVFe2+ species. The developed ceramics and glass-ceramics are promising for saturable absorbers of mid-infrared (2-3 μm) lasers.