Broadband NIR photoluminescence from Ni ²⁺ -doped nanocrystalline Ba–Al titanate glass ceramics

Nanocrystalline Ba–Al titanate precipitates from supercooled TiO2–BaO–SiO2–Al2O3 melts by catalyzed volume nucleation in the presence of Ni2+, forming a BaAl2Ti6O16 hollandite-type lattice. Ni2+-species are incorporated into the crystalline environment in octahedral coordination. Hollandite formation is accompanied by precipitation of tetrahedrally distorted BaTiO3 as a secondary crystal phase, where crystal species and habitus can be clearly distinguished by dark-field transmission electron microscopy. Resulting photoluminescence due to spin-allowed relaxation of 3T2g(3F) to 3A2g(3F) in VINi2+ occurs from three distinct emission centers. It spans the spectral range of 1.0 to 1.6 μm and exhibits a lifetime of about 60 μs, which suggests applications in tunable lasers and broadband optical amplifiers. Besides red and IR laser excitation, NIR photoemission can be excited with conventional near UV light sources, i.e. in the spectral range of 350–420 nm. Decay kinetics as well as position and shape of the emission band can be adjusted by dopant concentration and synthesis conditions.