Broadband NIR photoluminescence from Ni2+-doped nanocrystalline Ba–Al titanate glass ceramics

Abstract

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 ³T_2g(³F) to ³A_2g(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.