Abstracts

Fe³⁺ ions in solids exhibit luminescence in the spectral regions between red and near-infrared, depending on the local environment symmetry and distances between central ions and ligands. For a very long time, Fe-dopant was rather considered as being a luminescence quencher, however relatively recently it has been shown that it may be a very efficient luminescence activator. It is most probably related to pure doping with Fe in a 3+ valence state since Fe²⁺ ions act as the emission quenchers. The basic optical properties of this dopant are well described by the crystal field theory for d⁵ electronic configuration. According to this theory, all optical transitions at ambient conditions occur between the ⁶A₁ ground state and higher energy excited states with a different spin, thus being strongly spin-forbidden, having very small transition probability and long decay times of luminescence. Usually, the optical transitions, especially between the ground and the first excited level are also strongly coupled with the lattice, thus both absorption and luminescence spectra are broad, often without 0-phonon lines, even at very low temperatures. An externally applied pressure may strongly influence the spectral position of luminescence as well as the luminescence decay kinetics of the ions with a d⁵ electronic structure. In this report, we present the optical properties of LiGaO₂ doped with Fe³⁺. The 0.25% iron (Fe³⁺) doped LiGaO₂ phosphor, synthesized by a high-temperature solid-state reaction method, turned out to be a β polymorph of the LiGaO₂ with an orthorhombic crystallographic structure. The phosphor exhibits a photoluminescence band around 746 nm related to the ⁴T₁ → ⁶A₁ transition with a 28% quantum efficiency at ambient conditions. The luminescence of this material shifts towards longer wavelengths with pressure increase, which agrees very well with the Tanabe-Sugano crystal field theory. Evidence of pressure-induced phase transitions are observed in the luminescence spectra of LiGaO₂:Fe. At higher pressures, the luminescence is quenched due to reversible amorphization of the powder.Additional luminescence 0-ph line is observed at 695 nm wavelength, which is suspected to be related to Fe³⁺ ions in Li sites. To confirm this hypothesis several additional experiments were performed, including electron paramagnetic resonance and a ⁵⁷Fe Mossbauer spectroscopy study. The results of these experiments will be discussed.