Low-Temperature Luminescence of Lithium Aluminate Spinel LiAl5O8 Doped with Chromium Ions

Ceramic LiAl5O8 lithium aluminate spinel materials doped with trivalent chromium ions have been prepared by high-temperature solid-state synthesis. The intensity, spectral shift, and width of two zerophonon luminescence lines (R lines) due to transitions from the E-2 excited state to the (4)A(2) ground state of Cr3+ ions have been measured as functions of temperature in the range 80-295 K. Analysis of luminescence and luminescence excitation spectra of the ceramics points to severe local lattice distortion around the Cr3+ ions, which shows up, in particular, as large inhomogeneous broadening of the zero-phonon Cr3+ luminescence lines. The temperature dependences of the relative intensity, spectral shift, and width of the two zero-phonon lines can be sufficiently well described in terms of existing models for optical centers in crystals. The feasibility of using these temperature dependences for noncontact luminescence thermometry is assessed. Analysis of the data obtained in this study provides no way to uniquely identify the observed zero-phonon lines as due to transitions from two sublevels of the E-2 state of a single type of chromium center (that is, as R-1 and R-2 lines) or transitions of two inequivalent chromium centers coupled by nonresonant energy transfer.