Nature of the broadband luminescence center in MgO:Cr3+

The luminescence spectrum of MgO:${\mathrm{Cr}}^{3+}$ is characterized by a series of sharp features at around 7000 Å and a broad band which stretches from about 7000 Å to 1 μm. The nature of the centers responsible for this broadband luminescence is examined by a combination of phase-sensitive detection methods and polarized absorption and luminescence methods. The phase-sensitive detection method allows us to separate clearly the broadband luminescence from the remainder of the spectrum and a sharp no-phonon line associated with the broadband luminescence is uncovered. The centers responsible for the broadband luminescence are found to have axes of symmetry along [011]-type directions, supporting the conclusion that this luminescence originates on ${\mathrm{Cr}}^{3+}$ ions in rhombic centers. At high temperatures broad emission is also observed from cubic site ${\mathrm{Cr}}^{3+}$ ions. The cubic site luminescence is shown to behave in accordance with theoretical expectations.