Observations of the absorption, infra-red emission, and excitation spectra of Cr in

Photoluminescence and optical absorption in Cr-doped were observed for the first time and studied within the 300-800 nm spectral region and over a 7-300 K temperature range. `Nominally pure' crystals, with Cr impurities detected by means of EPR, and intentionally Cr-doped crystals grown by the top-seeded solution method were studied. Cr doping results in additional absorption over the whole visible region, increasing towards the band-edge area (at ), and wide absorption bands centred at about 470 nm, 575 nm, and 610 nm. At 74 K, exposure to optical excitation with results in luminescence in the near-infra-red region consisting of four sharp lines, (781.65 nm, ), (776.4 nm, ), (768.8 nm, ), (766.8 nm, ), and several additional weak emission lines. The luminescence excitation spectrum for each A line consists of two complex bands centred at 630 nm and at 400 nm for T = 74 K. With decreasing temperature, all of the sharp lines, such as the zero-phonon lines of in , shift to lower energies, which is opposite to the behaviour of such lines for ionic crystals. The thermal shift for the line is the largest, . Taking into consideration EPR data, temperature transformations, and the lifetime of the sharp emission lines ( at 70 K), we argue that the and lines are lines, i.e. originating from zero-phonon transitions of single ions replacing ions. The nature of the emission lines is not quite clear, but can be considered to originate from ions exchange coupled with other, unknown defects (including exchange-coupled pairs) or with -type transitions of centres perturbed by nearest-site hole polarons, or .