Optical Spectra of Cr3+ Impurity Ions in Ferroelectric LiNbO3 and LiTaO3

The optical absorption, luminescence, and excitation spectra of LiNbO3 and LiTaO3 doped with chromium impurities are examined as a function of temperature. Several interesting differences between these spectra and those of other chromium-doped oxides are noted. The most important differences are the large halfwidth of the Cr3+-ion “R” lines (∼50 cm−1 at 4.2°K) and the absence of any “R”-line luminescence down to 4.2°K. Instead, a broadband emission peaking at about 1 μ is observed. It is shown that these differences arise from the low value of the crystal-field parameter Dq at the Cr3+-ion site in LiNbO3 and LiTaO3, so that the zero-phonon T24 state of the Cr3+ ion lies at a lower energy than the E2 state. Point-charge calculations of Dq for several oxides, including LiNbO3 and LiTaO3, and other considerations suggest that the most probable sites of the Cr3+-ion impurities are the Nb/Ta sites and not the Li sites.