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

Abstract

The optical absorption,luminescence, and excitation spectra of LiNbO3 and LiTaO3doped 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 4 T 2 state of the Cr3+ ion lies at a lower energy than the 2 E 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.