Zntrodution The Cr3+-doped LiNbO, single crystal has potential application as a frequency-tunable laser material [l]. In the ferroelectric phase, the Li and Nb ions are surrounded by six oxygens in a distorted octahedron and lie along the C,-axis, but away from the centre of the octahedron. In the [lll] direction, the cations are distributed in the octahedron in the following sequence: Nb, structural vacancy, Li, Nb, structural vacancy, Li, etc. Negative sign of D (-0.392 cm-') has been determined for Cr3+ : LiNbO, at 5, 77, and 300 K from X-and Q-band studies. Therefore, to judge which result is suitable, it is necessary to examine the D-value from an independent experiment. In this work, we will firstly study the optical spectra. Secondly, based on the crystal-field theory, B, C, id, Dq, V, and V' can be estimated by using the room temperature optical spectra. Finally, all the parameters obtained are used to calculate the EPR D-value and the result is compared with those published in [lo] to evaluate the rationality of the crystal-field theory for calculating the D-parameter.
Optical measurements and resultsA large single crystal of Cr3+ : LiNbO, (0.8 wt%) is pulled from the melt by use of the Czochralski technique. The crystal axes are checked by X-ray analysis. Siu and Zhao [9] have studied the EPR spectra of the single ion and the Cr3+-Cr3+ pair. Here, we will study the optical spectra for Cr3+ : LiNbO,.The optical measurements are carried out on the Cr3+ : LiNbO, sample (2 x 2 x 0.1 cm3) by using UV-VIS-CZ-2 spectrophotometer (made in Institute of Earth Chemistry of Chinese Academy of Science) and Beckman UV-VIS-DU-8 spectrophotometer. Fig. 1 shows the polarized absorption spectra at room temperature, Fig. 2 the luminescence spectrum for the R-line. The results are listed in Table 1.Calculations of the spectralparameters It has been pointed out above that the spectral parameters B, C, Dq, V, and V ' should be determined from detailed spectral information.') Chengdu 610068, People's Republic of China.18 physica (b) 187/1
