High-pressure and magneto-optical studies of Cr-related defects in the lithium-rich LiNbO_3: Cr, Mg crystal

摘要

In this paper we present spectroscopic data clarifying the nature of the Cr~(3+) centers and Cr~(3+)-related defect in the LiNbO_3:Cr,Mg crystal containing an excess of Li~+ ions. Apart from quite well known lines associated with the ~4T_2 → ~4A_2 and ~2E→~4A_2 transitions of the Cr~(3+) ions, the luminescence spectrum of this crystal reveals some additional relatively narrow lines in the range of 750-860 nm. We carried out high-pressure and magneto-optical measurements of the crystallographically oriented samples of this crystal at temperature of 10 K. The measured spectra, decay times, pressure coefficients, and the dependence of the luminescence on high magnetic field show that the luminescence lines observed in lithium-rich LiNbO3:Cr,Mg originate from both Cr~(3+) ions and strongly antiferromagnetically coupled Cr~(3+)-Cr~(3+) first nearest-neighbor pairs. The ground-state levels of the pairs can be described by the isotropic exchange interaction H=JS_1·S_2+j(S_1-S_2)~2, where S_1 and S_2 are the total spins of each of the two chromium ions. The obtained results enabled us to determine the ground-state spin-spin exchange constants J and j as well as the excited-state zero field splitting constant D.