Spectrum of Exchange Coupled Cr3+ Pairs in YAlO3

Abstract

The optical spectrum and exchange splittings of the two types of nearest neighbor Cr3+ pairs in orthorhombic YAlO3 have been studied. The emission spectrum from 7200 to 7350 Å yields the ground state exchange −JS.S+j(S.S)2 with Ja=−24.7, ja=−0.66 cm−1 and Jb=−26.2 and jb=−0.91 cm−1. From the shift of the lines under uniaxial compression along the crystal z axis we conclude these values correspond, respectively, to pairs whose axes are in the xy plane and along the z axis. The energies of several pair levels with one ion in 2E and the other in 4A2, having S=1 and 2 have been obtained. Selective enhancement of nn pair luminescence is observed when the excitation energy is at about twice the single ion 2E energy. The excitation and absorption spectra indicate these are pair levels corresponding to both ions in the 2E state and have S=0 and 1. The single ion transitions are magnetic dipole in agreement with Cr3+ being at a center of inversion. The pair lines are electric dipole, and the strongest transitions in the infrared, and the pair lines in the ultraviolet obey the selection rule ΔS=0 indicating the pair transitions obtain their intensity from the spin dependent dipole moment mechanism.