Spectrum of Exchange Coupled Cr3+ Pairs in YAlO3

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 $-\mathrm{JS.S}\mathrm{+j(}\mathrm{S.S}{)}^2$ with $J_a{\text{=−24.7,\hspace{0.17em}j}}_a\text{=−0.66\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ and $J_b\text{=−26.2}$ and $j_b\text{=−0.91\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ . 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.