An orthorhombic chromium-center in the spinel MgAl2O4

In Cr-doped MgAl₂O₄ most of the Cr³⁺-ions occupy normal B-sites. In addition to this axial center we report here the presence of an orthorhombic Cr³⁺-center. The ESR-spectra of this new center can be described by the spin Hamiltonian:

$?=\mathit{D}{S}_z^2?\frac{1}{3}\mathit{S}\left(S+1\right)]+g_x\beta H_x{S}_x+g_y\beta H_y{S}_y+g_z\beta H_z{S}_z+\mathit{E}\left(S_x^2?S_y^2\right)$

, with S=3/2. The principal axes of the center are along 111]=Z, $111]=Z,1\overline{1}0]=Y$ and $\overline{1}\overline{1}2]=X$. The values of the parameters are: g_z=1·950±0·005, g_x=g_y=1·915±0·015, D=+0·903cm^?1±0·003 cm^?1 and E=+0·084 cm^?1±0·005 cm^?1. The intensity of the lines is about ten times lower than that of the axial Cr³⁺-center.