Isotopic Shift in the Electron-Spin-Resonance Absorption Spectrum ofCr3+in Magnesium Oxide

The fine and hyperfine structure spectra of trivalent ${\mathrm{Cr}}^{52}$ (I=0) and ${\mathrm{Cr}}^{53}(\mathrm{I}=\frac{3}{2})$ located substitutionally for magnesium in magnesium oxide at sites of axial symmetry, where the axial distortion is along a crystal [100] direction, have been re-examined at 3-cm wavelength. Spectral constants for both isotopes have been measured with a precision equivalent to determining absorption-line field positions to within ±10 mOe. From measurements taken at 77°K, it is concluded that the crystalline-field splitting energy for ${\mathrm{Cr}}^{52}$ is (0.10±0.01) Oe greater in magnitude than that for ${\mathrm{Cr}}^{53}$, which corresponds to a relative shift in this parameter of 1.1×${10}^{-4\mathrm{}}$. In addition, the components of the spectroscopic splitting tensor for the two isotopes are found to be the same to within the limits of experimental error. From data taken over the range 4.2 through 350°K, it is found that the isotopic shift decreases with temperature while the crystalline-field splitting energy increases with temperature. The origin of the isotopic shift and its temperature dependence are discussed in terms of a dynamical crystal structure.