An Investigation of the Ground State of Cr$^{2+}$ in MgO Based on Thermal Conductivity Measurements

Measurements of the thermal conductivity of MgO doped with chromium in the temperature range 0.4 to 300 K indicate the existence of resonant scattering by Cr$^{2+}$ ions at 5.2 and 21 cm$^{-1}$. These measurements confirm that the ion cannot be described by simple crystal field theory or by static Jahn-Teller theory, but they are consistent with the dynamic Jahn-Teller theory of Fletcher & Stevens (1969) with the parameters $\delta $ tunnelling frequency) = 16 cm$^{-1}$ and D (spin-orbit splitting) = 4 cm$^{-1}$. It is not possible to determine the sign of the anharmonic term, B, from the measurements. These parameters differ significantly from those determined by Fletcher & Stevens and those determined by Ham (1971) from acoustic paramagnetic resonance data for Cr$^{2+}$ in MgO. The data also appear to confirm earlier indications that strong coupled mode effects are present. The scattering in a Ni-doped specimen was found to be largely due to the difference in mass between the Ni and Mg ions, whereas that in Fe-doped specimens was largely due to a process resonant at ca. 100 cm$^{-1}$.