Nuclear magnetic resonance investigation of the metal-to-semiconductor transition in crystalline CdO

Measurements of the Cd¹¹³ nuclear-spin-lattice relaxation time (T₁) and resonance frequency shift (K) were carried out in several samples of polycrystalline n-type CdO over the temperature (T) range 1.4-300°K, frequency (ν) range 2-10 MHz, and carrier concentration (Nₑ) range 3.3-44×10¹⁸ cm⁻³. For Nₑ>10¹⁹ cm⁻³, we find that $T_1\propto {N_e}^{\frac{-2}{3}}{T}^{-1}$ and $K\propto {N_e}^{\frac{1}{3}}$, showing that the nuclei are interacting with degenerate conduction electrons. Furthermore, the Korringa relationship holds and the calculated electron density at a nucleus is almost the same as that in a free atom, indicating that the electrons are in the host-lattice conduction band. The Hall constant is independent of temperature, also suggesting that there is no separate impurity band. Conversely, for Nₑ<10¹⁹ cm⁻³ none of the above-mentioned relationships holds and we must invoke a model in which the electrons are concentrated near impurity centers, forming an impurity band, with nuclei near the centers experiencing strong contact interactions while nuclei far from the centers experience only indirect interactions through nuclear-spin diffusion. These two groups of nuclei do not maintain a common spin temperature and thus the recovery of the magnetization is nonexponential. Finally, the concentration (N) at which the impurity wave functions no longer form a band is estimated from the Mott criterion to be about 2×10¹⁸ cm⁻³, slightly below our lowest-concentration sample. Thus, assuming that Nₑ≃N, we find that for N<2×10¹⁸ cm⁻³ the impurity wave functions are localized, for 2×10¹⁸10¹⁹ cm⁻³ the Fermi level crosses into the host-lattice conduction band.