Electronic transport phenomena in single-crystal NiO and CoO

Abstract

Hall effect, conductivity and Seebeck measurements have been made on a series of NiO and CoO crystals containing up to approximately 0.6 at.% Li in the range 150-1100 °K. The results are interpreted in terms of 3d conduction in a narrow band and indicate that impurity conduction is dominant in many samples up to 200-300 °K, even when no Li is added. The isotropy of conduction in single domains of NiO suggests that 3d conduction depends on indirect overlap through the oxygen ions, rather than direct 3d overlap. The Hall and Seebeck data show that holes require an activation energy to get into this band, ranging from 0.16 to 1.2 ev and depending on the Li concentration. The quantity md*(μD/μH)2/3 = 145 mo in NiO and approximately 250 mo in CoO. In NiO and CoO the room temperature 3d Hall mobilities are approximately 0.4 and 0.1 cm2 v-1 sec-1 respectively and decrease with temperature. The drift mobility μD also decreases with temperature, but the calculated variation depends on assumptions about the bandwidth and its temperature dependence. In NiO spin-disorder scattering may be significant near the Neel point TN. The ratio μD/μH is unusually large (similar 10 or more) and in NiO shows a further anomaly at TN. It is concluded that a hopping model is not applicable to 3d conduction in these oxides. The results are discussed in terms of Holstein's theory of narrowband polaron conduction and indicate an electron-phonon coupling constant γ for NiO in the range 0.5-1.0.