Viologens 10 1 s ; VCH Verlu~,sKesellJchuft mhH, D-6940 Wcinheim, 1990 OY35-904Kj90/0f0l-0010 S 02.50/0 A&. Muter. 2 (1990) No. 1

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The ac conduction in N i 0 films has been investigated in the frequency range 10 Hz < V < 109 Hz and at temperatures between 10 and 300 K. The frequency and the temperature dependence of the electrical conductivity can be consistently explained within a model developed for the mechanism of charge transfer in amorphous semiconductors which proposes that charge carriers hop over potential barriers between defect sites, the height of the barriers being correlated with the intersite separation.Nickel oxide is a Mott-Hubbard insulator ' which crystallizes in a high-temperature rocksalt structure and exhibits antiferromagnetism below 523 K. Below the NLel temperature the lattice contracts along one of its body diagonal~ and undergoes a cubic to rhombohedral phase transformation.' N i 0 is a metal-deficient p-type semiconductor and the most thoroughly and most carefully studied narrow-band transition-meta1 oxide. However, despite a huge amount of conductivity, thermoelectric power and Hall mobility investigations it was impossible to arrive at definite conclusions concerning the electrical transport properties. The long-standing and persistent controversy, on whether the conduction in N i 0 is due to thermal excitation of the carrier concentration (band semiconductor) or due to thermal excitation of the mobility (small-polaron hopping) is documented in a number of review articles.Renewed interest arose due to the fact that hydrated N i 0 can be used as an active eiectrochromic material. There have been a number of proposals for the development of electrochromic devices such as displays, active optical filters, automotive rear-view mirrors with adjustable reflectance and smart windows with adjustable absorption and reflectance in the visible and near-infrared wavelength range.In this paper we report measurements of the frequericy V and temperature T dependence of the ac conductivity o(v,T) in reactively evaporated N i 0 films. We will provide evidence that both the frequency and the temperature dependence of o can be well described in the correlatedbarrier hopping (CBH) model. In this model it is assumed that the charge carriers hop between defect centers over the potential barrier W separating them. The barrier height is correlated with the separation R of the defect states. This model was introduced by pike6 to account for the dielectric loss in scandium oxide films. With a similar model Pollak and pike7 investigated the ac conductivity of glasses due to atomic and electronic hopping. Later on the CBH model was successfully applied by ~lliott' to explain the ac loss in chalcogenide glasses assuming that electron pairs hop between defect centers. In the singleelectron CBH model, the frequency dependence of the conductivity can be expressed by9.I0 o-W", withThe temperature dependence of a can, in the narrow-band limit, be given in power-law form as9.l0 o-T", with n =(1 -s)ln(llwro). W,,, is the barrier height for infinite intersite separation and thus corresponds to the energy to take the charge carrier from the defect ...

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