Nickel oxide magnetic susceptibility

The spectrum of carbon monoxide adsorbed on nickel oxide prepared at 200° may be divided into two regions (Table I, la) (60). The first includes two bands at 2060 and 1960-1970 cm i, the second three bands at 1620, 1575, and 1420-1440 cm-i. Bands at 2060 and 1960-1970 cm- are typical of carbonyl structures and are found in the spectrum of carbon monoxide on metallic nickel (61). It has been suggested by some authors (62) that, in our experiments, these bands were also produced by the adsorption on the metal, the oxide being supposed oxygen-deficient. Chemical analyses (30) have shown, however, that, NiO(200°) contains an excess of oxygen and magnetic susceptibility measurements (33) have demonstrated that the quantity of metal is very small. Since the intensity of these bands is strong, we believe that they are not produced exclusively by the chemisorption of carbon monoxide on the metal but mainly by the adsorption on exposed nickel ions. 

NiO(250°) contains more metallic nickel than NiO(200°). Magnetic susceptibility measurements have shown that carbon monoxide is adsorbed in part on the metal (33) and infrared absorption spectra have confirmed this result since the intensity of the bands at 2060 cm-i and 1960-1970 cm-1 is greater when carbon monoxide is adsorbed at room temperature on samples of nickel oxide prepared at temperatures higher than 200° and containing therefore more metallic nickel (60). Differences in the adsorption of carbon monoxide on both oxides are not explained entirely, however, by a different metal content in NiO(200°) and NiO(250°). Differences in the surface structures of the oxides are most probably responsible also for the modification of their reactivity toward carbon monoxide. In the surface of NiO(250°), anionic vacancies are formed by the removal of oxygen at 250° and cationic vacancies are created by the migration of nickel atoms to form metal crystallites. Carbon monoxide may be adsorbed in principle on both types of surface vacancies. Adsorption experiments on doped nickel oxides, which are reported in Section VI, B, have shown, however, that anionic vacancies present a very small affinity for carbon monoxide whereas cationic vacancies are very active sites. It appears, therefore, that a modification of the surface defect structure of nickel oxide influences the affinity of the surface for the adsorption of carbon monoxide. The same conclusion has already been proposed in the case of the adsorption of oxygen. 

Gallium-doped nickel oxide contains more metallic nickel than pure or lithiated nickel oxides (Table X). Concentrations of metal deduced from magnetic susceptibility measurements (23) are, moreover, in agreement with the results of chemical analyses (30). The following mechanism of incorporation explains these results (80) ... 

Removal of lattice oxygen from the surface of nickel oxide in vcumo at 250° or incorporation of gallium ions at the same temperature [Eq. (14)] causes the reduction of surface nickel ions into metal atoms. Nucleation of nickel crystallites leaves cationic vacancies in the surface layer of the oxide lattice. The existence of these metal crystallites was demonstrated by magnetic susceptibility measurements (33). Cationic vacancies should thus exist on the surface of all samples prepared in vacuo at 250°. However, since incorporation of lithium ions at 250° creates anionic vacancies, the probability of formation of vacancy pairs (anion and cation) increases and consequently, the number of free cationic vacancies should be low on the surface of lithiated nickel oxides. Carbon monoxide is liable to be adsorbed at room temperature on cationic vacancies and the differences in the chemisorption of this gas are related to the different number of isolated cationic vacancies on the surface of the different samples. 

Below 6 per cent nickel the first observation is that the Weiss constant is zero. The form of the susceptibility isotherm is thus in the case of nickel in no way related to the exchange interaction between adjacent nickel ions. This is not to say that the nickel ions are at infinite magnetic dilution. For nickel in massive nickel oxide the exchange integral, the paramagnetic neighborhood (z), and the number of unpaired electrons are smaller than they are for chromium ions in massive chromia. The quantity. A, is understandably smaller for the case of nickel, and it... 

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