Evidence of Two Forms of Cobaltous Oxide

The existence of two forms of cobaltous oxide, CoO(I) and CoO(II), has been established by Mössbauer, x-ray, and chemical techniques. CoO(I) has the NaCl structure as determined by a well-resolved powder x-ray pattern. Its density is measured to be 6.4 g/${\mathrm{cm}}^3$ from both x-ray and direct macroscopic experiments. ${\mathrm{Fe}}^{57}$ Mössbauer lines in CoO(I), doped with ${\mathrm{Co}}^{57}$, show only ${\mathrm{Fe}}^{2+}$ when determined by either the isomer shift or the hyperfine splitting below the Néel temperature. We find a Néel temperature of 288°K, which is 3°K less than the value reported by earlier workers. CoO(I) is completely inert in air at room temperature, and its stoichiometry does not change over time intervals as long as two months. CoO(II) could be prepared at low temperatures only. Form II also had the NaCl structure, but the x-ray line of the powder pattern was very broad. The density of CoO(II) is 3.2 (+1.0, -0.0) g/${\mathrm{cm}}^3$, about half of the value found for CoO(I). CoO(II) shows only ${\mathrm{Fe}}^{3+}$ lines in the Mössbauer pattern, with the appropriate isomer shift and hyperfine field. CoO(II) is very reactive with oxygen, at room temperature, although the x-ray pattern is not appreciably altered after oxygen absorption. CoO(II) has a Néel temperature of 270±10°K, slightly less than that of CoO(I), and an effective Mössbauer characteristic temperature of 320°K, substantially less than the 510°K value found for CoO(I). The results of studies of thermally activited transformation of II→I and pressure and quenching experiments are given. CoO(I, II), a mixture of forms I and II, is obtained by the usual chemical recipes. This mixed material was also studied. At low temperature, CoO(I, II) behaves as a simple mixture of I and II, but at temperatures above 200°K there is an interplay between the two forms which is responsible for the observed increase in the relative fraction of ${\mathrm{Fe}}^{3+}$ to ${\mathrm{Fe}}^{2+}$ observed in the Mössbauer patterns. This interplay is also connected with the shift in the apparent Néel temperature for ${\mathrm{Fe}}^{3+}$ in pure form II compared with the value obtained for CoO(I, II). A detailed model is proposed which explains all of the properties observed thus far. The data preclude the possibility of Auger after-effects in cobaltous oxide, as suggested by Wertheim, and they also show that stoichiometry fluctuations are not the principal cause of the observed ${\mathrm{Fe}}^{3+}$ Mössbauer line, as was recently claimed by Triftshäuer and Craig.