Comment on “Magnetism and structure of Li$_x$CoO$_2$ and comparison to Na$_x$CoO$_2$”

In the article [Phys. Rev. B 77, 075119 (2008)], effective magnetic moments 𝑝eff
per Co site have been determined from magnetic susceptibility measurements for a series of Li$_x$⁢CoO$_2$ samples. The nominal Li content 𝑥 (ranging theoretically from 1 to 0) determines the concentration of Co$^{3+}$ (𝑥) and Co$^{4+}$(1−𝑥) that occupy the Co sites. Assuming that the Co$^{3+}$ species is diamagnetic, the effective magnetic moment 𝑝eff per Co$^{4+}$ ion has been calculated from the experimentally determined effective magnetic moment 𝑝eff per Co site and the nominal Co$^{4+}$ concentration in the cited publication [Phys. Rev. B 77, 075119 (2008)] by assuming a purely linear relation between the effective magnetic moments and the concentrations. From the viewpoint of this comment's author, this linear approach is not valid and the linear effective magnetic moments 𝑝eff utilized in the formulas in the cited article should be replaced by their squared values 𝑝2 eff. This comment provides the correct formulas for the calculations aimed to be performed in [Phys. Rev. B 77, 075119 (2008)] and recalculates the published values. Since for low Co$^{4+}$ concentrations a high-spin species is discussed to exist in Li𝑥⁢CoO$_2$ in numerous publications, correct handling of effective magnetic moments is of general interest in this discipline of research.