Dioxocobaltate(II) Anions in Alkaline Earth-Lanthanum Germanate Apatites: Magnetic Anisotropy and Correlated Dual Slow Relaxation of Magnetization.

Cobalt(II) enters the trigonal channels of calcium-lanthanum and strontium-lanthanum germanate apatites, forming the nonlinear dioxocobaltate(II) ion [OCoO]2-. Samples reveal two magnetization relaxation processes: SR1 characterized by a Ueff of 47-48 cm-1 and SR2 with considerably extended relaxation times and a weak temperature dependence. The relaxation times of SR1 show an unusual steplike drop with increasing temperature, which correlates with the SR2 contribution to the magnetization. Temperature and field dependence of the magnetization are described by an axial zero-field splitting model, yielding negative D values corresponding well to the Ueff values obtained. Modeling the electronic structure of the dioxocobaltate(II) ion provides information about the Co-O distance ranges corresponding to two different ground states, a moderately anisotropic spin-only one with spin S = 3/2 and an extremely anisotropic non-Aufbau one with S = 3/2 and an orbital angular moment L = 3. Experimental D and g|| values correspond to certain Co-O distances in the spin-only state. The distances follow a qualitative relation with estimated Coulomb forces acting between ions in the host compound. These findings suggest that to convert the dioxocobaltate(II) anion into the desired non-Aufbau state, one must construct a host apatite crystal from ions with lower charges and larger sizes.