Dioxocobaltate(<scp>ii</scp>)-anion in calcium vanadate apatite: partial ordering, magnetic anisotropy, and slow relaxation of magnetization

Polycrystalline ceramic and small single-crystal samples of Co-containing calcium vanadate(V) with apatite structure were prepared for the first time. The Co²⁺ ions enter the apatite trigonal channels formally substituting protons of the OH^- groups and form separate O-Co-O atomic groups elongated in the <i>c</i> direction, Co being additionally weakly coordinated to an oxygen atom of a VO₄ group. At a high Co content, the hexagonal apatite structure undergoes a triclinic distortion followed by partial ordering of the Co²⁺ ions. The dc magnetic data fit well to a model of a zero-field split <i>S</i> = 3/2 state with a large negative <i>D</i> of -22 to -25 cm^-1, suggesting a strong easy-axis magnetic anisotropy. The ac susceptibility measurements below <i>T</i> = 10 K reveal a multichannel slow relaxation of the magnetization in non-zero dc field. The temperature dependence of the relaxation time can be described by an Orbach process with the remagnetization energy barrier <i>U</i>_eff being equal to experimentally determined 2|<i>D</i>|. Modelling of the electronic structure shows that, with a small increase of the crystal field strength, the high-spin Co²⁺ ion changes its ground state from one with an unquenched orbital moment <i>L</i> = 3 to a fully orbitally quenched one. Both states are characterized by easy-axis magnetization vectors directed approximately perpendicular to each other with a smooth rotation of the vector at intermediate crystal fields. The model explains the weak magnetic anisotropy observed in the triclinic single crystal as well as the earlier reported ability of the dioxocobaltate(II) ion to behave like a single-ion magnet with either a high <i>U</i>_eff of hundreds of cm^-1 or a moderate one of tens of cm^-1. To the best of our knowledge that represents the first instance of the conversion of the d-element ground electronic state from orbitally degenerate to non-degenerate by a slight variation of the crystal field.