Cluster-Glass Freezing with a Ferrimagnetic-like Component in KNa3Co5.74Cu1.26(PO4)6 and Antiferromagnetism in KNa3Cu7(PO4)6

The ANa3M7(PO4)6 series unites isostructural alkali A and transition metal M phosphates, arranged from diamond-like chains combined into a triperiodic metallic substructure. Two members of the family, a monoclinic, partially ordered solid solution KNa3Co5.74Cu1.26(PO4)6 and the first triclinic Cu variety KNa3Cu7(PO4)6, were obtained by spontaneous crystallization. Their morphology, chemical composition, and crystal structure were studied by means of electron microscopy, EDS analysis, and single-crystal X-ray diffraction. The symmetry lowering from monoclinic to triclinic in KNa3Cu7(PO4)6 is associated with Jahn–Teller distortion of Cu2+ polyhedra. Thermodynamic measurements have revealed that KNa3Co5.74Cu1.26(PO4)6 exhibits ferrimagnetic-like behavior below TG = 6.4 K. KNa3Cu7(PO4)6 exhibits long-range antiferromagnetic order at TN = 8 K, even though its Curie–Weiss temperature is very high in magnitude, Θ = −93 K. The analysis of the spin exchange interactions in KNa3Cu7(PO4)6 indicates that it is not caused by geometrical spin frustration, but by the fact that its spin–lattice has a layered character with weak interlayer interactions.