Crystal Growth and Magnetism of Transition Metal Pyrochlore Fluorides

Geometric magnetic frustration arises when the geometry of a structure prevents the simultaneous fulfillment of nearest-neighbor antiferromagnetic interactions and is commonly observed in lattices that exhibit a triangular topology, such as those found in the pyrochlore structure. Via a mild hydrothermal route, we have synthesized seven quaternary β-pyrochlore-related fluorides AM²⁺M³⁺_(2-F₆ (A = Cs and Rb; M²⁺ = Co²⁺, Ni²⁺, and Zn²⁺; and M³⁺ = V³⁺ and Fe³⁺). Crystal structures and compositions were determined using a combination of single-crystal X-ray diffraction and energy-dispersive spectroscopy. After adjusting the reaction conditions, phase-pure products of AM²⁺M³⁺_(2-F₆ were obtained. The magnetic susceptibility and isothermal magnetization data for all seven compounds were collected to interpret the magnetic behavior, which ranged from paramagnetic to antiferromagnetic with and without a ferromagnetic component. We found that the magnetic behavior of the AM²⁺V³⁺_(2-F₆ pyrochlore structures strongly depends on the presence or absence of unpaired electrons on the M²⁺ position. The titled pyrochlore compounds, with the exception of the Zn-analogue, can be considered frustrated materials, with frustration indices in the range of 6-13.