NaCaNi2F7: A frustrated high-temperature pyrochlore antiferromagnet with S=1 Ni2+

NaCaNi2F7 is an insulating, frustrated A(2)B(2)F(7) pyrochlore with magnetic S = 1 Ni2+ on the pyrochlore B site. Nonmagnetic Na and Ca are disordered on the A site. Magnetic susceptibility measurements made on an oriented single crystal, grown in a floating zone furnace, show isotropic behavior at temperatures between 5 and 300 K, with an effective moment of 3.7 mu(B)/Ni. Despite displaying a large Curie-Weiss theta (-129 K), spin-ordering- related features are not seen in the susceptibility or specific heat until a spin glass transition at 3.6 K. This yields an empirical frustration index of f = -theta(CW)/T-f approximate to 36. The spin glass behavior is substantiated by a shift of the freezing temperature with frequency in the ac susceptibility, bifurcation in the dc susceptibility, and by a broad maximum in the magnetic specific heat. The observations as made on large single crystals suggest that NaCaNi2F7 is likely a realization of a frustrated spin 1 pyrochlore antiferromagnet with weak bond disorder.