Glassy disordered ground states in the frustrated pyrochlore and fluorite antiferromagnets NaCdM2F7 (M = Ni2+, Mn2+)

Abstract

We report the crystal structures, magnetic and thermodynamic properties of two magnetically frustrated A’A”M₂F₇ -type antiferromagnets, NaCdNi₂F₇ and NaCdMn₂F₇. While NaCdNi₂F₇ forms a stable pyrochlore structure (SG: Fd-3m, #227) with magnetic S = 1 Ni²⁺ ions on the frustrated pyrochlore 16c site and fully disordered non-magnetic Na⁺/Cd²⁺ ions on the pyrochlore 16d site, NaCdMn₂F₇ favors the defect-fluorite structure (SG: Fm-3m, #225) with magnetic S = 5/2 Mn²⁺ and non-magnetic Na⁺ and Cd²⁺ ions fully disordered on the fluorite 4a site. This is a result of the Mn²⁺ ionic radius being too close to the average Na⁺/Cd²⁺ ionic radius, hindering the cationic site ordering towards the stable pyrochlore structure. In both cases, dominant antiferromagnetic interactions θ_CW,Ni = -91.2(5) K and θ_CW,Mn = -42.4(4) K are noted, with no magnetic transition until T_f,Ni = 3.2 K and T_f,Mn = 2.0 K, implying substantial frustration, with frustration indices f_Ni = 28.5 and f_Mn = 21. AC susceptibility measurements and bifurcation of ZFC/FC low-field magnetization indicate a spin-glass-like ground-state, precipitated by the magnetic-bond-disorder stemming from the inherent structural disorder.