Glassy disordered ground states in the frustrated pyrochlore and fluorite antiferromagnets NaCd$M_2$F$_7$ ($M$ = Ni$^{2+}$, Mn$^{2+}$)

Abstract

We report the crystal structures, magnetic and thermodynamic properties of two magnetically frustrated $A$'$A$"$M_2$F$_7$-type antiferromagnets, NaCdNi$_2$F$_7$ and NaCdMn$_2$F$_7$. While NaCdNi$_2$F$_7$ forms a stable pyrochlore structure (SG: $Fd \overline{3} m$, #227) with magnetic $S$ = 1 Ni$^{2+}$ ions on the frustrated pyrochlore 16$c$ site and fully disordered non-magnetic Na$^+$/Cd$^{2+}$ ions on the pyrochlore 16$d$ site, NaCdMn$_2$F$_7$ favors the defect-fluorite structure (SG: $Fm \overline{3} m$, #225) with magnetic $S$ = 5/2 Mn$^{2+}$ and non-magnetic Na$^+$ and Cd$^{2+}$ ions fully disordered on the fluorite 4$a$ site. This is a result of the Mn$^{2+}$ ionic radius being too close to the average Na$^+$/Cd$^{2+}$ ionic radius, hindering the cationic site ordering towards the stable pyrochlore structure. In both cases, dominant antiferromagnetic interactions $\theta_{CW,Ni}$ = -91.2(5) K and $\theta_{CW,Mn}$ = -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 glassy ground-state, precipitated by the magnetic-bond-disorder stemming from the inherent structural disorder.