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.

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