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.
Categories: cond-mat.str-el cond-mat.mtrl-sci