  We report neutron diffraction studies of TbCo$_2$Zn$_{20}$ and
TbFe$_2$Zn$_{20}$, two isostructural compounds which exhibit dramatically
different magnetic behavior. In the case of TbCo$_2$Zn$_{20}$, magnetic Bragg
peaks corresponding to antiferromagnetic order are observed below $T_N$
$\approx$ 2.5 K with a propagation vector of (0.5 0.5 0.5). On the other hand,
TbFe$_2$Zn$_{20}$ undergoes a ferromagnetic transition at temperatures as high
as 66 K which shows a high sensitivity to sample-to-sample variations. Two
samples of TbFe$_2$Zn$_{20}$ with the same nominal compositions but with
substantially different magnetic ordering temperatures ($T_c$ $\approx$ 51 and
66 K) were measured by single crystal neutron diffraction. Structural
refinements of the neutron diffraction data find no direct signature of atomic
site disorder between the two TbFe$_2$Zn$_{20}$ samples except for subtle
differences in the anisotropic thermal parameters. The differences in the
anisotropic thermal parameters between the two samples is likely due to very
small amounts of disorder. This provides further evidence for the extreme
sensitivity of the magnetic properties of TbFe$_2$Zn$_{20}$ to small sample
variations, even small amounts of disorder.
