  Nabokoite family compounds ACu$_7$(TeO$_4$)(SO$_4$)$_5$Cl (A=Na, K, Cs, Rb)
are candidates for the experimental realization of highly-frustrated 2D square
kagome lattice (SKL). Their magnetic subsystem includes SKL layers decorated by
additional copper ions. All members of this family are characterized by quite
high Curie-Weiss temperatures ($\sim 80-200$ K), but magnetic ordering was
reported only for Na and K compounds at a much lower temperatures below 4 K. We
report here results of the study of high-frequency ($\sim 10$ GHz) dielectric
properties of this family of compounds. Our study revealed presence of the
strong dielectric anomaly both in the real and imaginary parts of
high-frequency dielectric permittivity for Na and K compounds approx. 100 and
26 K, correspondingly, presumably related to antiferroelectric ordering.
Additionally, much weaker anomalies were observed at approximately 5K
indicating possible interplay of magnetic and lattice degrees of freedom. We
discuss possible relation between the structure rearrangements accompanying
dielectric anomalies and a delayed magnetic ordering in the nabokoite family
compounds.
