  The structural and magnetic properties of the geometrically frustrated
spin-$1/2$ capped-kagome antiferromagnet (RbCl)Cu$_{5}$P$_{2}$O$_{10}$ are
investigated via temperature dependent x-ray diffraction, magnetization, heat
capacity, and $^{31}$P NMR experiments on a polycrystalline sample. It
undergoes a structural transition at around $T_{\rm t} \simeq 310$ K from a
high temperature trigonal ($P\bar{3}m1$) to a low temperature monoclinic
($C2/c$) unit cell, where the low temperature structure features the
capped-kagome geometry of Cu$^{2+}$ ions. Interestingly, it shows the onset of
two successive magnetic transitions at $T_{\rm N1} \simeq 20$ K and $T_{\rm N2}
\simeq 7$ K. The shape of the $^{31}$P NMR spectra unfold the possible nature
of the transitions below $T_{\rm N1}$ and $T_{\rm N2}$ to be of incommensurate
and commensurate antiferromagnetic type, respectively. A large value of the
Curie-Weiss temperature as compared to $T_{\rm N1}$ sets the frustration
parameter $f \simeq 8$, ensuring strong magnetic frustration in the compound.
From the $^{31}$P NMR spin-lattice relaxation rate, the leading
antiferromagnetic exchange coupling is estimated to be $J/k_{\rm B} \simeq 117$
K. These unusual double magnetic transitions make this compound beguiling for
further investigations.
