  We report the single-crystal X-ray analysis and magnetic properties of a new
double-layered perovskite antiferromagnet, Cs$_3$Cu$_2$Cl$_4$Br$_3$. This
structure is composed of Cu$_2$Cl$_4$Br$_3$ double layers with elongated
CuCl$_4$Br$_2$ octahedra and is closely related to the Sr$_3$Ti$_2$O$_7$
structure. An as-grown crystal has a singlet ground state with a large
excitation gap of $\Delta/k_{\rm B}\simeq 2000$ K, due to the strong
antiferromagnetic interaction between the two layers. Cs$_3$Cu$_2$Cl$_4$Br$_3$
undergoes a structural phase transition at $T_{\rm s}\simeq330$ K accompanied
by changes in the orbital configurations of Cu$^{2+}$ ions. Once a
Cs$_3$Cu$_2$Cl$_4$Br$_3$ crystal is heated above $T_{\rm s}$, its magnetic
susceptibility obeys the Curie-Weiss law with decreasing temperature even below
$T_{\rm s}$ and does not exhibit anomalies at $T_{\rm s}$. This implies that in
the heated crystal, the orbital state of the high-temperature phase remains
unchanged below $T_{\rm s}$, and thus, this orbital state is the metastable
state. The structural phase transition at $T_{\rm s}$ is characterized as an
order-disorder transition of Cu$^{2+}$ orbitals.
