  Cation ordering is most common process detected in
A$_{3}$B'B"$_{2}$O$_{9}$-based complex perovskites. Some important physical
features of this system are due to the B-site ordering at long and short range.
For microwave applications as filters and resonators, the 1:2 order is more
appropriate. Otherwise, the oxygen vacancies and 1:1 order are considered the
main factors behind the good performance of nonstoichiometric
A$_{3}$B'$_{1+x}$B"$_{2-x}$O$_{9-\delta}$-based ceramics as proton conductors.
Until now, however, there are no available reports regarding the isolated
effects of B-site order at long range on the electrical properties of
stoichiometric systems. This work reports the preparation of 1:1 and 1:2
fully-ordered Ba$_{3}$CaNb$_{2}$O$_{9}$ ceramics. Here, we combine the Raman
scattering and group-theory calculations to distinguish the fingerprints of the
1:1 and 1:2 orders. The electric properties of the ordered
Ba$_{3}$CaNb$_{2}$O$_{9}$ are analyzed in terms of a phenomenological model
based on a parallel combination of a resistor, constant phase element, and
capacitor. In particular, the conductivity relaxation ascribed to the grain is
due to the oxygen vacancies. Besides, we found that the 1:1 order increases the
dc conductivity, but not enough to account the good performance reported for
the non-stoichiometric Ba$_{3}$Ca$_{1+x}$Nb$_{2-x}$O$_{9-\delta}$-based proton
conductors.
