Answer
The $\mathrm{Cl}$ atom is smaller than the $\mathrm{Br}$ atom. As a result, the central P atom can accommodate only four $\mathrm{Br}$ atoms, as in the tetrahedral $\mathrm{PBr}_{4}^{+}$ ion. However, the central $P$ atom can accommodate four $\mathrm{Cl}$ atoms in case of the tetrahedral $\mathrm{PCl}_{4}^{+}$ ion or six in case of the octahedral $\mathrm{PCl}_{6}^+$ ion.
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