What is hybridisation of central atom of anionic part of ${\mathrm{PBr}}_5$ in crystalline state?

The hybridization of ${\mathrm{PBr}}_5$ is ${\mathrm{sp}}^3\mathrm{d}$ because one electron enters the s orbital, three occupy the p-orbital, and the final electron enters the d-orbitals of the central atom.