Expression of relativistic expectation values of powers of $r$ in terms of Clebsch-Gordan coefficients

It is shown that, in the relativistic hydrogenic approximation, expectation values of the type $⟨n\mathrm{\ell }j\mathrm{\mid }{r}^k\mathrm{\mid }n\mathrm{\ell }j⟩$ can be expressed in terms of Clebsch-Gordan coefficients or $3jm$ symbols. This generalizes the results obtained by Varshalovich and Karpova [Opt. Spectrosc. 118, 1-5 (2015); Opt. Spektrosk. 118, 3-7] in the non-relativistic hydrogenic case.