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 $\langle nlj|r^k|nlj\rangle$ can be expressed in terms of Clebsch-Gordan coefficients or 3$jm$ symbols. This generalizes the results obtained by Varshalovich and Karpova [Opt. Spectrosc. $\mathbf{118}$, 1-5 (2015); Opt. Spektrosk. $\mathbf{118}$, 3–7 (2015)] in the non-relativistic hydrogenic case.