Crystal-field splitting of Pr$^{3+}$ ion multiplets in YPO$_4$ with allowance for interconfigurational interaction

The crystal-field splitting of the Pr$^{3+}$ ion multiplets is analyzed with regard to the influence of excited opposite-parity configurations $4f^{ (N - 1)}5d$ and a configuration with charge transfer. Such an approach makes it possible to (i) refine the characterization of the Stark structure of multiplets by 39% compared with the approximation of a weak configurational interaction and (ii) determine the covalence parameters and the parameters of an odd-symmetry crystal field from experimental data for the Stark structure. The covalence parameters thus determined coincide in order of magnitude with the respective parameters calculated for other ligands using microscopic models.