[en] Here we study the homogeneity of Eu³⁺-doped La_1-xGd_xPO₄ (x = 0, 0.11, 0.33, 0.55, 0.75, 0.92, 1) monazite-type solid solutions by a combination of Raman and time-resolved laser fluorescence spectroscopies (TRLFS) with complementary quasi-random structure-based atomistic modeling studies. For the intermediate La_0.45Gd_0.55PO₄ composition we detected a significant broadening of the Raman bands corresponding to the lattice vibrations of the LnO₉ polyhedron, indicating much stronger distortion of the lanthanide cation site than the PO₄ tetrahedron. A distortion of the crystal lattice around the dopant site was also confirmed in our TRLFS measurements of Eu³⁺ doped samples, where both the half width (FWHM) of the excitation peaks and the ⁷F₂/⁷F₁ ratio derived from the emission spectra increase for intermediate solid-solution compositions. The observed variation in FWHM correlates well with the simulated distribution of Eu···O bond distances within the investigated monazites. The combined results imply that homogenous Eu³⁺-doped La_1-xGd_xPO₄ monazite-type solid solutions are formed over the entire composition range, which is of importance in the context of using these ceramics for immobilization of radionuclides. - Highlights: •Homogenous Eu³⁺-doped La_1-xGd_xPO₄ monazite-type solid solutions have been synthesized. •Solid solution formation is accompanied by slight distortion of the LnO₉ polyhedron. •Raman and laser spectroscopic trends are observed within the monazite series. •Results are explained with atomistic simulations of Eu-O bond distance distribution.