[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.

[en] The order-disorder phase transition in the Nd_xZr_1_−_xO_2_−_0_._5_x system is studied by complementary techniques which include wet chemical synthesis of a series of compositions with various Nd/Zr ratios with the final annealing at 1873 K, X-ray diffraction, oxide melt solution calorimetry and ab initio thermodynamic modeling. Our structural data indicate the transition from ordered to disordered pyrochlore at x ∼0.31 at a temperature of 1873 K. Our calorimetric data show a transition enthalpy of ∼30 kJ/mol, which corresponds to an entropy of disordering of ∼16 J/K/mol. The latter value is significantly smaller than the configurational entropy of transition computed under the assumption of complete disorder in a fluorite phase, indicating a substantial degree of order remaining in the fluorite phase at the temperature of synthesis. The considered phases are computed ab initio using a series of special quasi-random structures that emulate the complete or partial disorder. The results of our calculations and thermodynamic modeling are in good agreement with the measured lattice parameters, the Nd content at the order-disorder transition, and the measured formation and transformation enthalpies. Thus our combined experimental and modeling results provide valuable insight into the disordering of this pyrochlore phase and of other pyrochlore materials.