The question of calculating the substitution limits in solid solutions of monazites of rare-earth elements with structure La_1–xLn_xPO₄, where Ln = Pr – Dy.

Abstract

Earlier, the authors proposed (Get’man, E. I.; Radio, S. V. Inorg. Mater. 2017, DOI: 10.1134/S0020168517070044) to quickly determine equilibrium solubility limits at given temperatures or measure decay temperatures for specified compositions based on the diagrams of thermodynamic stability (dependences of La_1–xLn_xPO₄ decay temperatures from rare-earth elements number). However, in case of intermediate compositions (at x other than 0.02, 0.05, 0.1, 0.2, 0.5), we could find only approximate values, since the measurements were carried out by interpolation in the regions of nonlinear dependence of solid solution temperature from its composition. In the paper, we propose to determine the solubility limits of REE in La_1–xLn_xPO₄ only based on the decomposition temperature or measure the decomposition temperature only based on solubility limits without using any other additional data from the graphical interdependencies described by the R. Becker equation. We also analyzed the values of interaction parameters (Q, kJ/mol) in La_1–xEu_xPO₄ and La_1–xGd_xPO₄ systems, obtained using different methods by different authors during the period of 2007–2017 (Ab initio, Li et al., 2014; Strain energy, Mogilewsky, 2007; Drop calorimetry, Popa et al., 2007; Ab initio/strain energy, Kowalski and Li, 2016; Drop solution, Neumeier et al., 2017).

V. S. Urusov crystallochemical method for determining solid solution mixing energy and R. Becker equation for regular solutions were used to represent the graphs for quick determination of substitution limits in solid solutions of La_1–xLn_xPO₄ with monazite structure, where Ln represents rare-earth elements in Pr – Dy series. The proposed graph dependencies make it possible to determine substitution limits for orthophosphates La1–xLnxPO4 with monazite structure (Ln = Pr – Dy; 0.005 < х < 0.99) based on the specified decomposition temperatures of solid solutions or determine the decomposition (stability) temperatures of solid solutions based on the specified substitution limits without any other parameters using. Graph dependencies can be useful both for specialists working in the field of immobilization of radioactive waste to determine the maximum amount of radioactive material absorber contained in the structure, as well as in the field of creating new luminescent, laser and other materials containing a small amount of activator.