High-temperature calorimetry of (La1−xLnx)PO4 solid solutions

Excess properties of the (Ln2−2xCaxThx)(PO4)2 (Ln=La, Ce) solid solutions

“…This suggests that the behaviour of the solutions is regular and the results can thus be fitted to the function H E = A 0 x(1 À x), yielding A 0 = (29.3 ± 1.4) kJ Á mol À1 for La 2À2x Ca x Th x (PO 4 ) 2 and A 0 = (23.0 ± 1.6) kJ Á mol À1 for Ce 2À2x Ca x Th x (PO 4 ) 2 . Figure 5 shows the A 0 values as a function of the molar volume difference (at T = 298.15 K) between the end-members of the binary solid solutions, together with the results for the LaPO 4 -LnPO 4 solid solutions (with Ln = Nd, Eu, Gd) as reported by us earlier [18]. It can be seen that the A 0 values for the solutions with cheralite show a different dependence compared to the LnPO 4 solutions.…”

“…The non-ideal behaviour can be caused by lattice strains resulting from the ion size effects of substitution of ions of different sizes on the cation site. Our measurements for the LaPO 4 -LnPO 4 solid solutions (with Ln = Nd, Eu, Gd) clearly demonstrated that the enthalpy {H (T) À H (298.15 K)} does not follow the ideal Neumann-Kopp additivity rule but shows an excess strain enthalpy that varies with the size of the Ln 3+ ions [18]. Whereas in that study a simple Ln 3+ -La 3+ substitution was studied, it is of interest to investigate the effects of the substitution of the Ln 3+ cation in the monazite structure with (Ca 2+ + Th 4+ ).…”

Evidence for Lattice Strain and Non-ideal Behavior in the (La1−xEux)PO4 Solid Solution from X-ray Diffraction and Vibrational Spectroscopy

“…Thust et al (2015) measured the low-temperature heat capacity of the (La 1−x Eu x )PO 4 solid solution, but the variation observed was solely attributed to the difference in the electronic configuration and the concomitant Schottky effects. Li et al (2014) derived mixing enthalpies for the (La 1−x Eu x )PO 4 solid solution from ab initio density functional theory (DFT) computations that very well agree with the drop calorimetric measurements of Popa et al (2006a) and Konings et al (2008). However, drop calorimetric measurements are nonreacting and thus do not yield a mixing enthalpy, as erroneously assumed by Li et al, but are mainly a measure for the vibrational (phonon) excess enthalpy.…”

“…In the monazite structure, the Ln-O lattice modes are the most affected, as they exhibit the largest excess broadening. They form intertwining dispersive phonon branches throughout the Brillouin zone (Silva et al, 2006) and have also the largest influences on the thermodynamic properties such as enthalpy and entropy (Kieffer, 1979a,b), and can thus explain very well the excess enthalpy that we observed for the (La 1−x Eu x )PO 4 solid solution (Popa et al, 2006a;Konings et al, 2008).…”

“…Enthalpy increment measurements in the (La 1−x Eu x )PO 4 solid solution series at T = 1000 K revealed a significant excess enthalpy, which has been attributed mainly to local strain fields in the lattice due to the cationic mismatch (Popa et al, 2006a;Konings et al, 2008). Thust et al (2015) measured the low-temperature heat capacity of the (La 1−x Eu x )PO 4 solid solution, but the variation observed was solely attributed to the difference in the electronic configuration and the concomitant Schottky effects.…”