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

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.…”

Crystal chemistry of the monazite structure

“…Podor demonstrated from Raman spectra that REE, Ca and U elements are randomly distributed in the cation site for La 1−2x 230 Moreover, Konings et al performed calorimetry measurements in the LnPO 4 -CaTh(PO 4 ) 2 binaries and concluded to a deviation of the solid solutions formed from the ideality, this deviation increasing with the size of the Ln 3+ cation. 228 Type 2: Another mechanism for the coupled incorporation of divalent elements and large tetravalent cations (i.e. cerium and actinides) was described in the literature through the preparation of single phase M II 0.5 M IV 2 (PO 4 ) 3 compounds by the Russian team of Orlova et al 209,210,[231][232][233][234] .…”

Crystal chemistry of MIIM′IV(PO4)2 double monophosphates

“…Based on X-ray powder diffraction and drop calorimetry, Konings et al highlighted that Ln 1-2x Ca x Th x PO 4 solid solutions show deviation from ideal behavior due to lattice strains induced by ion size effects of substitution of Ln 3 þ with 1/2(Ca 2 þ þ Th 4 þ ) [65].…”