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

The monazite-type solid solution of LaPO4 and EuPO4 has been studied by X-ray diffraction, infrared (IR) and Raman spectroscopic techniques. A substantial excessmolar volume has been derived from the X-ray data, and the Raman and IR spectra show band broadening typical for mixing of cations of different size on the cation sublattice. The IR spectra were interpreted by the autocorrelation method and the excess autocorrelation parameter 1corr shows clear deviation from ideal solution behavior, similar to the observed broadening of the Raman bands. The results can be interpreted in terms of local lattice strains resulting from the ion size effects of substitution of La3+ by Eu3+, and correlate very well with calorimetric measurements of the excess enthalpy that was previously measured.