Synthesis, structure refinement and vibrational spectroscopy of new rare-earth tricalcium phosphates Ca9RE(PO4)(7) (RE = La, Pr, Nd, Eu, Gd, Dy, Tm, Yb)

Rare earth elements doped (beta-tricalcium phosphates Ca9RE(PO4)(7) (RE = La, Pr, Nd, Eu, Gd, Dy, Tm, Yb) were synthesized by solid-state reaction at T = 1200 degrees C. The obtained RE-doped beta-TCP phases were studied by a combination of Scanning Electron Microscopy (SEM) equipped with energy dispersive X-Ray spectroscopy (EDS), X-ray diffraction (XRD), Fourier Transform Infra Red (FTIR) and Raman spectroscopies. SEM morphological analyses revealed the presence of sub spherical micro crystalline aggregates, while EDS semi quantitative analyses confirmed the nominal RE/Ca composition for all the phases. The unit cell and the space group were determined by X-ray powder diffraction data showing that all phases in the series crystallize in the rhombohedral R3c whitlockite-type structure; the unit cell constants are a linear function of the dimension of the substituting rare-earth element, and range from a = b = 10.4695(3) angstrom, c = 37.500(3) angstrom and V= 3559.7(2) angstrom(3) (La) up to a = b = 10.4073(2) angstrom, c = 37.2725(2) angstrom, V = 3496.2(2) angstrom(3) (Yb). The analysis of each compound was completed with the structure model refinement by the Rietveld method and the distribution of RE within the available structural sites is discussed according to the results of the Rietveld refinement. The FTIR and Raman spectra show slight band shifts of the phosphate modes correlated to the evolving size of the RE element.