Segregation of Sr in Sr-doped LaPO4 ceramics

Lanthanum phosphate (LaPO4) powders with 1, 2 and 5 mol.% Sr substitution were prepared and sintered at 1450 °C. Bulk, grain boundaries and surfaces of different origins were investigated using powder x-ray diffraction (XRD), SEM, electron probe microanalysis (EPMA), transmission electron microscopy (TEM) and XPS in order to determine the bulk solubility of Sr and extent of segregation and precipitation. A secondary phase (Sr3La(PO4)3) distributed in matrix was found by powder XRD, SEM and EPMA in the 2% and 5% samples. The EPMA indicates that the bulk solubility of Sr is approximately 1%. A crystalline secondary phase (possibly Sr2P2O7) at the triple points and some Sr enrichment at the grain boundaries were found by TEM in the 2% and 5% samples. The XPS taken with Mg anode x-rays on different surfaces showed Sr enrichment in thermally annealed surfaces, as well as in intergranular fractures. Features in XPS spectra of these fractures and surfaces support the presence of a thin La-free or La-poor film of a few atomic layers. Ion etching removed the Sr-rich layers in both cases.