Segregation of Sr in Sr-doped LaPO4 ceramics

Lanthanum phosphate (LaPO4) powders with 1, 2 and 5 mol,% Sr substitution w ere prepared and sintered at 1450 degrees C, Bulk, grain boundaries and sur faces of different origins were investigated using powder x-ray diffraction (XRD), SEM, electron probe microanalysis (EPMA), transmission electron mic roscopy (TEM) and XPS in order to determine the bulk solubility of Sr and e xtent of segregation and precipitation. A secondary phase (Sr3La(PO4)(3)) d istributed 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 similar to 1 %. A crystalline secondary phase (possibly Sr2P2O7) at the triple points an d some Sr enrichment at the grain boundaries were found by TEM in the 2% an d 5% samples. The XPS taken with Mg anode x-rays on different surfaces show ed Sr enrichment in thermally annealed surfaces, as well as in intergranula r fractures. Features in XPS spectra of these fractures and surfaces suppor t the presence of a thin La-free or La-poor film of a few atomic layers. Io n etching removed the Sr-rich layers in both cases. Copyright (C) 2000 John Wiley & Sons, Ltd.