Synthesis and characterization of monazite-type Sr:LaPO4 prepared through coprecipitation

A coprecipitation method was used to obtain La(1--x)SrxPO4 (x=0, 0*025, 0*05, 0*10) powders from the following precursors: La(NO3)3*6H2O, Sr(NO3)2 and (NH4)2HPO4. Several analysis methods were applied in order to study powder morphology and particle size, thermal evolution and sintering behavior of green compacts. Although the electrical properties of these materials have been studied, their synthesis and characterization are still open and barely studied issues in the literature. XRD patterns of the as--prepared powders display the typical peaks of phase--pure LaPO4 (monazite, standard PDF 32--0493 pattern), and also for all the studied compositions and calcination temperatures. TGA data exhibit a 20% weight loss, up to 250 deg C, that can be attributed to the residual water and the decomposition of NH4NO3, a byproduct of the coprecipitation. The presence of NH4NO3 was also confirmed by DTA data and Nitrogen content analysis of the powders. It was found that strontium additions enhanced densification. After sintering at 1200 deg C for 4h in air, the materials showed relative densities higher than 99% of the theoretical density. None of the analyses performed showed any significant difference between Sr--doped and the undoped powders, except for the dilatometric studies. No second phases are observed in the sintered materials. The stability of the sintered samples was studied in high water--vapor partial pressure. The samples were stable in the essayed conditions (pH2O 2*7--11*5atm, 120--170 deg C for 48h) as confirmed by XRD, weight measurements and scanning electron microscopy coupled to energy dispersive X--ray spectroscopic analysis (SEM--EDS).