Two cerium-monazites containing excess P were synthesized by a precipitation method. The Ce-phosphate selected samples were doped with 1.3 and 4.7wt%Li and sintered in dry air. The phase evolution as a function of temperatures was investigated. The X-ray diffraction and microstructural studies reveal that Ce-monazite appears at low temperature (350C). Lithium was incorporated into the monazite lattice up to 1000C. The material achieved at this temperature was a composite constituted by mainly CePO4 monazite grains, surrounding by a minor Li-Phosphate glassy phases. Precipitation of cubic ceria, and Li3PO4 as secondary phases were also provided. Li introduction into the Ce-monazite lattice was accounted due to the fact that the main X-ray diffraction monazite peaks, was significantly shifting towards higher diffraction angles. For the larger Li content sample, when temperature increased (1250C), the cerium oxide were the major phase depicted. The total electrical conductivity plots obtained from impedance spectra, between 25 and 900C, in dry air, indicated that the Ce-monazite decomposition starts at 1000C and it was evident at 1250C. The total conductivity decreased as precipitated Ceria phases increased. Furthermore, the total conductivity data in dry air, evidenced that both Li ionic and electronic-hole transport govern the conduction process, similarly to what occurred in doped Sr-lanthanide based monazites. Li-doped ceria-monazite may promote electronic-hole formation by a trapping–de-trapping process at Ce3+ and Ce4+ sites. Li ionic conduction contribution comes from the presence of Li3PO4, but it was restricted due to the fact that Pt block Li electrodes were used for impedance electrical tests. The optical measurements for the Li doped monazites show the Ce3+ luminescence band when Li was introduced that band intensity decreased providing Ce4+, which confirms hole-electronic formation. The Li role was to catalyzes the ceria-monazite crystallization and also induces the monazite Ce3+ oxidation. Finally CeO2 was the major phase found at 1250C.

Microstructural and Electrical Features of Lithium Ce-Monazite. T.Hernández, R.Vila, J.R.Jurado, E.Chinarro, J.Molla, P.Martín: Solid State Ionics, 2008, 179[7-8], 256-62