Microstructural and electrical features of lithium Ce-monazite

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