Preparation and Characterization of Rare Earth Phosphates,1-LaPO4 and Its Electrical Properties

摘要

THIS WORK aims to prepare a series of LaPO4 samples by different methods depending on the precursor material and to choose the most proper method of preparation.Chemical analysis,IR spectra,X-ray diffraction,Thermogravimetric analysis(TGA,DTA)scanning electron microscope(SEM)and electrical conductivity measurements were used for characterization.Six samples I(a,b,c,d),II and III were prepared from LaCl3 and H3PO4 using ammonia,urea and ethylene oxide as gelating agents in both aqueous and alcoholic media.Sample II was.prepared by dissolution of La2O3 in HNO3 and precipitation by(NH4)2HPO4.Sample III was prepared using solid-solid reaction by mixing La2O3 and(NH4)2HPO4 in stoichiometric ratio and firing at 1300°C for 2hr.The chemical analysis shows that method lb is the most suitable one for preparing LaPO4 as the calculated and experimental ratios of La/P are in agreement.This may be attributed to the decomposition of urea to NH3 and CO2 and releasing OH~-.For the other samples the deviation of La/P ratios may due to the unreacted materials.IR spectrum shows that absorption peaks characterizing the phosphate salts were appeared on the IR charts,but for sample III.Additional bands characteristic to metaphosphates were observed.X-ray patterns charts indicate that NH3 or ethylene oxide improve the crystallinity of LaPO4.0.5 H2O than urea.Also,it is observed that the sample prepared in alcoholic medium is in the amorphous state.The effect of firing temperature indicates that the amorphous LaPO4 is converted to crystalline state by heating.Thermal analysis indicates that the LaPO4 is thermally stable in the range up to 1300°C.Scanning electron microscope results show that the morphology of LaPO4 samples is in agreement with X-ray diffraction and it depends on methods of preparation and firing temperature.Electrical conductivity measurements indicate that LaPO4 behaves as a semiconductor material.