Fluorite structure oxides have a property of deviating from stoichiometry as a function of temperature and/or pressure. CeO 2 has a fluorite structure and a wide range of nonstoichiometry. 1 The material deviates from stoichiometry with increasing temperature and decreasing oxygen partial pressure, leading to a high concentration of defects. The concentration of defects can also be controlled by doping the oxide with impurities. Since CeO 2 exhibits a wide range of solubility 2 for rare earth elements, the concentration of defects may be controlled by dopant concentration. CeO 2 is a mixed conductor, and both electronic and ionic conductivities have been investigated in several studies. 3-10 It is generally agreed 11 that the electronic conductivity of undoped CeO 2 is of the n-type. Since the ionic conductivity in this material is directly related to oxygen diffusion, it is important to understand the diffusion characteristics. However, the characteristics of oxygen diffusion in CeO 2 remain unclear. In particular, the oxygen diffusion in undoped CeO 2 must be studied in detail, because it is useful in understanding the behavior of oxygen diffusion and electrical conductivity for doped CeO 2 .The oxygen self-diffusion coefficients in oxides having fluorite structure are summarized by Ando et al. 12 and Kamiya et al. 13 The characteristic in fluorite structure is that the absolute value of the oxygen diffusion coefficients in stoichiometric oxides, i.e., UO 2 , 14,15 PuO 2 , 16 ThO 2 , 17,18 and CeO 2 , 19 are lower than those of nonstoichiometric oxides, i.e., UO 2ϩx and CeO 2 doped with Y. In addition, the activation energy for oxygen diffusion in stoichiometric oxides is large (202 kJ mol Ϫ1 for PuO 2 and 248-273 kJ mol Ϫ1 for UO 2 ). However, the absolute values of the oxygen diffusion coefficients of nonstoichiometric oxides are higher than those of stoichiometric oxides, and the activation energy for oxygen diffusion in nonstoichiometric oxides is small (77-89 kJ mol Ϫ1 for CeO 2 doped with Y).In the literature, two results, one by Floyd 19 and the other by Kamiya et al., 13 have been presented for oxygen diffusion coefficients in undoped CeO 2 . The activation energy reported by Floyd for undoped CeO 2 was 104 kJ mol Ϫ1 and was close to the value for nonstoichiometric UO 2 and for CeO 2 doped with Y. In contrast, the activation energy indicated by the data by Kamiya et al. (322 kJ mol Ϫ1 ) was close to the value of stoichiometric UO 2 , ThO 2 , and PuO 2 and the absolute value of the oxygen diffusion coefficient of CeO 2.00 was found to be similar to the value of other stoichiometric oxides having fluorite structure. Consequently, Kamiya et al. concluded that their result corresponds to stoichiometric CeO 2 (Ce-1). In their study, the oxygen self-diffusion coefficient for stoichiometric cerium oxide was obtained using gas-phase analysis. Direct measurement of the diffusion coefficient can be performed via secondary ion mass spectroscopy (SIMS). One of the objectives of the present study was to investigate ...

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An oxalate coprecipitation method was applied to produce rare-earth-doped ceria powders (Ce0 .8R0.201.9, R=Yb , Y, Gd, Sm, Nd and La). The powders calcined at 600•Ž were isostatically compacted and sintered at 1600 •Ž in air to reach a relative density exceeding 98%. The electrical conductivity of rare-earth-doped ceria ce ramics was dominated by the migration of oxygen ions in the bulk and was in the range of 6.7 to 13.6S•Em-1 at 800•Ž. The activation energies for diffusion of oxygen ions in the bulk ranged from 75.3 to 93.4kJ/mol. The lattice energy theory for the fluorite structure was applied to estimate the activation energy. The calcu lated activation energy was close to the measured value.

Three kinds of chemical processes, citrate gel process, acetate gel process, and coprecipitation route, have been applied to the synthesis of homogeneous metastable tetragonal (t 0 ) and cubic solid solutions of ZrO 2 -X mol % CaO (X 4-20). From a Raman scattering study, the citrate gel process based on the gelation of the aqueous solution of citric acid containing Zr and Ca ions was found to produce compositionally homogeneous samples in comparison with the other two methods. The axial ratio c͞a decreases with increasing concentration of CaO and becomes unity around 8-10 mol % CaO composition.M. Yashima et al.: Synthesis of metastable tetragonal (t 0 ) zirconia-calcia solid solution

Efficient La-doped TiO2photocatalysts were prepared by sol-gel method and extensively characterized by various sophisticated techniques. The photocatalytic activity of La-doped TiO2was evaluated for the degradation of monocrotophos (MCPs) in aqueous solution. It showed higher rate of degradation than pure TiO2for the light of wavelength of 254 nm and 365 nm. The rate constant of TiO2increases with increasing La loading and exhibits maximum rate for 1% La loading. The photocatalytic activities of La-doped TiO2are compared with La-doped ZnO; the reaction rate of the former is ~1.8 and 1.1 orders higher than the latter for the lights of wavelength 254 nm and 365 nm, respectively. The relative photonic efficiency of La-doped TiO2is relatively higher than La-doped ZnO and commercial photocatalysts. Overall, La-doped TiO2is the most active photocatalyst and shows high relative photonic efficiencies and high photocatalytic activity for the degradation of MCP. The enhanced photocatalytic activity of La-doped TiO2is mainly due to the electron trapping by lanthanum metal ions, small particle size, large surface area, and high surface roughness of the photocatalysts.

An extensive X‐ray study of CeO2–Nd2O3 solid solutions was performed, and the densities of solid solutions containing various concentrations of NdO1.5 were measured using several techniques. Solid solutions containing 0–80 mol% NdO1.5 were synthesized by coprecipitation from Ce(NO3)3 and Nd(NO3)3 aqueous solutions, and the coprecipitated samples were sintered at 1400°C. A fluorite structure was observed for CeO2–NdO1.5 solid solutions with 0–40 mol% NdO1.5, which changed to a rare earth C‐type structure at 45–75 mol% NdO1.5. The change in the lattice parameters of CeO2–NdO1.5 solid solutions, when plotted with respect to the NdO1.5 concentration, showed that the lattice parameters followed Vegard's law in both the fluorite and rare earth C‐type regions. The maximum solubility limit for NdO1.5 in CeO2 solid solution was approximately 75 mol%. The relationship between the density and the Nd concentration indicated that the defect structure followed the anion vacancy model over the entire range (0–70 mol% NdO1.5) of solid solution.

Since the beginning of life on earth, the environment has been polluted by waste, both natural as well as synthetic (man-made). In the case of natural waste however, the environment by itself controls the effect of contamination. But in the case of synthetic materials, the pollution is higher even in trace concentration and this may go on accumulating, leading to disastrous effects on the environment and ecology. Industrial wastes containing heavy metals and pesticide residues fall in this category. The major point sources of pesticide pollution are wastewater from agricultural industries and pesticide manufacturing and formulating plants. Hence the waste from these sources must be removed or destroyed before discharge to the environment. There have been several methods practiced for the treatment of wastewater. However many of them are not able to achieve hundred percent satisfaction. Environmental pollution and destruction on a global scale as well as lack of sufficient clean and natural energy sources have drawn much attention and concern to the vital need for ecologically clean chemical technology, materials and processes. This is one of the most urgent challenges facing chemical scientists. Recently, light-induced photocatalytic reactions have gained a lot of attention for the purification of wastewater.