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ThO2-Tm2O3 (TDT) solid solutions containing 1,3,5,8,10, and 15 mol% Tm2O3 were synthesized from spectroscopically pure ThO2 and Tm2O3 polycrystalline powders. The TDT solid solutions were indentified to the fluorite structure by the X-ray powder technique. The values of the lattice parameter were decreased with increasing amount of Tm2O3 incorporated. But, there was no linearity for the samples containing 8, 10, and 15 mol% Tm2O3. It was concluded that these samples became incomplete solid solutions. From the intensity analyses of X-ray diffraction patterns, the residual factor was found below 0.13 even for the 15 mol% TDT system. lt was confirmed from the DTA and TGA analyses that any phase transitions did not occur under the experimental condition executed. Comparing the pycnometric density with the lattice parameter obtained from XRD, it was suggested that the predominant defect model be an oxygen vacancy
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