X-ray powder diffraction investigation of new oxynitride precursors: rare earth oxide compounds of fluorite- and sheelite-type structures in the Yb-(Zr, W)-O system

摘要

Nanocrystallized oxide precursors of colored (oxy)nitrides related to the system Yb-(Zr-W)-O have been successfully prepared using a chimie douce process—the amorphous citrate route. The process involves first a formation of fine and homogeneous powdered solids obtained by calcination at 600 ℃, a temperature much lower than that of the conventional solid-state method. At this stage, the X-ray diffraction patterns exhibit large line broadening effects. Finally, two well-crystallized and pure quaternary oxides have been readily obtained by heating and under annealing conditions at 850 and 900 ℃ for 12 h. For one of the patterns, all the X-ray diffraction lines can be easily indexed to a cubic phase with the fluorite structure conforming to the Fm3m space group [Yb_2Zr_(1.21)W_(0.41)O_(6.65)□_(1.35) called C-phase: a = 5.1864(2) A]. The second phase adopts the sheelite-type structure [Yb_2ZrWO_8 called T-phase: space group I4_1/a, a = 5.1584(5), and c = 10.8246(6) A]. By taking into account the present compositions determined by EDS measurements, Rietveld structure refinements produce final R_B factors of 0.015 and 0.044, and R_(wp) factors of 0.069 and 0.089, respectively. In order to characterize the microstructure of the materials (crystallite size and lattice distortion) at the nanometer scale, a study based on diffraction line broadening analysis applying the whole pattern refinement method was also undertaken with confidence. The results show smooth angular variations of the values of FWHMs, indicating that the microstructural properties are isotropic for the cubic and tetragonal oxides. More precisely, the results indicate that whatever the profile fitting approach used ("profile matching" procedure and Rietveld method), the reliability factors R_(wp) are systematically better with a combined size strain than with zero strain considerations. The strain magnitudes observed for the C-phase-850 ℃ as well as for the T-phase-900 ℃ should be viewed as realistic strain.