Fluorite-pyrochlore phase transition in nanostructured Ln2Hf2O7\mathrm{Ln_{2}Hf_{2}O_{7}} (Ln = La-Lu)

Complex oxides of the Ln$_{2}$Hf$_{2}$O$_{7}$ (Ln = lanthanide) series undergo a fluorite to pyrochlore phase transformation. We have studied the whole process of the crystal and local atomic structure realignment during the crystallization and the phase transition in the series of Ln$_{2}$Hf$_{2}$O$_{7}$ (Ln = La-Lu) samples synthesized by the coprecipitation method with the subsequent annealing of mixed hydroxides (precursors). The study employed a combination of x-ray diffraction (normal and anomalous), x-ray absorption spectroscopy, analysis of atomic pair distribution function and Raman spectroscopy. The starting and ending temperatures of the fluorite-pyrochlore phase transition for Ln$_{2}$Hf$_{2}$O$_{7}$ compounds have been determined along the lanthanide series La-Dy. The scheme summarizing structure types (amorphous, fluorite and pyrochlore) for the whole Ln2Hf2O7 (Ln = La-Lu) series as a function of the Ln cation radius (or the r$_{Ln3+}/r_{Hf4+} ratio) and the annealing temperature has been refined

Features of the Phase Preferences, Long- and Short-Range Order in <i>Ln</i>₂(WO₄)₃ (<i>Ln</i> = Gd, Dy, Ho, Yb) with Their Relation to Hydration Behavior

The effect of synthesis conditions on the features of the long- and short-range order of Ln2(WO4)3 (Ln = Gd, Dy, Ho, Yb) powders synthesized via coprecipitation of salts has been studied by a complex of physico-chemical techniques including synchrotron X-ray powder diffraction, X-ray absorption spectroscopy, Raman and infrared spectroscopy, and simultaneous thermal analysis. It was found that crystallization of amorphous precursors begins at 600 °C/3 h and leads to the formation of the monoclinic structure with sp. gr. C12/c1(15) for Ln2(WO4)3 (Ln = Gd, Dy) and with sp. gr. P121/a1(14) for Ln = Yb, whereas crystallization of Ho precursor requires even higher temperature. After annealing at 1000 °C, the P121/a1(14) phase becomes the dominant phase component for all heavy lanthanoid types except for Ln = Gd. It was shown that the Ln (Ln = Dy, Ho, and Yb) tungstates with the P121/a1(14) monoclinic structure correspond to trihydrates Ln2(WO4)3·3H2O formed due to a rapid spontaneous hydration under ambient conditions. It was concluded that the proneness to hydration is due to a specific structure of the P121/a1(14) phase with large voids available to water molecules. Modifications in the local structure of Ln-O coordination shell accompanying the structure type change and hydration are monitored using EXAFS spectroscopy

Influence of Synthesis Conditions on the Crystal, Local Atomic, Electronic Structure, and Catalytic Properties of (Pr_1−<i>x</i>Yb_<i>x</i>)₂Zr₂O₇ (0 ≤ <i>x</i> ≤ 1) Powders

The influence of Yb3+ cations substitution for Pr3+ on the structure and catalytic activity of (Pr1−xYbx)2Zr2O7 powders synthesized via coprecipitation followed by calcination is studied using a combination of long- (s-XRD), medium- (Raman, FT-IR, and SEM-EDS) and short-range (XAFS) sensitive methods, as well as adsorption and catalytic techniques. It is established that chemical composition and calcination temperature are the two major factors that govern the phase composition, crystallographic, and local-structure parameters of these polycrystalline materials. The crystallographic and local-structure parameters of (Pr1−xYbx)2Zr2O7 samples prepared at 1400 °C/3 h demonstrate a tight correlation with their catalytic activity towards propane cracking. The progressive replacement of Pr3+ with Yb3+ cations gives rise to an increase in the catalytic activity. A mechanism of the catalytic cracking of propane is proposed, which considers the geometrical match between the metal–oxygen (Pr–O, Yb–O, and Zr–O) bond lengths within the active sites and the size of adsorbed propane molecule to be the decisive factor governing the reaction route