High-temperature structural properties of the refractory rare-earth tungsten oxides

phase transformations;in situ X-ray diffraction;rare earth tungstate;thermal expansion;thermal analysis

Apostolov, Zlatomir

关键词: phase transformations;    in situ X-ray diffraction;    rare earth tungstate;    thermal expansion;    thermal analysis;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/49733/Zlatomir_Apostolov.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

This thesis presents an investigation of the high-temperature structural behaviour of two types of rare-earth tungsten oxide compounds. Included are first-time descriptions of phase transformation and thermal expansion phenomena in Ln6WO12 (Ln= Y, Ho, Er, Yb) and Ln2WO6 (Ln= Gd, Dy, Y, Ho, Er, Yb) obtained through high-temperature in situ X-ray diffraction and thermal analysis techniques. The phase transformations are presented and classified according to thermodynamic, mechanistic and kinetic criteria, while thermal expansion behaviour is considered in terms of both the conventional lattice parameter approach and thermal expansion ellipsoid analysis. Both phenomena are studied in conditions identical to, or exceeding, the demands of real-world applications. Characterization is done in terms of fundamental crystal lattice constituents and property trends are extended across the compositional spectrum. Two phase transformations were discovered and analysed in the Ln6WO12 systems - both were determined to be first-order reconstructive processes, with slow kinetics and small but noticeable changes in the volume and shape of the unit cells of the phases which were involved. The irreversible lower temperature transformation was from an fcc to a rhombohedral structure and occurred in the range 1250-1300 C. In the higher temperature transition the rhombohedral phase transformed back into an fcc structure, which was initiated at approximately 1600 $^{\circ}$C. The transformation process in this case was reversible on slow cooling.The thermal analysis of the monoclinic Ln2WO6 compositions demonstrated a series of consistently repeatable and reversible phenomena in all but Gd2WO6 and Yb2WO6. For Dy2WO6, Y2WO6 and Er2WO6 the starting and ending phase after two measurement cycles up to 1500 C were identical, with all three compounds displaying a similar sequence of endothermic and exothermic peaks, which occurred on heating and cooling correspondingly. The peak shape and formation kinetics strongly suggested a displacive transformation as a possible description of the observed processes. This explanation was further reinforced by the comparison and close similarity of the observed Ln2WO6 peak shapes with those of the peaks reflecting the monoclinic to tetragonal displacive transformation in ZrO2.Because of the structural resemblance of the fcc and rhombohedral phases of the Ln6WO12 system, their thermal expansion behaviour was very similar. The noticeable expansion anisotropy of the rhombohedral phase was ascribed to the layered disposition of anions and cations, which caused the resulting polyhedra to expand more along the (111) plane and less in the [111] direction. Analysis of the thermal expansion ellipsoid of the monoclinic Ln2WO6 revealed a continuous change in both its shape and orientation, with the latter effect strongly manifested in the (010) plane. A noticeable reversal of the relative expansion rates between the 'a', and 'b' and 'c' lattice parameters was attributed to the rotation of the ellipsoid cross section in this plane, bringing the larger of the two eigenvectors closer to 'a', while the smaller one to 'c'. Investigating the structural dynamics of the constituent layers suggested that the Ln polyhedron centered on the only general position in the unit cell may be the reason for the rotating CTE ellipsoid.