Ln2Zr2O7 ( Ln = La、Sm、Eu)和(La,A)Zr2O7 (A = Dy、Y)的結晶相與製備溫度之關聯

Abstract

In this research, five series of oxides were prepared, their nominal compositions were Ln2Zr2O7 (Ln = La, Sm, Eu) and (LaA)Zr2O7 (A = Dy, Y). Samples prepared at lower temperatures have a fluorite structure; those prepared at higher temperatures have pyrochlore phase. Transition temperature depends on the ionic radius ratio of the R(Ln3+)/R(Zr4+). Cation and anion ordering of the pyrochlore samples increases with increasing the preparation temperatures, in other words, the amount of the Ln/Zr cation exchange and the O(3) site occupancy decrease. Activation energies calculated on the log (σT/Scm-1K) vs. (1/T) for all the samples on the cation ordering are in the range of 0.23-0.40 eV, and on the O(3) site are 0.20-1.44 eV. They also depend on the ionic radius ratio. The Zr K-edge XANES spectra near the 18005 eV, a pre-edge peak assigned to the absorption of the forbidden transition of Zr (1s)-->Zr (4d), shift to higher energies if the samples are prepared at higher temperatures, indicates a stronger repulsion force between the Zr atom and the ligands as samples change phase from fluorite to pyrochlore. In the Zr L-edge and the O K-edge absorption spectra, crystal field splitting energy (delta End) of the Zr atom increases with increasing the preparation temperatures. It also shows that interaction between the O (2p) and Zr (nd) is stronger in the pyrochlore than in the fluorite phase. In conclusion, the coordination covalent bonds of the Zr-O in the pyrochlore phase is stronger than that in the fluorite. As a result, bond lengths in the former are shorter than that in the latter.