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| Title: | Ln2Zr2O7 ( Ln = La、Sm、Eu)和(La,A)Zr2O7 (A = Dy、Y) 的結晶相與製備溫度之關聯 |
| Other Titles: | Correlation of the preparation temperature with crystalline phase in ln2zr2o7 (ln = la, sm, eu) and (la,a)zr2o7 (a = dy, y) |
| Authors: | 邱志煒;Chiu, Chih-wei |
| Contributors: | 淡江大學化學學系碩士班
高惠春;Kao, Huey-chuen I. |
| Keywords: | 焦綠石;螢石;X-光粉末繞射;X-光吸收近邊緣結構;pyrochlore;fluorite;XRD;XANES |
| Date: | 2010 |
| Issue Date: | 2010-09-23 16:12:28 (UTC+8) |
| Abstract: | 本研究製備五系列氧化物，樣品起始組成為Ln2Zr2O7 (Ln = La、Sm、Eu)和(LaA)Zr2O7 (A = Dy、Y)。低溫製備樣品具有螢石結構；高溫製備者為焦綠石結構，相轉變溫度隨著摻雜的稀土元素離子半徑變大而變小。當樣品為焦綠石結構時，隨著鍛燒溫度的上升，焦綠石相的有序程度增加，亦即Ln、Zr陽離子互換程度下降；O(3)的佔有率減少。陽、陰離子有序的活化能分別為0.23-0.40 eV與0.20-1.44 eV，也是隨著稀土元素離子半徑而變。Zr K-edge XANES光譜在18005 eV附近的前緣吸收峰為Zr (1s) → Zr (4d)的電子躍遷，隨著樣品製備溫度的上升，此峰會往高能量位移，表示螢石結構d軌域能量會隨著相變成為焦綠石結構而使得配位基O與Zr原子的作用力增加而上升；在Zr L-edge與O K-edge吸收光譜圖中，Zr的晶場分裂能(delta End)亦隨
著樣品鍛燒溫度上升而增加，顯示O (2p)與Zr (nd)的作用力增強，因
此焦綠石相中的Zr-O配位鍵比螢石相中的鍵強，這與Rietveld 精算
結果相符。
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. |
| Appears in Collections: | [Graduate Institute & Department of Chemistry] Thesis
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