過渡金屬化合物離子價數與電荷有序之吸收光譜研究

Abstract

本論文利用同步輻射中心X光源進行隨著溫度改變之O K-edge、Mn L-edge和Mn K-edge X光近緣吸收結構(XANES)實驗，探討La1-xCaxMnO3 (x=0.6、0.8)塊材在電荷有序(charge ordering)中錳離子是否會分成Mn+3與Mn+4交替排列，或只是分成較少差異的Mn+(3+y)與Mn+(4-y)(0<y≦0.5)。我們的結論是Mn K-edge的研究無法回答這個問題。在變溫下La1-xCaxMnO3 (x=0.6、0.8)的O K-edge光譜與Mn K-edge光譜之pre-edge發現了與La0.7Ca0.3MnO3光譜相反的變化，原因是電荷有序也就是相干極化子(Correlated Polaron)使得電荷載子(charge carrier)侷域(localize)在Mn+3晶格點上，為了降低其能量而產生了Jahn-Teller distortion效應所造成的結果。由La1-xCaxMnO3 (x=0.6、0.8)與其它同系列樣品(x= 0、0.3、1)之Mn L-edge和Mn K-edge X光吸收光譜發現，在室溫時(T = 300K) La1-xCaxMnO3 (x=0.6、0.8)的錳離子為平均價數，隨著溫度降至低溫(T = 18K)，Mn L-edge和Mn K-edge光譜幾乎無任何變化。

另一部份，我們使用同步輻射中心X光源測量一系列有關鉻的標準樣品之X光吸收光譜，分析其氧化價數對Cr K-edge吸收邊緣能量位置的變化，完成鉻化合物價數圖，亦用此圖來判斷Cr3(PO4)2、Cr2S3、CdCr2S4與CuCrO4中Cr的價數，並以L-edge加以佐證。實驗結果為：一. Cr2S3、CdCr2S4是Cr+2，CdCr2S4在光譜中呈現接近單一2+價數，然無價數分離之現象發生。二. Cr3(PO4)2是Cr+2，Cr3(PO4)2與CrF2在Cr L-edge光譜中極為相似，已知CrF2是高自旋態(high spin state)，推論Cr3(PO4)2的自旋組態為相同的高自旋態(high spin state)，並以磁性量測加以佐證。三. CuCrO4是Cr+6，可以輔助說明其Cu存在價數分離之現象。

In this thesis, x-ray absorption near edge spectroscopy (XANES) experiments were conducted in a range of temperatures upon La1-xCaxMnO3 (x=0.6, 0.8) using synchrotron radiation from the National Synchrotron Radiation Research Center (NSRRC). The O K-edge, Mn L-edge and Mn K-edge absorption spectra at different temperatures were obtained. Further analysis probes the nature of the charge ordering of Mn ions in La1-xCaxMnO3 (x=0.6、0.8) samples. The Mn K-edge spectra were meant to determine the degree of charge disproportion in charge ordered samples. As temperature changed, opposite trends developed in the O K-edge spectra and the Mn K-edge pre-edge spectra of charge ordered La1-xCaxMnO3 (x=0.6, 0.8) and La0.7Ca0.3MnO3 without charge ordering. They were results of the charge ordering. The correlated polaron localizes charge carriers in Mn+3 lattice points and hence results in Jahn-Teller distortion effect.

In another series of XANES experiments, the x-ray absorption spectra of standard samples of chromium were obtained. The correlation between the oxidation state and the Cr K-edge absorption edge energy was investigated. A chromium valence state vs. K-edge energy chart was obtained, from which the valence states of Cr in Cr3(PO4)2, Cr2S3, CdCr2S4 and CuCrO4 were determined and verified with L-edge absorption spectrum. The valence of Cr was found to be +6 in CuCrO4, and+2 in Cr3(PO4)2, Cr2S3, CdCr2S4. Furthermore, Cr3(PO4)2 is found to be in high spin state, as its L-edge spectrum highly resembles that of CrF2. This result was testified by the magnetic measurements. Lastly, the valence of Cr was found to be +6 in CuCrO4, which aids the explanation of the charge disproportion of Cu in this compound.