利用核磁共振解析NaxCoO2的超晶格與磁性結構

Abstract

我們利用核磁共振技術來探討NaxCoO2 單晶樣品中所出現的超晶格(supercell) 結構級磁性行為。我們量測了Na0.825CoO2 中Na與Co的頻譜分別發現了6種不同Na的晶格位置與四種不同Co的位置，並參考x-ray 所量測到超晶格大小的數據，成功的解析出超晶格結構。另外也量測了在不同溫度與不同磁場下的頻譜，發現樣品在進入22K反鐵磁態之前，就已在60K附近出現了磁相干 （magnetic correlation） 的現象，並觀察到在鐵磁與反鐵磁態的磁相變（magnetic transition） 過程中展現了自旋玻璃 （spin glass）行為。

我們也比較了Na2/3CoO2 ，Na0.71CoO2 ，Na2/3CoO1.98 單晶樣品中的晶格結構，探討它們是否有穩定的超晶格結構。結果顯示Na2/3CoO2單晶樣品中並未出現像文獻中樣品所報導長程有序（long range order）的結構態。但是，Na0.71CoO2與Na2/3CoO1.98的樣品中，我們發現與粉末Na2/3CoO2 的樣品類似的超晶格結構。我們認為Na2/3CoO2本質上應具有穩定的超晶格結構。

Nuclear magnetic resonance was used to explore the superlattice structure and magnetic behavior in the NaxCoO2 single crystals. Measurements of the sodium (Na) and cobalt (Co) spectra of Na0.825CoO2 revealed 6 and 4 distinct lattice positions for Na and Co, respectively. Based on the supercell size data from x-ray diffraction experiments, superlattice structure was determined. Moreover, the spectra obtained for various temperature and magnetic field conditions showed that before the samples entered an antiferromagnetic state at 22 K, magnetic correlation already appeared at approximately 60 K. Spin glass behavior was also observed during the magnetic transition from the ferromagnetic to the antiferromagnetic state.

The lattice structure in Na2/3CoO2, Na0.71CoO2, and Na2/3CoO1.98 samples was investigated to determine whether they have superlatice structure or not. The Na2/3CoO2 single crystal did not exhibit the long-range ordered structure indicated in the literature for powder sample. However, the Na0.71CoO2 and the oxygen-deficient samples exhibited similar superlattice structures to that of the Na2/3CoO2 powder sample. We thus, conclude that Na2/3CoO2 should possess a stable superlattice structure.