以吸收光譜研究鈣鈦礦結構中鈷氧化物自旋態與鈷離子價數

Abstract

在許多過度金屬氧化物之中，鈷氧化物在價態與自旋態上展現了 非常豐富的變化。本實驗是以Re1-xCaxCoO3-δ(Re=Eu、Sm ; x=0, 0.1, 0.2, 0.3, 0.4)與等樣品進行一系列變溫之O K-edge、Co K-edge 和Co L-edge 之X 光近緣吸收光譜(XANES)，研究Re1-xCaxCoO3-δ(Re=Eu、Sm ; x=0, 0.1, 0.2, 0.3, 0.4)光譜權重隨鈣參雜之含量變化，進而探討在電子自旋 態與價態變化之機制。由結果推論鈷離子在低摻雜(x=0.1, 0.2)時，價 數與氧缺陷共同平衡離子價數，因此可以同時觀察到價數增加與自旋 態改變的特徵；而高摻雜樣品，鈷離子價數不在隨著鈣含量而增加， 平衡離子價數的工作則全由氧缺陷貢獻，進而使自旋態由低自旋轉變 為高自旋態。 本論文亦在ReCo0.5Mn0.5O3(其中Re=Eu、La 和Pr)樣品以及 PrSrCoMnO6-δ 樣品進行Co 和Mn 的L、K-edge 及O K-edge XANES 光譜實驗，探討不同稀土族化合物其過渡金屬離子之電子組態，並討 論是否有可能在鈣態礦結構中發現Co+3 離子高自旋態的貢獻。由實 驗結果判斷，ReCo0.5Mn0.5O3(其中Re=Eu、La 和Pr)樣品鈷、錳離子 價數分別為+2 價與+4 價。但當我們利用Sr 取代Pr 後可以觀察出， 錳離子維持在+4 價，而鈷離子價數已轉變為+3 價，並且自旋態也與 低自旋態之光譜形狀大為不同。

Soft x-ray absorption spectroscopy at the Co L2,3- ,K-edge and O K-edges was employed as an idea tool to examine the valence states and spin configurations of the Co ions in the perovskite compounds, such as Re1-xAxCoO3 (Re is Eu or Sm, A is Co). Depending on the substitution level x of calcium, our spectra show the existence of low-spin and high-spin Co+3 and a few high-spin Co+4 ions associated with octahedral and pyramidal oxygen coordinations in the compounds. The valence changes of Co ions are not propotional to the levels of calcium (x). We also measured Co and Mn L, K-edge XAS on Re2CoMnO6 (Re=Eu, Pr, and La) and PrSRCoMnO6-δ samples. Our XAS spectra clarify the valence states of Co and Mn ions in the samples. We also found that the valence states of the transition-metal ions is Mn+4/Co+2 which cannot be understood by the simple estimation of ionization energy. And the valence states of the Co ions in PrSRCoMnO6 is +3 , and the spin state is high spin different from low spin in Pr2CoMnO6.