新穎材料 Cs2Nb4O11 與 MexMn1-xS ( Me = Co, Gd ) 之光譜性質研究

Abstract

們研究Cs2Nb4O11單晶樣品的光譜性質。Cs2Nb4O11於165 ℃展現反鐵電相轉順電相的相變，第一原理理論計算顯示，氧八面體中心的Nb(4)與Nb(5)的位移為相變主要機制，NbO6八面體沿著c-axis位移而造成氧原子週遭的環境不同，受到其它原子的吸引或排斥，所以高溫橢圓偏振光譜 ( T = 165 ℃) 顯示氧原子2p軌域上的電子躍遷至鈮原子3d軌域所需要的能量( 4.95 eV與6.07 eV )受到了影響。 其次，我們研究MexMn1-xS (Me = Co, Gd)多晶樣品的變溫拉曼散射光譜。隨著摻雜Co離子濃度增加，CoxMn1-xS樣品顯現值得注意的重點包括(i) 840 cm-1拉曼峰有藍移的現象；(ii) 840 cm-1拉曼峰隨著溫度降低有紅移的現象。我們推測840cm-1拉曼峰與小極化子機制有緊密的關連性，經由小極化子的理論分析顯示出其束縛能隨著居里溫度的降低而增加。此外，位於1000 cm-1的拉曼峰在溫度低於尼爾溫度時，其拉曼峰權重會上升。以上這些實驗結果驗證材料為具有複雜自旋-聲子交互作用的系統。

We study the infrared and optical properties of Cs2Nb4O11 single crystal. This material displays an antiferroelectric to paraelectric transition at 165 ℃. The results of first-principles theoretical calculations suggest that antiferroelectricity is caused by an antiparallel ionic displacement of Nb(4) and Nb(5) along the c-axis. The observed charge-transfer transitions between O 2p and Nb 3d states at about 4.95 and 6.07 eV electronic transitions exhibit anomalies at 165 ℃ and sensitively gauge the changes of Nb ionic displacements. We further investigate the Raman-scattering spectra of polycrystalline MexMn1-xS (Me = Co and Gd). With increasing cobalt concentration in CoxMn1-xS, most notable are (i) an increase in the center frequency of one Raman excitation at about 840 cm-1, and (ii) a red shift of this peak as the temperature is lowered. An analysis of the small-polaron picture shows that the polaron binding energy increases with decreased Curie temperature in CoxMn1-xS. Additionally, a resonance mode at about 1000 cm-1 shows an enhancement of spectral weight below TN. All of these observables suggest a complex nature of spin-phonon coupling in these materials.