奈米級尺寸顆粒 Bi1-xDyxFeO3 多鐵材料之光譜性質研究

Abstract

本論文研究奈米級尺寸顆粒 Bi1-xDyxFeO3（x = 0.00、0.05、0.10、0.15、0.20、0.30、0.40）多鐵材料的全頻反射與拉曼散射光譜。隨著摻雜鏑離子濃度的增加，紅外光與拉曼活性振動模的變化符合x 光繞射能譜的晶格結構分析：（i）0.00 ≤ x ≤ 0.05 屬於空間群 R3c 菱形晶系結構；（ii）0.20 ≤ x ≤ 0.40屬於空間群 Pnma 正交晶系結構；（iii）x = 0.10與 0.15 顯示兩相共存狀態。此外，高頻紅外光吸收與拉曼散射光譜展現多倍磁振子的貢獻，代表鏑離子的摻雜和奈米級尺寸顆粒導致Bi1-xDyxFeO3本身磁性結構的改變。更有趣地是低頻拉曼散射光譜顯現擴散響應，我們認為鏑離子的摻雜引起晶格局部扭曲，降低電荷的漂移率，造成Bi1-xDyxFeO3的電性傳導屬於電荷躍遷機制。 高溫拉曼散射光譜顯示：（i）在預期的尼爾溫度附近，各拉曼峰的參數（頻率位置、半高寬、及強度）並未發生明顯地異常變化，這代表自旋與聲子的交互作用微弱；（ii）擴散響應之半高寬隨溫度升高而變小，Bi1-xDyxFeO3電荷彼此之間的碰撞率降低，暗指其電性傳導愈佳化。We present the results of infrared, optical reflectivity and Raman-scattering measurements of nano-sized Bi1-xDy xFeO3 (x = 0, 0.05, 0.10, 0.20, 0.30, and 0.40) polycrystalline samples. It is found that when doping with Dy on Bi-site, the variations of infrared and Raman-active phonon modes are consistent with the analysis of x-ray powder diffraction spectra: (i) rhombohedral space group R3c as 0.00 ≤ x ≤ 0.05；(ii) dominant orthorhombic group Pnma as 0.20 ≤ x ≤ 0.40; and (iii) rhombohedral and orthorhombic mixings as x = 0.10 and 0.15. Moreover, multimagnon excitations are observed in both infrared absorption and high-frequency Raman-scattering spectra, indicating Dy doping and the nano-sized grains modify the magnetic structures of these materials. Interestingly, low-frequency Raman-scattering spectra exhibit diffusive response, reflecting the substitution of Dy for Bi induces the local lattice distortion and a concomitant reduction in the carrier mobility which manifest in the carrier hopping mechanism in Bi1-xDy xFeO3. With increasing temperature, there are two important features to the Raman-scattering spectra: (i) no detectable phonon anomalies are observed near the Néel temperature, suggesting the spin-phonon coupling is weak; and (ii) the scattering rate of diffusive hopping of the carriers is decreasing, indicating the enhancement of conductivity in these materials.