La(2-x)/3NaxMg1/2W1/2O3微波介電材料之拉曼光譜與延伸X光吸收精細結構研究

Abstract

本文利用三種光學方法：拉曼散射、X光繞射和延伸X光精細結構吸收譜，測量結構為A(B’1/2B”1/2)O3之鈣鈦礦陶瓷La(2-x)/3Nax(Mg1/2W1/2)O3的氧八面體結構，以探討缺陷結構晶體之氧八面體與其微波性質的關聯性。此系列樣品共有六個，x代表鈉原子的濃度，從0、0.1、0.2、0.3、0.4到0.5。隨著鈉原子濃度上升，則晶格空缺比例減少，樣品之晶體結構從斜方結構轉變為單斜結構，介電常數與品質因子Qxf值均為逐漸下降之趨勢。從X光吸收譜可以明確地探測出以鎢原子為中心周圍六顆氧原子的距離與分佈，當鈉原子摻雜濃度增多，造成鎢與氧原子的平均鍵長變小，證實了氧八面體的扭曲造成其體積壓縮，使得介電常數下降。拉曼光譜實驗顯示專屬氧八面體伸縮振動的A1g(O)振動模具有聲子頻率紅移的現象，表示氧八面體周圍A-site密度隨著質量遠小於鑭原子的鈉原子摻雜逐漸變小，使得氧八面體伸縮振動變得輕易，因此氧八面體體積雖然為壓縮，但是整個八面體網路結構卻是變鬆散，造成A1g(O)聲子頻率位移產生紅移。由於缺陷結構的品質因子與晶體結構之有序性及對稱性成正相關，將聲子半高寬乘上鎢與氧原子距離之標準差值，可得與Qxf值成反比之關係。從拉曼實驗及X光吸收譜實驗分析顯示本文樣品的微波介電特性與氧八面體微觀結構直接相關。

The structure and microwave dielectric properties of 1：1 ordered La(2-x)/3Nax(Mg1/2W1/2)O3 (x is Na+ ion concentration, from 0 to 0.5) ceramics were investigated by the X-ray diffraction, Raman scattering, and Extended X-ray Absorption Fine Structure measurement. XRD phase analysis shows that all La-substituted samples exhibit single phase and the crystal symmetry change from orthorhombic to monoclinic phase. The Q×f value and the dielectric permittivity decrease as x increase, although the estimated concentration of A-site vacancies decreases with increasing x. EXAFS analysis indicates that the W-O average bond length decreases with the larger addition of Na+ ion. Therefore the volume of oxygen-octahedron is decreasing due to the slight distortion of the oxygen octahedral influenced by Na+ ion-doped. This result is consistent with the decrease of dielectric permittivity. The strongest peak near 850 cm-1 in Raman spectra is A1g(O) mode which is in the stretching mode of oxygen octahedron. The reduce of A-site density for small atomic weight of Na+ ion-doped results in softening of WO6 stretching, as A1g(O) phonon undergoes redshifted. Such behavior indicates that the structure of three-dimensional oxygen-octahedral framework become relaxed for high Na doped sample, although the volume of oxygen-octahedron is decreasing. The microwave Qxf value is strongly correlated with the order of A/B-site atom and the symmetry of crystalline structure. Hence, the microwave performance of doped 1：1 ordered ceramics reflects the changes in the oxygen octahedral structure.