xLa(Mg1/2Sn1/2)O3-(1-x)La(Mg1/2Ti1/2)O3微波陶瓷材料之拉曼光譜與延伸x光吸收精細結構分析

Abstract

本文利用拉曼散射、X光繞射和延伸X光精細結構吸收譜等光學方法來測量1：1結構A(B’1/2B”1/2)O3陶瓷家族中x La(Mg1/2Sn1/2)O3-(1-x) La(Mg1/2Ti1/2)O3鈣鈦礦陶瓷中氧八面體結構與其微波性質的關連性。這系列樣品共有五個，x代表錫原子的濃度，從0、0.25、0.5、0.75到1。在不同濃度的錫原子摻雜，Q×f值跟隨改變，隨著錫原子濃度的上升，介電係數下降。從X光吸收譜可以很明確知道錫原子是取代鈦原子的晶格位置，由於錫原子離子半徑較鈦原子大，所以當錫原子掺雜濃度越多時，錫與氧的鍵長增加，驗證氧八面體體積隨著錫原子濃度上升而增加，而在A位置的鑭原子與氧原子的鍵長隨著錫濃度之增加而減少，也就是鑭與氧形成的LaO8體積減少。從拉曼實驗顯示高頻部份與A1g(O)振動類似的振動模有聲子頻率紅移的現象，這也證明了錫與氧的鍵長增加，氧八面體體積增加，但是摻雜的錫原子較重使得氧八面體內部密度變大，造成介電常數減少。另一方面，部分鑭跟氧振動有聲子頻率藍移的現象，是與鑭原子形成之氧LaO8體積變小有關。Q×f值與聲子半寬度成反比，並且在錫原子與鈦原子掺雜比例相同為1：1時為最低，波的傳遞最差。從拉曼實驗及X光吸收譜實驗都顯示本文材料的微波特性與氧八面體微觀結構直接相關。

The 1:1 ordered xLa(Mg1/2Sn1/2)O3-(1-x)La(Mg1/2Ti1/2)O3 microwave ceramics with x = 0, 0.25, 0.5, 0.75 and 1 were examined by Raman scattering and Extended X ray Absorption Fine Structure(EXAFS) to reveal the correlation of the micro-structure with the microwave dielectric properties. The microwave Qxf value increases with x, i.e. the concentration of the La(Mg1/2Sn1/2)O3, while the dielectric constant decreases with Sn concentration. EXAFS found that the Sn atoms are sitting at Ti lattice site, and the Sn-O bond length increases with Sn concentration. This is mainly due to the larger ionic radius and heavy mass of Sn4+, as compared with Ti4+ ions. Therefore, the volume of oxygen-octahedron increasing with Sn concentration, as predicted by the x-ray diffraction, is expected. However, the La-O bond length decreases with Sn concentration, that is the volume of LaO8 decreases, as revealed by EXAFS. Raman measurement found that the oxygen-octahedral vibrations mostly are redshifted by Sn doping, and the vibrations regarding to the La-site vibration are mostly blueshifted with Sn concentration. Raman measurement is consistent with the EXAFS results. EXAFS indicates the volume of oxygen-octahedron is increased with x, however, the density of oxygen-octahedron is also increased with x due to the heavy mass of Sn4+. Therefore, it is the main reason that causes the dielectric constant to reduce with x. The width of oxygen-octahedral phonon is strongly correlated with the microwave Qxf value, which indicates the propagation of the microwave is similar to the propagation of oxygen-octahedral phonon.