"Ba2SmNbXMo1-XO6 氧化物 (X=1, 0.9, 0.5, 0.4, 0.3, 0.2, 0.1) 之介電性質研究"

Abstract

對於過渡元素鈮(Nb)和鉬(Mo)來說，兩者擁有相當接近的離子半徑 (Nb=0.69 A, Mo+4=0.68 A, Mo+6=0.65 A) ，此兩元素所形成之雙鈣鈦礦系統 (Ba2SmNbO6, Ba2SmMoO6)在晶體結構上都屬於正方晶系(Cubic structure) (Ba2SmNbO6 :a=7.463 A, Ba2SmMoO6 : a=8.2 A )，晶體常數也相近，以容忍因子(tolerance factor)角度來看，此二系統各離子在結構上之排列應是相當類似，但在介電係數,磁性,結構相變的行為卻截然不同。Ba2SmNbO6 之介電係數在20k~300k只在180~200間變化，晶體結構從20k~400k都是正方晶系，從2k~400k此系統都是順磁性; 然而Ba2SmMoO6之介電系數卻可到達104之最大值，且在TN=130k，磁性從反磁轉變為順磁，從文獻得知此為ㄧJahn – Teller效應造成之反鐵相變，且因其相變溫度對應到ㄧ個在tan δ 圖的峰值，Ba2SmMoO6應是ㄧ多鐵材料(Multiferroics)。為了瞭解造成此巨大差異之背後機制，本研究製備Ba2SmNbxMo1-XO6 (X=0, 0.1, 0.2, 0.3, 0.4, 0.5,0.9,1)摻雜系統，透過固態化學反應法,不同之燒結溫度,退火過程以及不同通氣鍛燒時間等製程以控制氧化程度，藉由X-ray粉末分析，接著以溫度和頻率作變數量測介電常數(ε’)，介電損失(tan δ)，複數modulus (M, M)來分析這些氧化物之行為。

The synthesis, characterization and dielectric properties of the polycrystalline rare earth oxides Ba2SmNbXMo1-XO6 compounds were investigated. Powder X-ray diffraction patterns show that all these samples are single phase without any impurity. The dielectric properties of these compounds were studied in the frequency range from 20 Hz to 1 MHz between 20 K and 320 K. The Ba2SmNbXMo1-XO6 samples exhibit a high dielectric permittivity (ε’~104）at room temperature with frequency dispersion. The corresponding tanδ(T) curves exhibit two relaxations. The peak temperature TP of tanδ shifts to a higher temperature as the frequency increases in both relaxations, indicating thermally activated relaxation. The overall behavior of high temperature ε’ and tanδ is similar to that observed in giant dielectric constant materials. The observed giant dielectric response was explained in terms of internal (grain boundary) barrier layer capacitance (IBLC) effect. Complex modulus analysis was used to distinguish the grain and grain boundary resistance in all compositions. The observed results may provide a better understanding of the dielectric properties of the Ba2SmNbXMo1-XO6 system, and might be extended to advanced applications.