xSr(Mg1/3Ta2/3)O3-(1-x)Ba(Mg1/3Ta2/3)O3微波介電材料的光譜研究

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2009

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我們研究摻入不同Sr濃度的Ba(Mg1/3Ta2/3)O3 (xSr(Mg1/3Ta2/3)O3- (1-x)Ba(Mg1/3Ta2/3)O3,縮寫為xSMT-(1-x)BMT,x = 0.0~1.0) 的光譜特性,包含X光繞射(XRD)、延伸X光吸收精細結構(EXAFS)、拉曼散射及紅外反射光譜,並從光譜特性中找出與微波介電特性之關聯。在X光繞射實驗中,我們發現在x = 0.25之後(範圍在2θ = 42°~44.5°)會出現單斜結構 (-2 6 0) 的繞射峰,因此從X光繞射並不容易觀察出xSMT-(1-x)BMT的相變行為。在延伸X光吸收光譜中,不論是在以Ta為中心還是以Sr為中心的χ(R)光譜中,皆可看出光譜大致上有三階段的變化,證實xSMT-(1-x)BMT的晶格結構隨Sr摻入濃度x的增加產生改變。拉曼散射從單斜結構所主導之拉曼散射峰的出現,預測在x = 0.625時可能已經產生相變。紅外反射光譜的波形不容易看出相變,但從聲子的頻移分析可以得到相變發生的事實。 我們發現B位置 (Mg和Ta) 的有序程度與微波品質因子有很高的關聯性,而兩個A1g(O)聲子的寬度亦與微波品質因子有密切關係。相變之後(x ≥ 0.65),BO6才是決定微波介電常數趨勢的重要因素,因此我們推論x ≤ 0.5時A位置對微波介電常數的影響是顯著的。異常紅外聲子和微波介電特性是有關聯的,但是需要更多來自第一原理的資訊才能有清晰的解釋。
X-ray diffraction (XRD), extend x-ray absorption fine structure (EXAFS), Raman scattering, and infrared reflection spectroscopy were adopted to analyze the correlation of microwave properties with the microstructural parameters of xSr(Mg1/3Ta2/3)O3-(1-x)Ba(Mg1/3Ta2/3)O3 perovskite ceramics (xSMT-(1-x)BMT) with x = 0.0 to 1.0. We found the XRD peak (-2 6 0) of monoclinic phase near 2θ = 42°~44.5° appeared when x = 0.25, so the phase transition does occure, but it does not clearly find when it happens by XRD pattern. By EXAFS results, we found a three-stage variation in the χ(R) spectra for the Ta core or the Sr core, and this confirms the minor change of crystal symmetry through the slight displacement of atomic rearrangement as x increases. In Raman scattering experiments, the dominant scattering peaks of monoclinic phase appeared when x ≥ 0.625, and we think it transfers from trigonal symmetry to monoclinic phase for x ≥ 0.625. The shape of peak is not easy to recognize the phase transformation in infrared reflection spectrum, but we can find the at x ≥ 0.625 phase transformation from analyzing shift of phonon peaks. The strong correlation of ordering parameter of B site (Mg and Ta) is obtained by XRD and microwave quality factor such correlation also exists between widths of two Raman A1g(O) phonons and microwave quality factor. After the phase transformation (x ≥ 0.65), the BO6 oxygen octahedra dominate the trend of microwave dielectric constant, so we predict that A site effect on the microwave dielectric properties is remarkable when x ≤ 0.5. The anomaly of the infrared active phonon modes may be connected with microwave dielectric properties, but we need more information from first-principles calculations to get the precise picture of the phase transition by the IR measurements.

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X光繞射, 延伸X光吸收精細結構, 拉曼散射, 紅外反射光譜, 相變, X-ray diffraction, extend X-ray absorption fine structure, Raman scattering, infrared reflection spectrum, phase transition

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