微波介電材料 La(Mg1/2Ti1/2)O3 和 (A2+1/3B5+2/3)1/2Ti1/2O2 (A2+ = Mg, Ni, and Zn, B5+ = Nb and Ta) 之光譜性質研究

Abstract

我們研究固態法和檸檬酸鹽法兩種不同方法製成的 La(Mg1/2Ti1/2)O3 以及不同原子取代的 (A2+1/3B5+2/3)1/2Ti1/2O2 (A2+ = Mg, Ni, and Zn, B5+ = Nb and Ta) rutile系列的光譜性質，並探討紅外光光譜特徵與其微波特性的關聯。 首先，我們觀察到 (i) x 光繞射能譜顯示兩種不同製程方法的 La(Mg1/2Ti1/2)O3 塊材具有相同之晶格結構；(ii) 兩個樣品都有 12 個拉曼聲子振動模，但固態法樣品峰值的半高寬較窄，暗示其比檸檬酸鹽法樣品具有較好的晶格同調性；(iii) 兩個樣品皆有 10 個紅外光聲子振動模，並推算出離子晶格貢獻的介電常數與品質因子。固態法的介電常數較高，且固態法在 1 THz 的品質因子也較好。 接著，我們發現到 (i) 六個 rutile 系列樣品的 x 光繞射峰皆對應到 rutile 結構的峰值，且沒有雜相存在；(ii) 六個樣品的紅外振動模頻率位置有差異，應與氧八面體中陽離子與氧的鍵結強度有關；(iii) 在高頻及微波頻段量測介電常數和品質因子以 (Mg1/3Nb2/3)1/2Ti1/2O2 及 (Mg1/3Ta2/3)1/2Ti1/2O2 趨勢最為符合；(iv) (Zn1/3Ta2/3)1/2Ti1/2O2 品質因子最高，但介電常數最小，(Zn1/3Nb2/3)1/2Ti1/2O2 介電常數最高，但品質因子最差。 最後，(Mg1/3Nb2/3)1/2Ti1/2O2 及 (Mg1/3Ta2/3)1/2Ti1/2O2 樣品隨著溫度的下降，我們發現聲子軟化的現象，此結果符合賴登-沙哈-泰勒關係式。

We report the optical properties of La(Mg1/2Ti1/2)O3 prepared by either solid state or citric-acid method and (A2+1/3B5+2/3)1/2Ti1/2O2 (A2+ = Mg, Ni, and Zn, B5+ = Nb and Ta). Our goal is to understand the correlation between their Raman-scattering and infrared characteristics and microwave properties. The x-ray powder diffraction spectra show La(Mg1/2Ti1/2)O3 ceramics prepared by two different methods possess the same crystal structure. Twelve Raman-active phonon modes are observed in both samples, displaying similar frequency positions. The linewidth of La(Mg1/2Ti1/2)O3 prepared by solid state method is narrower than that of citric-acid one, implying that its coherency in lattice vibration modes is better. In addition, ten infrared-active phonon modes contribute the dielectric constant. La(Mg1/2Ti1/2)O3 grown by solid state method exhibit higher dielectric constant and better Q × f factor than those of citric-acid one. The x-ray powder diffraction spectra also show six rutile samples are well crystallized. The variation of frequency positions of infrared-active phonon modes in six samples is mainly due to different bond strength between cation and oxygen in the octahedron. The trend of dielectric properties of (Mg1/3Nb2/3)1/2Ti1/2O2 and (Mg1/3Ta2/3)1/2Ti1/2O2 in microwave and high-frequency regime is very similar. (Zn1/3Ta2/3)1/2Ti1/2O2 has the highest Q × f factor, but the lowest dielectric constant. In contrast, (Zn1/3Nb2/3)1/2Ti1/2O2 has the highest dielectric constant, but the lowest Q × f factor. With decreasing temperature, (Mg1/3Nb2/3)1/2Ti1/2O2 and (Mg1/3Ta2/3)1/2Ti1/2O2 demonstrate the effect of phonon softening, consistent with the prediction of Lyddane-Sachs-Teller relation.