多鐵電氧化物及硫化物材料之光譜性質研究

Optical properties of multiferroic oxides and sulfides

林美如(Mei-Ru Lin)

指導教授 ： 劉祥麟

國立臺灣師範大學/理學院/物理學系/碩士(2006年)

摘要

我們研究多鐵電體材料(Tb1-xNax)MnO3(x = 0.0、0.03及0.05)、DyMnO3及CdCr2S4之光譜性質。TbMnO3的室溫拉曼光譜與之前的論文[Phys. Rev. B 73, 064302 (2006)]結果完全吻合，以Na+離子部分取代Tb3+離子時，拉曼振動峰呈現微小的擾動，可能與氧缺陷產生局部晶格扭曲有緊密的關聯性，而當Dy3+離子完全取代Tb3+離子，拉曼振動峰顯現藍移的現象且半高寬變窄，印證了中心離子變小造成單位晶胞體積縮小的趨勢，另一類鐵電材料CdCr2S4的四室溫拉曼光譜與之前論文[Phys. Rev. B 21, 1316 (1980)]結果亦相吻合。
其次，我們研究這些樣品的變溫光譜響應，隨著溫度的降低，(Tb1-xNax)MnO3(x = 0.0、0.03及0.05)及DyMnO3在380 cm-1的聲子吸收峰顯示紅移現象且伴隨權重強度的增加，可能與電荷重新分布有關；其次，CdCr2S4接近相變溫度84 K時，兩個325和381 cm-1的聲子吸收峰有微小的紅移現象，這是由於Cr-S-Cr與Cr-S-Cd-S-Cr超交換作用所引起的自旋-聲子耦合。

關鍵字

多鐵電材料

並列摘要

We study the optical properties of single crystalline multiferroic materials, such as (Tb1-xNax)MnO3 (x = 0.0, 0.03, and 0.05), DyMnO3, and CdCr2S4. The room-temperature Raman spectra of TbMnO3 are similar with the previous results of Phys. Rev. B 73, 064302 (2006). The position and intensity of Raman-active phonons are perturbed to some extent by Na doping. It is likely that monovalent Na+ ions were used to create oxygen vacancies and to induce the local lattice distortions. When doped with Dy on Tb, the phonon frequencies are hardening and their linewidths are narrowing, which is due to the chemical substitution of the smaller Dy ions into the bigger Tb ionic sites. The room-temperature Raman spectra of CdCr2S4 are also similar with the previous results of Phys. Rev. B 21, 1316 (1980).

With decreasing temperature, overall infrared-active phonon spectra of (Tb1-xNax)MnO3 and DyMnO3 remain unchanged. However, the phonon mode at about 380 cm-1 show a softening and an increased intensity at low temperature, which is possibly due to the redistribution of effective charges in the unit cell. In the ferromagnetic phase below 84 K, the phonon peaks of CdCr2S4 near 325 and 381 cm-1 show a shift to lower frequencies, indicating the spin-phonon coupling via the superexchange interactions of Cr-S-Cr and Cr-S-Cd-S-Cr.

參考文獻

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