在混合相的鈣鈦礦材料中常有ㄧ些特殊物理特性，像很高壓電係數、介電常數等等。這些現象可能來自於多相的耦合或有一中間相。本研究中，我們探討受鋁酸鑭(LaAlO3，簡稱LAO)基板壓縮應力所形成的混合相鐵酸鉍(BiFeO3，簡稱BFO)磊晶薄膜樣品。受基板應力，在鐵酸鉍混合相中形成類似菱長晶(R)和類似長方晶(T)的斜方晶結構，具有週期性的結構，並且隨著厚度增加而釋放應力，R結構比例增加。我們利用拉曼散射研究混合相晶格結構，從不同R/T比例的拉曼光譜顯示了隨厚度的晶格扭曲變化。本研究中，探討了從溫度室溫到600 ℃的相變化。此外，利用奈米級顯微探針探討電壓對結構的相變影響也可在拉曼光譜中量測到。R結構的振動峰在最厚的薄膜中明顯增強，在厚度增加期間，Bi-O鍵結間距拉長，氧八面體沿著[110]軸持續傾斜。菱長晶結構振動峰225、 238 和 362 cm-1在200 ℃ R-T相變點消失，相反的，T結構一直到600 ℃都還存在且Bi-O間距隨溫度縮短。外加電壓下，Bi-O距離縮短，Fe-O相關的振動峰強度下降，結構相變成T結構。

Significant changes of physical properties are usually observed in perovskite materials with mixed phases. This phenomenon may come from the couplings between the multiple phases or the intermediate structures. In this study, we investigated the BiFeO3 (BFO) films with mixed phases driven by compressive stress from the substrate. In these films, different phases, including monoclinic rhombohedral-like phase (R), and tetragonal-like phase (T), formed periodical strained patterns, and the R/T ratio relaxed with the film thickness. We explored the BFO crystal structures by Raman scattering. The Raman spectrum of samples with different R/T phase ratios showed the change of lattice distortion with thickness. The evolution of phonon behaviors during phase transformation from room temperature to 600 ℃ was investigated. Moreover, the phase transformation with the bias, which was confirmed at nanoscale by scanning probe microscopy, was also revealed by the Raman spectrum. The enhancement of the rhombohedral phonon was observed in the thickest film. In the processes of increasing thicknesses, the Bi-O bonds became longer and the octahedra rotated around [110] axis. The R-phonon about 225, 238 and 362 cm-1 disappeared at the R-T phase transition at low temperature. In contrast, the T phase in the Raman spectrum last to our highest measured temperature 600 ℃ and the Bi-O bonds distance decreased. Under the electric field, the Bi-O bonds shrank and the intensity of Fe-O octahedra-related phonons decreased, which corresponded with the phase transformation of R to T.

[1] C. H. Ahn, et al., Science 33, 488 (2004).
[2] Rijnders G. et al., Nature 306, 5 (2004).
[3] L. P. Yong et al., Mater. Lett. 61, 542 (2007).
[4] Ma et al., Appl. Phys. Lett. 92, 182902 (2008).
[5] B. Noheda, Current Opinion in Solid State and Materials Science 6,(2002), 27-34.
[6] R. J. Zeches et al., Science 326, 977 (2009).
[7] M. E. Lines, et al., Principles and Applications of Ferroelectrics and Related Materials, Clarendon Press, Oxford (1977).
[8] C. H. Ahn, et al., Science, 303, 488 (2004).
[9] G. Srinivasan, et al., Solid State Commun. 128, 261 (2003).
[10] G. A. Smolenskii , et al., SOVS. PHYS. USPEKHI.25, 475-493 (1982).
[11] H. Schmid, Int. J. Magn. 4, 337 (1973).
[12] N. A. Hill, Filippetti A, J Magn Magn Mate. 242–245,976. (2002).
[13] N A. Hill, et al., Phys. Rev. B 59, 8759-8769 (1999).
[14] Baettig P and Spaldin N A, Appl. Phys. Lett. 86, 012505 (2005).
[15] Filippetti A and Hill N A J. Magn. Magn. Mater.236 176 (2001).
[16] B. B. Van Aken, et al., Phys. Rev. B 69, 134113 (2004).
[17] B. B. Van Aken, et al., Nat. Mater. 3, 164 (2004).
[18] T Goto, et al., Phys. Rev. Lett. 92, 257201 (2004).
[19] L. C. Chapon, et al., Phys. Rev. Lett. 93, 177402 (2004).
[20] D. V. Efremov, et al., Nat.Mater. 3, 853 (2004).
[21] John R. Ferrara, INTRODUCTORY RAMAN SPECTROSCOPY.
[22] Manoj K. Singh et al.,PRB 72, 132101 (2005).
[23] Jornal of APPLIED PHYSICS 103, 093532 (2008).
[24] Mariola O. Ramirez et al., APPLIED PHYSICS LETTERS 92, 022511 (2008).
[25] Y. Repelin et al., Journal of Physics and Chemistry of Solids 60 (1999) 819-825.
[26] Alison J. Hatt, et al., Physical Review B 81, 054109 (2010).
[27] 汪建民, 材料分析.