NaNbO<sub>3</sub>的高压拉曼光谱研究

doi:10.3964/j.issn.1000-0593(2020)03-0738-06

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Abstract Perovskite oxide NaNbO₃ is an environmentally friendly piezoelectric material with great potential applications. Thus, it has been studied by many researchers using various methods. Although several high pressure studies on structural phase transition of NaNbO₃ have been carried out but there are still many disputes regarding the phase transition sequence and the crystal structure of the phase transition near 2 GPa and above 12 GPa. Previous Raman studies on structural phase transition of NaNbO₃ were mainly focused on the high frequency side of the spectrum related to the internal vibration of NbO₆, and did not cover the lattice vibration in low frequency region. Therefore, we studied the structural phase transition of NaNbO₃ by Raman spectroscopy at high pressure using diamond anvil cell technique from 0~22 GPa. In this study, we used a mixture of 16∶3∶1 methanol, ethanol and water as the pressure transmitting medium. We obtained the Raman spectra from 40 to 1 000 cm^-1, so that the obtained spectrum fully covered the phonons related to the Na+ displacement, librational, translational and vibrational modes of NbO₆ in the unit cell. Our results showed that the Raman spectra of NaNbO₃ under pressure drastically changed near 2, 7 and 9 GPa, which was related to the structural phase transition. During compression, the intensity of three peaks at 180~210, 221.2 cm^-1 increased rapidly, whereas the two shoulder peaks at 204.1 and 252.8 cm^-1 disappeared near 2 GPa. Also the ν₁ and ν₃ modes showed softening at the same pressure. These results indicated that NaNbO₃ transformed from Pbma phase to HP-Ⅰ phase at 2 GPa. Further increasing pressure to 6.6 GPa, the Raman modes at 122.3, 155.5, 196.2, 228.2 and 279.4 cm^-1 at ambient pressure disappeared, while the peak intensity of high frequency Raman modes decreased and the peaks became broad, indicating that the second structural phase transition (HP-Ⅰ~Pbnm) of NaNbO3 occurred near 7 GPa. At 9.7 GPa, Raman modes below 125 cm^-1 disappeared completely, showing a strong background like feature. However, at intermediate frequency range, new peaks at 182.2, 261.4 and 517.7 cm^-1 appeared, whereas the Raman mode at 559.1 cm^-1 disappeared, indicating another structural phase transition from Pbnm to HP-Ⅲ phase near 9 GPa. Up to 22 GPa, the Raman spectra did not change much as a function of pressure, and showed very sharp spectral characteristics, indicating that the HP-Ⅲ phase remained stable up to the 22 GPa. Thus, our result did not support the appearance of the cubic paraelectric phase above 12 GPa as reported by other researchers. From our result, we can estimate the Tc temperature decreased from 614 ℃ to RT at least at the rate of dT/dP=27.9 ℃·GPa, much less than that calculated by Shen et al. During decompression, below 7 GPa, the Raman spectra of HP-Ⅰ phase were significantly different from the spectra observed at increasing pressure cycle. This result showed the irreversibility of the structural disorder caused by Na⁺ displacement, indicating the crystal structure within this pressure range may be coexistence of HP-Ⅰ and Pbnm phase. After releasing the pressure, the ambient structure of NaNbO₃ was basically recoverable. Therefore, it can be seen that the lattice vibration induced by Na⁺ displacement in low frequency range has a great importance for the high-pressure phase transition studies on NaNbO₃, which can provide a reference for the future study on structural phase transition of other perovskite materials.

Key words：Sodiumniobate; Raman scattering; High pressure; Structural phase transition

Received: 2019-03-06 Accepted: 2019-07-05

ZTFLH:

O521.+23

Corresponding Authors: Anwar Hushur E-mail: anwar.hushur@outlook.com

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https://www.gpxygpfx.com/EN/10.3964/j.issn.1000-0593(2020)03-0738-06 OR https://www.gpxygpfx.com/EN/Y2020/V40/I03/738