钙钛矿氧化物NaNbO3是一种极具应用前景的环保型压电材料, 为此科研人员已经对其进行了大量的科学研究。 近年来, NaNbO3高压结构相变的研究有很多, 但是2 GPa附近和12 GPa以上的结构相变序列和晶体结构依旧存在很多争议。 前人利用拉曼光谱研究NaNbO3的结构相变主要关注的是NbO6内部振动引起的结构变化, 没有详细研究低频段的晶格振动对结构相变的影响。 因此, 以体积比为16∶3∶1甲醇、 乙醇和水的混合液作为传压介质, 在准静水压力条件下, 在0~22 GPa压力范围开展NaNbO3粉末的结构相变研究。 实验测试了更广波数区间（40~1 000 cm-1）的拉曼光谱, 详细分析Na+位移和NbO6振动在升压和卸压过程中对NaNbO3结构的影响。 研究表明, 压力诱变下NaNbO3的拉曼光谱图在2, 7和9 GPa附近观察到结构的转变。 升压过程, 2 GPa附近, NaNbO3从室温Pbma转变成HP-Ⅰ相, 具体表现在180～210 cm-1的3个峰的强度快速增大, 221.2和252.8 cm-1峰消失以及ν1和ν3模快速软化; 6.6 GPa以上, 原来122.3, 155.5, 196.2, 228.2和279.4 cm-1峰消失以及高频段的峰强度减弱、 对称性变低等一系列的显著变化, 标志NaNbO3在7GPa附近发生第二次结构相变（HP-Ⅰ→Pbnm）; 9.7 GPa的拉曼谱线显示出125 cm-1以下的峰完全消失, 形成一个很强背景, 182.2, 261.4和517.7 cm-1处出现新的峰以及559.1 cm-1峰消失, 表明NaNbO3在9 GPa附近从Pbnm转变成HP-Ⅲ相。 直到22 GPa, NaNbO3的拉曼谱线再没有变化并且呈现非常显著的光谱特征, 说明HP-Ⅲ相在这一压强范围内保持稳定, Tc温度至少以dT/dP=27.9 ℃·GPa的速率从614 ℃降至室温, 远远小于Shen等计算的结果。 卸压过程, 7 GPa以下, HP-Ⅰ相的拉曼光谱图与升压时存在显著差异, 表现在Na+位移引起的结构无序化具有不可逆性, 导致在这一压强范围内的晶体结构可能是HP-Ⅰ和Pbnm的共存相。 完全卸压后, NaNbO3的相结构基本恢复。 由此可见, 低频段Na+位移引起的晶格振动对NaNbO3的高压相变的影响很大, 可以为以后研究其他钙钛矿型材料的结构相变提供参考。

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.