Pressure-induced phase transitions of multiferroic BiFeO3

Pressure-induced phase transitions of multiferroic BiFeO3 have been investigated using synchrotron radiation X-ray diffraction with diamond anvil cell technique at room temperature. Present experimental data clearly show that rhombohedral （R3c） phase of BiFeO3 first transforms to monoclinic （C2/m） phase at 7 GPa, then to orthorhombic （Pnma） phase at 11 GPa, which is consistent with recent theoretical ab initio calculation. However, we observe another peak at 2θ=7° in the pressure range of 5-7 GPa that has not been reported previously. Further analysis reveals that this reflection peak is attributed to the orthorhombic （Pbam） phase, indicating the coexistance of monoclinic phase with orthorhombic phase in low pressure range.     

Quasi-hydrostatic Limit of LiF as a Pressure Transmitting Medium and Its Equation of States

Quasihydrostatic limit of LiF as a pressure transmitting medium is investigated by synchrotron radiation x-ray diffraction combined with the diamond anvil cells technique up to 60 GPa at room temperature. The equation-of- state parameters of LiF are determined to be Vo = 65.7（2）A3 Bo = 58（3） GPa and B＇0 = 4.9（2） in the silicon oil environment; Vo = 67.4（3） A3, Bo = 51（3） GPa and B＇0 = 4.7（2） without pressure transmitting medium. The full width at half maximum of LiF （111） peak increases with the increase of pressure in two independent experiments. The pressure distribution in the sample chamber is estimated by line-scanning x-ray diffraction measurements across the chamber＇s center, which presents as homogeneous with Pmax - Pmin Of about 1 GPa below 40 GPa.     

高压下α-石英和柯石英的相变行为

利用金刚石压腔和同步辐射X射线衍射技术,对α-石英和柯石英在常温高压下的相变行为进行了研究。实验结果表明:α-石英在约23 GPa开始发生结构相变,在约44 GPa相变完成,直至59 GPa仍能观察到结晶态;柯石英在约22 GPa转变为柯石英-Ⅱ相,高于36 GPa时,继续发生结构转变,直至59 GPa仍有结晶态;氖气和氩气所提供的不同静水压条件对α-石英和柯石英的高压相变行为影响不大。实验结果为进一步厘清二氧化硅物相的压致相变行为和相变机制提供了实验支撑。     

双向活塞圆筒装置19 mm外径样品组装的压强和温度标定

在高温高压实验中,样品所处位置的压强、温度及样品腔温度分布情况对实验结果分析十分重要,因此使用高温高压实验装置前需对所用组装进行压强和温度标定。针对双向活塞圆筒装置19 mm外径样品组装,进行了压强与温度标定。利用氯化钠(NaCl)在高压下的熔化曲线对压强进行标定,当下油缸油压出现大幅度下降时,样品腔内的NaCl发生熔化,将此时热电偶测量的温度与文献报道的NaCl在高压下的熔化曲线进行比较,确定样品腔内的实际压强。压强标定结果显示,实际压强与目标压强满足线性关系。采用双热电偶法对19 mm外径样品组装样品腔的中部和上部进行测温,发现样品腔中心温度高于样品腔上部温度,温度梯度随温度升高而增大,随压强升高而减小。在二次加压升温时,样品腔内的温度梯度高于第一次加压升温实验测量结果。所得压强和温度标定结果对今后使用19 mm外径样品组装开展高温高压实验研究具有参考价值和指导意义。     

BiFeO_3高压拉曼光谱研究

BiFeO_3是少数在室温下同时具有铁电有序和铁磁有序的多铁材料之一。BiFeO_3在室温下呈扭曲的三方钙钛矿结构,满足R3c空间群。通过六面顶大压机在高温高压下合成纯的BiFeO_3粉晶,并结合高压拉曼光谱测试技术,得到了0~44GPa压强下BiFeO_3的拉曼光谱。结果表明:随着压强的增加,低波数的拉曼峰逐渐向高波数迁移并展宽,位于145、177和231cm-1的拉曼峰强度逐渐减弱。第一次相变发生在5GPa左右,位于145cm-1左右的拉曼峰消失,与此同时在217cm-1左右出现新的拉曼峰。当压强增加到11GPa时,明显观察到位于340cm-1左右的拉曼峰出现,而低波数(200cm-1)的拉曼峰则全部消失,标志BiFeO_3发生第二次相变,转为正交晶系的Pnma结构。压强增加到38GPa时,BiFeO_3发生第三次相变,位于340cm-1左右的拉曼峰消失,谱线上不存在明显的拉曼峰,BiFeO_3可能由正交晶系的Pnma结构转变为Pnmm结构或高对称的立方晶系结构。