Phase Transition Kinetics of Ge from dc Phase to β-Sn Phase under High Pressure
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摘要: 室温常压下锗是一种具有高载流子迁移率和窄带隙的半导体材料。在高压下，锗具有多种与硅相似的晶相，其有趣的高压行为如压致金属化和超导电性转变引起了高压研究领域的广泛关注。然而，其核心的高压相变动力学机制却鲜有深入研究。利用先进同步辐射光源的高通量X射线衍射结构诊断手段，结合基于金刚石压砧的快速动态压缩技术，研究了锗在高压相变过程中的结构演化机理。采用气膜与压电陶瓷相结合的快速加载方法，实现了数十太帕每秒的压缩速率。采用第三代同步辐射高通量粉光X射线衍射技术，实现了数十微秒时间分辨的结构解析。在相变过程中，新旧相中不同晶面的衍射强度变化存在一定的先后顺序，证实了锗的半导体相（金刚石立方结构）到金属相（β-Sn结构）的转变是位移型相变。此外，通过与静态压缩X射线衍射数据的对比，证实了在此相变过程中不同晶面消失/出现存在先后顺序的行为只能通过动态压缩和动态探测手段观察。Abstract: Germanium is a semiconductor with good performances of high carrier mobility and narrow band gap at ambient conditions. Under high pressure, it undergoes serials of polymorphs similar to the case of silicon, and the attractive characteristics in its high pressure phases such as metallization and superconducting transition make it one of the most appealing materials in high pressure research. However, its fundamental phase transition kinetics has been rarely studied. In this work, we present our experimental observations on the phase transition of germanium via a novel designed rapid compression tool and ultrafast time-resolved X-ray diffraction (XRD) acquisition system. The compression rate reaches to tens of TPa/s which is realized by combining gas membrane and piezoceramics compression methods in a symmetrical dynamic diamond anvil cell (dDAC). The time-resolved XRD with high resolution in microseconds is achieved by integrating the high flux pink beam diffraction, an X-ray scintillator to convert diffracted X-rays to visible lights and a high-speed optical camera. It is found that there is a time sequence for diffraction planes disappearing and appearing of dc and
$\,\beta $ -Sn phases, showing a displacive feature for this phase transition. In addition, the XRD evolution under static compression is also given for comparing with the dynamic compression, the results demonstrate our novel designed rapid compression and ultrafast time-resolved XRD setup shows a great potential for studying the high pressure phase transition kinetics. -
图 1 动态压缩实验装置示意图
Figure 1. Schematic diagram of experimental setup for dynamic compression
图 2 静高压下Ge从dc到
$\,\beta $ -Sn相变过程的原位单色XRD积分曲线（图中标注了不同衍射峰对应的晶面指数，以及所用X射线的波长$\lambda $ 、晶相和所属对称群）Figure 2. In-situ monochromatic XRD integral curves of the phase transition of Ge from dc phase to
$\,\beta $ -Sn phase under static high pressure (The figure shows the crystal plane index corresponding to diffraction peaks, the wavelength ($\lambda $ ) of the X-ray used, the crystal phase and the symmetry group.)
图 3 动态压缩前后的粉光XRD谱（(a)和(c)分别为Ge多晶压缩前、后的XRD谱，对应的晶面用虚线圆弧标记，并用黑色字符标出，明亮的衍射斑点来自样品中的大尺寸晶粒；(b)和(d)为对应的衍射积分图）
Figure 3. Pink-beam XRD patterns before and after dynamic compression ((a) and (c) are the XRD patterns of Ge polycrystals before and after compression, respectively. The corresponding crystal planes are marked with dashed arcs and black labels. The bright diffraction spots come from large-sized crystal grains in the sample. (b) and (d) are the corresponding diffraction integral patterns.)
图 4 动态加载过程中Ge的XRD谱以及压力随时间的变化：(a) 动态XRD的时间堆叠图，(b) 压力与时间的关系
Figure 4. XRD patterns of Ge during dynamic compression and the relationship between pressure and time: (a) the time-stacked diagram of dynamic XRD patterns; (b) the relationship between pressure and time
图 5 (a) 动高压下Ge从dc到β-Sn相变过程的原位粉光XRD积分曲线，(b) 动态加载过程中Ge的dc相和 β-Sn相的不同晶面衍射峰归一化强度随时间的变化
Figure 5. (a) In-situ pink-beam XRD integral curve of the Ge phase transition from dc phase to β-Sn phase under dynamic compression; (b) the normalized intensity of the diffraction peaks of Ge in the dc phase and β-Sn phase during dynamic compression changes with time
表 1 静态压缩下10.75 GPa时Ge的晶胞参数
Table 1. Unit cell parameters of Ge under static compression at 10.75 GPa
Method Medium Phase a/Å c/Å V/Å3 (Vdc−Vβ)/Vdc Present
experimentSilicone oil dc phase 5.4692±0.0003 20.450±0.002 0.182±0.001
(10.75 GPa)$\,\beta $-Sn 4.9496±0.0008 2.7322±0.0007 16.735±0.005 Other
experiment[28]Methanol-ethanol mixture
(methanol∶ethanol=4∶1)0.189±0.007
(10.60 GPa)
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