液态Ga高压局域结构演化研究

发布时间：2019-05-21 10:12

【摘要】：Ga在极端条件下的结构研究是一个长期的重要基础研究。作为具有丰富多形相的单质元素,Ga由于其所具有的共价键和金属键混合共存的特殊成键形式表现出了较为罕见的物理特性。稳定的Ga-I相是稀有的准分子金属的特例,而高压固体相Ga-II和Ga-III相则具有普通金属的特性。该元素另一个引人关注的要点源于体系液态结构的研究,Ga不但在常压下极易形成过冷液体,且液态密度比稳定的Ga-I密度高3%,表现了反常的熔化曲线。另外,液态Ga曾被报道可能存在液液相变现象,但相关研究仍存在较大争议。尽管对液态Ga进行了大量的实验和理论研究,但是对其特性和结构的认识仍然十分有限,例如,由于液态Ga体系缺乏长程有序,因此高压条件下密度的直接实验测量一直是具有挑战性的工作,有待于进一步的研究。本论文以明晰Ga体系中存在较大争议的特性和结构为目的,研究了液态Ga压力条件下的局域结构、密度和液液相变现象,主要获得了以下结果。(1)采用原位高压同步辐射X射线3D成像技术结合能量色散X射线衍射实验研究了液态和固态Ga在不同温度压力下相对体积变化。由3D成像图样数据重构直接获得了液态Ga的P-V关系曲线,同时利用状态方程拟合得到了300 K-3.02 GPa和330 K-3.63 GPa压缩条件下的等温体积模量分别为23.6(0.5)GPa和24.6(0.4)GPa。并通过反常压缩现象发现液态Ga在330 K-2.44 GPa条件下存在潜在的液液相变。(2)采用原位高压高能X射线散射实验研究了1.9 GPa压力作用下的液态Ga的局域结构,并以测量密度为基本输入参数对体系的双体分布函数进行了逆向蒙特卡罗拟合。实验中所施加的压力区间涵盖了Ga体系Ga-I到Ga-II固态相变的范围。以前的研究根据已知晶体结构对实验获得的PDF数据进行拟合显示在该区域内的液态Ga的局域结构是由Ga-I和Ga-II的局域团簇混合构成的,然而这种拟合方法的准确性一直存在较大的争议。我们的研究显示了与其截然不同的结果,即液态Ga的局域结构与其高压晶体相Ga-II和Ga-III的原子排列较为类似。并根据自由体积理论建立了Ga的熔化模型,指出在体系熔化瞬间Ga-I相迅速打破原有结构进行原子重排,获得了足够的自由体积进而形成液体状态。(3)通过以上实验确定了液态Ga的原子间距统计分布函数和体系密度,结合幂律对体系的分形特征进行了研究。通常唯一的非整数幂指数D_f能够根据幂律表征金属玻璃的分形特征,但是对于同样长程无序的液态Ga体系,应用幂律得到的结果显示体系具有多重幂指数并且其近邻四个配位球的幂指数数值均大于欧几里得维数3,该结果表明应用幂律无法确定液态Ga所具有的唯一的分形维数。事实上,液态Ga的最近邻配位数随压力增加的现象导致其最近邻配位球在不同的压力作用下不存在几何上的自相似性,也就是说液态Ga的分形维数在压力的所用下将产生变化。基于渗逾模型,体系的多重幂指数行为随压力增加被限制在11.65-11.38?的关联长度范围内。超出这个范围的液态Ga体系呈现均匀状态,分形维数D_f=3。(4)利用X射线散射对Ga 330 K压力达3.7 GPa条件下的结构进行了研究,P-T范围涵盖了Ga体系的液态和固态区域。在温度和压力的作用下,液态Ga在3.4-3.7 GPa压力区间晶化,呈现由Ga-II和Ga-III构成的混合孪晶相。该晶化相与相图所显示的Ga-III相存在较大的偏差,表明该温压范围为亚稳区域。根据RMC拟合结果显示Ga熔体、Ga-II和Ga-III存在相似的结构模块,导致液态Ga能够形成Ga-II相和Ga-III相,同时根据经典成核理论可知Ga的亚稳行为与Ga-II和Ga-III可比较的成核几率相关。此外,330 K条件下还发现了较为罕见的2.4-3.4 GPa的液态亚稳区。两次实验中,在相同温压条件不同温压路径下观察到了不同的Ga相,表明Ga体系的液态和固态的亚稳行为与温压路径存在较密切的关系。综上所述,本论文首次通过X射线成像实验提供了直接测量得到的高压下液态Ga的密度,并发现在330 K温度条件下存在潜在压致液液相变;对液态Ga的高压局域结构进行了研究,并根据自由体积理论建立了Ga熔化模型;指出了液态Ga体系不同尺度范围内存在多重幂指数关系;发现了Ga在330 K温度条件下同时存在固态和液态亚稳区。本论文的研究成果进一步促进了Ga高压结构和物理特性的研究,有助于更深刻的了解这个特殊的元素。
[Abstract]:The study of the structure of Ga under extreme conditions is a long-term important base study. As an elementary element with a rich multi-form phase, Ga shows a rare physical characteristic in the form of a special bond in which the covalent bond and the metal bond coexist. The stable Ga-I phase is a special case of the rare excimer metal, and the high-voltage solid phase Ga-II and the Ga-III phase have the characteristics of the common metal. The other interesting point of this element is the study of the liquid structure of the system, Ga not only can form the subcooled liquid at normal pressure, and the liquid density is 3% higher than the stable Ga-I density, and the abnormal melting curve is shown. In addition, liquid phase change may be present in that liquid Ga, but there is still a great deal of dispute in the related study. Although a large number of experimental and theoretical studies have been conducted on liquid Ga, the understanding of its characteristics and structure is still very limited, for example, direct experimental measurements of the density under high pressure conditions have been challenging due to the lack of long-range order of the liquid Ga system, There is a need for further research. In this paper, we study the local structure, density and liquid-liquid phase change of the liquid Ga system under the condition of liquid Ga pressure, and the following results are obtained. (1) The relative volume change of liquid and solid Ga at different temperature was studied by using in-situ high-voltage synchrotron radiation X-ray 3D imaging technique in combination with energy dispersive X-ray diffraction. The P-V relation curve of liquid Ga is obtained directly by 3D imaging pattern data reconstruction, and the isothermal volume modulus of the liquid Ga is 23.6 (0.5) GPa and 24.6 (0.4) GPa, respectively, by using the equation of state to obtain the compression conditions of 300 K-3.02 GPa and 330 K-3.63 GPa, respectively. The liquid phase change of liquid Ga under the condition of 330 K-2.44 GPa was found through abnormal compression. (2) The local structure of liquid Ga under the pressure of 1.9 GPa is studied by in-situ high-energy X-ray scattering experiment, and the two-body distribution function of the system is simulated by using the measurement density as the basic input parameter. The pressure interval applied in the experiment covers the range of the Ga-I to Ga-II solid phase transition in the Ga system. Previous studies are based on the known crystal structure, and the local structure of the liquid Ga displayed in the region is composed of the local clusters of Ga-I and Ga-II, but the accuracy of such a fitting method has always been large. Our studies show a distinct result, that is, the local structure of liquid Ga is similar to that of the high-voltage crystalline phase Ga-II and Ga-III. The melting model of Ga is set up according to the free-volume theory, and it is pointed out that the atomic rearrangement of Ga-I phase quickly breaks the original structure in the melting of the system, and a sufficient free volume is obtained to form a liquid state. (3) The distribution function and the system density of the atomic distance of the liquid Ga are determined by the above experiments, and the fractal characteristics of the system are studied by the power law. the only non-integer power exponent d _ f can be used to characterize the fractal characteristics of the metal glass according to the power law, but for the same long-range disordered liquid ga system, The result of the power law shows that the system has a multiple power index and the power exponent values of the four adjacent ligands are greater than the Euclidean dimension 3. The results show that the power law is unable to determine the unique fractal dimension of the liquid Ga. In fact, the phenomenon that the nearest neighbor coordination number of the liquid Ga increases with the pressure leads to the absence of geometric self-similarity of the nearest neighbor coordination ball under different pressures, that is, the fractal dimension of the liquid Ga will change under the pressure. Based on the percolation model, the multi-exponential behavior of the system is limited to 11.65-11.38 with the pressure increase. Within the range of the associated length. The liquid Ga system which is out of this range presents a uniform state, and the fractal dimension D _ f = 3. (4) The structure of Ga 330 K under the condition of 3.7 GPa is studied by X-ray scattering, and the P-T range covers the liquid and solid state of the Ga system. Under the action of temperature and pressure, the liquid Ga is crystallized in the pressure region of 3.4-3.7 GPa, and the mixed twinning composed of Ga-II and Ga-III is presented. The crystallization phase and the phase diagram show a large deviation from the Ga-III phase, indicating that the temperature and pressure range is the metastable region. The results show that Ga melt, Ga-II and Ga-III have similar structural modules, resulting in the formation of Ga-II phase and Ga-III phase in the liquid Ga. At the same time, according to the classical nucleation theory, the metastable behavior of Ga is related to the nucleation probability of Ga-II and Ga-III. In addition, a rare 2.4-3.4 GPa liquid metastable zone was also found under the condition of 330 K. In the two experiments, different Ga-phases are observed under different temperature and pressure paths under the same temperature and pressure conditions, indicating that the metastable behavior of the liquid and solid state of the Ga system is closely related to the temperature and pressure path. In conclusion, for the first time, the density of liquid Ga under high pressure is directly measured by X-ray imaging experiment, and the liquid phase change of potential pressure is found at the temperature of 330 K. The high-voltage local structure of liquid Ga is studied. In this paper, a Ga melting model is established according to the free volume theory, and the relation of the multiple power index in the different scale range of the liquid Ga system is pointed out, and the solid state and the liquid metastable zone exist at the same time of the Ga at the temperature of 330 K. The research results of this paper further promote the study of the high-voltage structure and physical characteristics of Ga, and help to understand this particular element more deeply.
【学位授予单位】：哈尔滨工业大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TG146.43

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