【作者】 余飞；

【导师】 孙久勋；

【作者基本信息】 电子科技大学 ， 凝聚态物理， 2011， 博士

【摘要】 随着高新技术的不断发展和进步,出现了许多新的功能材料,对这些新型功能材料的研究受到了广泛的关注。压力作为一个重要的热力学参数,对于调节材料的物理性能能起到重要的作用。基于此,研究压力对新型功能材料结构和物理性质的影响对于我们更好地了解和改善材料的性能以及设计和合成具有良好应用前景的新材料都将起到重要的作用。本文运用第一性原理计算方法对几种新型功能材料的高压性质进行了研究。全文共分为八章。第一章介绍了对新型功能材料高压性质进行研究的研究背景和意义;回顾了本文所关注的几种新型功能材料的研究现状。第二章首先简单介绍了采用赝势平面波表述的第一性原理计算方法的基本原理。其次,对相变基本理论进行了介绍,其中重点关注的是相变的分类、压致相变的特点和高压相的理论预测方法。第三章对NaBH₄在高压下的结构相变进行了研究。本文的计算首次从理论上证实了NaBH₄高压相的BaSO₄型结构,并正确的预测到了实验上观察到的从β-NaBH₄(P42_1c)到γ-NaBH₄（BaSO₄-type, Pnma）的相变,计算得到的相变压力（9.66 GPa）与Kumar等人通过X射线衍射实验得到的结果（8.9 GPa）符合较好。最后,我们的研究还表明常温下实验上发现的从β-NaBH₄到γ-NaBH₄的高压相变在低温下也能够完成,从而在一定程度上解决了以往理论计算和实验结果间的分歧。第四章研究了Mg₂X（X=C, Si, Ge, Sn）在高压下的结构相变、电子结构和光学性质。对结构相变的研究表明,在压力作用下Mg₂X（X=C, Si, Ge, Sn）将经历从反萤石结构到反氯化铅结构,再到Ni₂In型结构的两次结构相变。对Mg₂C而言,从反萤石结构到反氯化铅结构的相变是一级相变,而从反氯化铅结构到Ni₂In型结构的相变没有发现体积突变是二级相变。对Mg₂Si,Mg₂Ge和Mg₂Sn来说,两次高压结构相变都是一级相变。四种材料在由反氯化铅结构转变到Ni₂In型结构的相变过程中,反氯化铅结构Mg₂X（X=C, Si, Ge, Sn）的晶格常数的变化表现出了明显的非线性特征,这种现象可以认为是相变的前导。对电子结构的研究表明,Mg₂C的带隙宽度会随压力的增加而增加,与之相反Mg₂Si,Mg₂Ge和Mg₂Sn的带隙宽度会随压力的增加而减小,后三种材料在高压下表现出了金属的性质。对光学性质的研究表明,四种材料的光学性质会随压力的增加而急剧变化。对Mg₂C而言,压力作用下介电函数虚部ε2的吸收峰会随着带隙宽度的增加而往高能级方向移动。而对Mg₂Si,Mg₂Ge和Mg₂Sn来说,在高压下ε2的谱线中出现了许多新的吸收峰。第五章研究了压力对CaMgX（X=Si, Ge, Sn）结构稳定性和电子结构的影响。通过理论计算我们成功地预测到了三种材料在压力作用下从pnma结构到Ni₂In型结构的连续相变。对电子结构的研究表明,常压下pnma结构的CaMgSi和CaMgGe表现出半金属的性质;而pnma结构的CaMgSn则表现出了金属的性质。压力的作用将导致CaMgSi和CaMgGe发生从半金属到金属的电子结构的转变。而CaMgSn的电子结构对压力不敏感,其主要变化是能带宽度随压力的增大不断增加。第六章从理论上对实验上观察到的TiS₂高压相的结构作出了预测。结果表明,TiS₂将经历一次从1T结构到氯化铅结构的一级压致相变。计算得到的相变压力为16.20 GPa,与实验上测得的20.7 GPa的相变压力符合较好。与常压下的1T结构相比,高压下氯化铅结构的TiS₂有更紧凑的结构和更大的体变模量。另外,我们还对TiS₂的电子结构进行了研究。结果表明从1T结构到氯化铅结构的压致相变伴随着从半金属到金属的电子结构的转变。第七章对晶化BeF2的高压相变进行了研究,发现在50 GPa的压力范围内,BeF₂将先后经历从α-石英型结构到柯石英型结构,再到金红石型结构,最后到α-PbO2型结构的三次结构相变;对各种结构的BeF₂的电子结构进行了比较,发现其电子结构对特定的晶体类型并不敏感,其电子结构主要由晶体中的BeF₄四面体结构（或BeF₆八面体结构）决定。第八章是全文研究工作的总结和对下一步研究工作的展望。更多还原

【Abstract】 With the development of science and technology, many new functional materials emerged. For its extensive use, new functional materials have attracted intensively attention in recent years. It is well known that pressure is an important parameter to tune physical properties. High-pressure research on new functional materials is now helping us to better understand and improve the physical properties of materials and providing useful information in the design and synthesis of new materials. In the present study, high-pressure properties of several new functional materials are investigated by using first-principles calculation method. The whole thesis is divided into eight chapters.In chapter 1, the background and significance of the high-pressure research on new functional materials are briefly introduced, and then the recent research progresses of several new functional materials studied in this thesis are reviewed.In chapter 2, a brief introduction to the first-principles calculation methods based on plane wave functions and pseudopotential is given firstly. Then the phase transition theory are introduced, such as the classification of phase transition, the features of pressure-induced phase transition and the methods for crystal structure prediction of high-pressure phases.In chapter 3, the pressure induced structural transition of NaBH₄ fromβ-NaBH₄ （P421c） toγ-NaBH₄ （BaSO₄-type, Pnma） is investigated. The BaSO₄-type structure of high-pressure phase is testified theoretically for the first time. The calculated transition pressure is 9.66 GPa and agrees reasonably well with the experimental results （⁸.9 GPa）. Our results correctly predict the experimental observed phase transition fromβ-NaBH₄ toγ-NaBH₄ and demonstrate that this high-pressure transition may occur at low temperature. The poor agreement between previous theoretically prediction and experimental results have been settled in a certain extend.In chapter 4, the phase transitions, electronic structures and optical properties of Mg₂X （X=C, Si, Ge, Sn） under high pressure are investigated. The calculated results demonstrate that Mg₂X （X=C, Si, Ge, Sn） undergo two pressure-induced phase transitions from the anti-fluorite to anti-cotunnite and then from the anti-cotunnite to the Ni₂In-type structures. For Mg₂Si, Mg₂Ge and Mg₂Sn, the two high-pressure phase transitions are first-order. While, for Mg₂C, the previous phase transition is first-order and the later is second-order. When approaching the phase transition, the changes of lattice parameters of the anti-cotunnite Mg₂X （X=C, Si, Ge, Sn） show noticeable nonlinearities. This can be considered as a precursor of the phase transition. The electronic structure calculations show that the band gaps of Mg₂C become broader with the increase of the pressure. But for Mg₂Si, Mg₂Ge and Mg₂Sn, the reverse is true. The results show that they have become metallic at high pressure. Finally, the imaginary and real parts of the dielectric function for different structures Mg₂X （X=C, Si, Ge, Sn） are calculated. The results show that the optical properties of Mg₂X （X=C, Si, Ge, Sn） change drastically with increasing pressure.In chapter 5, the pressure effects on the structural stabilities and electronic properties of CaMgX （X=Si, Ge, Sn） are discussed. Our results successfully predict a continuous phase transition from pnma to Ni₂In-type structure for CaMgX （X=Si, Ge, Sn）. In addition, we discuss the electronic structures of both the pnma and Ni₂In-type CaMgX （X=Si, Ge, Sn）. At ambient pressure, the pnma structure CaMgSi and CaMgGe display semimetal behaviors and the pnma structure CaMgSn displays metal behaviors. At high pressure, a semimetal to metal electronic transition is found for CaMgSi and CaMgGe. While, for CaMgSn, the electronic structures are found to be quite insensitive to pressure, the significant changes are the bands become broader at high pressure.In chapter 6, a theoretical investigation on the structural stabilities and electronic properties of TiS₂ under high pressures has been performed. The results show that TiS₂ undergoes a first-order pressure-induced phase transition from its 1T-type structure to cotunnite-type structure. The calculated transition pressure 16.20 GPa agrees quite well with the experimental finding （20.7 GPa）. Compared with 1T-type structure, the cotunnite-type high-pressure phase has a more compact structure with a large bulk modulus. In addition, we discussed the electronic structures of TiS₂. Our results suggest that the structural phase transition of TiS₂ from 1T-type to cotunnite-type structure at high pressure is followed by a semimetal to metal electronic transition. In chapter 7, high-pressure behaviors of BeF₂ are investigated theoretically. The results demonstrate that the sequence of the pressure-induced phase transitions of BeF₂ under 50 GPa is from theα-quartz, to coesite, rutile, andα-PbO2-type structures. Moreover, the electronic properties of different crystal structures BeF₂ are compared. The results show that the electronic structures of BeF₂ are fairly insensitive to the particular crystal structures, which determined mainly by the BeF4 tetrahedron （or BeF6 octahedra）.In chapter 8, the contents of this dissertation are summarized and future directions of research are given.更多还原