Theoretical Studies On The Phase Stabilities And Phase Transitions In Functional Crystal Materials

Difference in relative phase stabilities of polymorphs can lead to phase transitions in functional crystal materials, affecting the physical properties of materials. Therefore, the study of phase stabilities and phase transitions is of great importance from the point views of both basic science and practical application. However, the problem of phase stabilities and phase transitions in functional crystals remains one of the most challenging topics at the interface between condensed mater physics and chemistry. In this area, the problems of the linkage type between polyhedra in multi-element composite functional crystals and the material behavior in extreme conditions are especially noteworthy. Through the investigation of polyhedyal linkage type, the essential origin of the phase stability can be clarified. Based on the study of material behavior in extreme conditions, the process and mechanism of phase transitions can be explained. In this dissertation, in terms of these two key problems, we study the phase stabilities and phase transitions of several important functional crystals through molecular dynamics, lattice dynamics and electronic properties based on density functional theory and density functional perturbation theory. The main research contents and results are as follows:The first chapter introduces the significance of the phase stabilities and the phase transition theory on the mechanism and transformation path, and reviews their recent research progresses. Finally, we put forward our scientific problems:the effect of polyhedral linkage type on phase stabilities and the behavior of materials in extreme conditions. On this basis, we briefly introduce the research ideas and constants of this dissertation.Chapter 2 briefly introduces the basic theoretical method employed in the thesis. Density functional theory does not only provides a method simplifying a multi-electron system into a single-electron system, but also gives approximations for exchange-correlation functional, such as LDA, GGA and so on. We also introduce the computing method of lattice dynamics-density functional perturbation theory (DFPT), as well as the computational software packages we employed in present resarch.In Chapter 3, we apply the theoretical methods to real material systems and begin to investigate the effect of polyhedral linkage type on phase stabilities of materials at ambient pressure. In general, the presence of shared edges of polyhedra for high-valence low coordinated small cations is rarely seen except under extreme conditions such as high pressure. However, the ambient pressure synthesis of the KZnB3O6 built of edge-sharing BO4 tetrahedra is contrary to this. Based on the experimental structural data of KZnB3O6 and the analogous evolution of KCdB3O6, we build the structures for both edge-sharing (es) and "corner-sharing (cs)" KZnB3O6, in order to make a comparison between their phase stabilities and seek the phase stability origin of es-KZnB3O6. Molecular dynamic simulations based on DFT show that from 100K to 1000K, the es-KZnB3O6 is stable enough to be preserved, while the "cs-KZnB3O6 deforms with bond-stretching, indicating the instability in ";cs-KZnB3O6". From the analysis of lattice dynamics, we infer that the vibration modes of edge-sharing BO4 tetrahedra are dynamically stable and all modes have real frequencies in es-KZnB3O6. In the case of "cs-KZnB3O6", a soft phonon mode at the G point (Bu) with an imaginary frequency (11.7i cm-1) is observed through both two different computing methods. The eigenvector analysis of the soft mode shows that the linkage of ZnO5 polyhedra is dynamically unstable. Adjacent ZnO5 polyhedra move in opposite directions when relaxing along the eigenvectors of the soft mode. It results in the breakage of the longest Zn-O bonds and finally the separation of the two ZnO5 polyhedra. The electronic structures of es-KZnB3O6 and "cs-KZnB3O6 are quite different due to the different coordination of B and Zn atoms. In a BO4 polyhedron, one B atom binds to four O atoms with four stable a bonds, whose states form from the B sp(?)-hybridized states and O 2p states. The longest B-O σ bonds which connect the edge-sharing BO4 polyhedra are stable enough to provide a solid framework for es-KZnB3O6. In the case of "cs-KZnB3O6, two edge-sharing ZnO5 polyhedra and five BO3 triangles are connected to the same Zn atom, and they repel each other due to the steric hindrance and Coulomb repulsion effect, resulting in the stretching of the Zn-O5 bond. The longest Zn-O bond possesses the smallest covalent nature and the least orbital overlap in a ZnO5 polyhedron, indicating the instability in it. It is exactly this overlong Zn-O bonds that cause the decrease of force constant, leading to the soft mode of ZnO5 polyhedra in "cs-KZnB3O6". That is, the structure instability of "cs-KZnB3O6" comes from the linkage type of the ZnO5 polyhedra rather than from the BOx polyhedra, which is consistent with the MD analysis. The results of this study strongly support the clarification of the structural stability origination of KZnB3O6, indicating that the approaches of lattice dynamics and electronic properties analysis for polyhedra networks are valid. As supplements to the Pauling’s rules, these approaches will help design new structures of borate materials.In Chapter 4, we turn our attention to the material behavior at high pressure, where the pathways of pressure-induced structural transformation from four-coordinated analogous zinc-blend (a-ZB) and analogous wurtzite (a-WZ) to six-coordinated rocksalt (RS) LiInSe2 phase were investigated. As one of the Ⅰ-Ⅲ-Ⅵ2type compounds, LiInSe2 crystal has great potential application in nonlinear optics. Research on its phase transformations under extreme conditions will help to analyze its service condition. In this chapter, we study the process of a-ZB/RS and a-WZ/RS phase transformations in LiInSe2 together with their critical transition pressure, transformation pathways and barriers via density functional theory. Molecular dynamics (MD) and constant-pressure geometry optimization show that both a-ZB and a-WZ structures transform into an analogous RS (a-RS) phase with a space group of I41amd. From the variation of cell shape, atomic position and bond angle during the transformation process, we can infer that for both a-ZB/RS and a-WZ/RS transformations the anion moves directly from the tetrahedral center to the octahedral center, in which way the transition structures are of orthorhombic symmetry. Further study gives the critical transition pressures as 4.12 GPa and 4.31 GPa for a-ZB/a-RS and a-WZ/a-RS respectively, which are consistent with experimental data. At the critical transition pressure, the barriers of orthorhombic pathway for a-WZ/a-RS and a-WZ/a-RS transformation are 0.30 eV and 0.32 eV respectively. The latter is the lowest among three proposed paths, confirming the conclusion drawn from MD and constant-pressure geometry optimization. For both transformations, two a-RS phases with different cation arrangements caused by different displacement directions form simultaneously and intertwine in the crystal, leading to the final RS phase with disordered cation arrangement. In the reverse transformation, the barrier of a-RS/a-ZB is lower than that of a-RS/a-WZ, explaining why annealing of RS phase below 1 GPa results in a transformation to the metastable a-ZB rather than the stable a-WZ phase.In Chapter 5, we focus on the influence mechanism of pressure on the phase transition. The transition from rutile-to CaCl2-type SnO2 phase is one of the most common transition behaviors in rutile-type AB2 compounds. Experiments showed that the transition is caused by an optical and an acoustic soft mode, but the coupling mechanism between these two modes are still not clear. In this chapter, the phase transition mechanism is investigated by density functional theory combined with Landau theory of phase transition. From the frequency calculation, we determine the phase transition pressure as 8.2 GPa, which agrees well with the experiment. According to the Curie principle and the analysis of O atom position as a function of pressure, we confirmed the displacement of O atoms along the direction of soft mode eigenvector rather than the octahedral rotation as the order parameter Q. The softening mechanism of the Big mode and TA mode together with the coupling mechanism between them are clarified. It is found that with the increase of pressure and the decrease in volume, the Sn-O-Sn bending caused by Big mode effectively reduces the excess energy increase caused by bond shortening, whereas always causes the SnO6 octahedral distortion however. During the phase transition process, the other structural change-lattice strain ε, which is induced by the soft TA mode, minimizes the octahedral distortion. The coupling term still effectively lowers the whole energy even considering the increase of the lattice distortion term. The soft TA mode interacts with the soft Big mode to drive the phase transition and further increase the stability of system.In chapter 6, we summarize the conclusions and innovative points of this dissertation, and preview the further studies.