The First Principle Studies On The Phase Transformation Of Transition Metal Compounds

Transition metal compounds have attracted much attention because of their outstanding physical chemical properties. There are two kinds of transition metal compound materials which are researching widely. Bulk materials and two-dimensional materials. Bulk materials have prefer mechanical properties and are applied to mechanical field, such as metallurgy. IVB transition metal nitrides have high electrical conductivity, large phononic band gap, small real permittivity, high abrasion corrosion resistance and thermal stability. These excellent properties make them very promising candidates for use as dyesenditized solar cells, alternative plasmonic materials, decorative coatings as well as light-emitting-diode devices. In addition, two-dimensional transition metal dichalcogenides have attracted much attention too. The preparation of single layer transition metal dichalcogenides with layered structure have been implemented in experiment. Subsequently, structural transition,electronic properties and optical properties of two-dimension transition metal dichalcogenides were systematically investigated. To date, these materials have been responsible for the designing of microelectronic devices.The density functional theory calculations were performed to study the phase transition of Hf1-x N and the transition pathway, potential barrier for single-layer MTe2(M=Mo and W) with different phases. By calculating the formation energy, density of states, electronic energy band structure and charge distribution, the structure stability and electronic properties were analyzed.1. We report investigation of the structural phase transition and electronic properties of Hf1-x N(0≤x≤0.25) using the first principles calculations. When the ratio of Hf to N is 1:1, the stable structure is rocksalt structure. When the ratio of Hf to N is 3:4, the stable phase is cubic structure reported by experiments. But at the ambient, the orthorhombic structure is the stable predicted by first principles calculations. We build a serials of Hf1-x N structure models by importing Hf vacancies. We compared the phase stability by calculating formation energy. The defective rocksalt structure with Hf vacancies is found to be stable over a large phase region. The formation energy of Hf3N4 with orthorhombic structure increased dramatically when the stoichiometric ratio of Hf to N deviates from 3:4. The electronic and optical properties of Hf1-x N can be control by varying the concentration of Hf vacancies. The full depletion of excess free electrons from Hf atoms results in the structural phase transition of Hf3N4.2. Based on the density functional theory calculations, we studied the pathway and the energy barrier of phase transition between 2H and 1T’ for Mo Te2 and WTe2 monolayers. We suggested that the phase transition is controlled by the movement of metal atoms and Te atoms in their plane without the intermediate phase 1T. The energy barrier is not so high that the phase transition is dynamically possible. The barrier is less than 0.9e V per formula cell. In addition, the relative stability of both 2H and 1T’ phases and the energy barrier is decreased by applying the strain. The phase transition between 2H and 1T’ controlled by the strain can be used to modulate the electronic properties of Mo Te2 and WTe2.