Defect And External Pressure Effects On The Physical Properties Of Some Typical Materials

Booming advances of computational technology make it possible to explore physical properties of materials, even design new materials. It is of vast importance to know deep into the composition, interactions and response to environment of a material via numerical calculations, since many body problems are impossible to be solved analytically. This paper systematically studied problems in solid state matter in three aspects using first principles method: the behavior of H and He in niobium, the existence of BCN ternary compounds and elastic stability of crystals.The problem of impurities in metals is continuous attractive, because not only the improvements on mechanical and electrical properties of a metal, but also the failures or malfunctions, especially for light elements as hydrogen and helium. Embrittlement and excessive ion implantation in metals will make the first wall of nuclear reactor shorten its mechanical lifetime. Recently, there is a trend to use hydrogen as a medium to make the full use of reproducible energy resources applicable. All these promote the researches on the behavior of hydrogen/helium in metals. On the other hand, thermonuclear reactors promote both the behavior of H and He in metals.The typical transition metal niobium is selected as the host. Different density of hydrogen is doped into Nb till niobium monohydride is formed. The interactions between Nb and H are studied, focusing mainly on the phase of NbH, the quantium behavior of H in Nb and the thermodynamic properties of both H and He doped Nb.The result shows that hydrogen has less effect on host lattice even for 1:1 dopant, because of the strong nd metallic bonding of transition metals. Since the mass of H is by far smaller than Nb, there exhibit considerable quantum effects. H is easier to be excited up, hovering in Nb lattices.Four types of NbH derived fromβ-phase is studied and all structures are shown to be dynamically stable.β-phaseis the lowest in total enthalpy, next D2d-1, D2d-9 and D4h-7. The electronic structure (DOS) of any structure is not in consistent with experimental EDC data. The DOS of NbH governed not only by nearest H-H distance, but also the local geometry. This implies that NbH may be the mixture of the four structure phase.Conventional cell of Nb₂H is used to calculate the potential experienced by H in Nb by directly evaluate the total energy at different H configurations. Schrodinger equation of H is solved numerically to obtain the energy levels and eigenstates of H, the 4T and 6T ring excited states are identified. The low density impure Nb lattice of 1/16 H dopant under pressures are studied, the lattice relaxation, electronic structure and optical absorption spectra are obtained.The behavior of helium in niobium is simulated using Car-Parrinello molecular dynamics methods. Two types of local He concentration are considered. The result shows that He behaves vast electronic stable such that the space occupied by He is a forbidden area for near free electrons of Nb. The existence of denser He damages the metallic bonding of Nb, producing vacancy/interstitial defects. Under ambient pressure, helium bubble formation depends on the local concentration and the existence of native vacancy. In perfect host lattice, high He content results in dislocations of neighboring Nb, lattice distortion and production of vacancy/interstitial of Nb and the bubble formation at the vacancy; a helium bubble will form direct at a nearer native vacancy with no new vacancy produced and less host lattice distortion. Under high pressures, no bubbles are formed due to the strong background interactions between Nb atoms and the relatively weakened He effects.Another type of impure systemβ-C₃N₄ is also studied. Before constructiong impure system, cubic C₃N₄ andβ-C₃N₄ are pre-studied. Cubic phase C₃N₄ is shown to be conductive, whileβ-C₃N₄ is semiconductive with a band gap of about 3.4eV(LDA). The chemical potential of cubic C₃N₄ relative to raw materials is by far larger than that ofβ-C₃N₄, resulting in the difficulty in the synthesis of cubic C₃N₄, although it may be the only one expected to be harder than diamond. The two structures ofβ-C₃N₄ suggested by Cohen's group and Hemley's Group are different in symmetry but similar in electronic structure.Defected system ofβ-C₃N₄ including host vacancy, inter-substitution, native interstitial and group III metallic interstitial are studied. The interstitial gallium is found to have the most doping effect on the electronic structure and optical response ofβ-C₃N₄. Three deep impurity energy levels are observed in the main gap, with one of which stands on the Fermi Level. The impurity induced electronic bands enables electron transitions in almost all impurity band transition models. The doped system is active to electromagnetic field from far infrared to ultraviolet, with a reflect window of about 0.2eV at frequency 1.5eV. The doped system may be a potential optic material with high optical efficiency.Thirdly, the existence, stability and the probability to be synthesized of zinc-blende crystal are studied using constant volume enthalpy, in addition to total energy, atomic forces and lattice vibration calculations.Up to date, the probability to obtain a new material depends mainly on the optimized structure (atom coordination and lattice parameters), the relative enthalpy and lattice vibration properties for stability investigation. But not all materials predicted by theory are successfully synthesized, although they are confirmed to be stable and probable to be synthesized at certain conditions by the methods above mentioned. On the other hand, considerable phase synthesized experimentally are not well interpreted by theory.In the present work we have gone further. The properties of a superhard material (B₃CN₄) are studied using ab initio pseudopotential methods within density functional framework. In addition to total energy, electronic structure and lattice dynamics, the normal vibration mode dependency of constant volume enthalpy (CVE) is systematically investigated in a wide pressure range up to about 200GPa. The new compound is shown to be conductive and comparable to BC₂N and cBN in basic mechanical properties, but dynamically stable in a narrow range from～80GPa to～120GP. The instability is confirmed by the acoustic mode softening around q-vector [0.5 0.5 0.0] and monotonic variation of CVE at normal vibration mode A₁. The lattice vibration mode dependence of CVE is expected to filter out the most stable structure of a material from the predictions based on total energy, atomic forces and lattice vibration calculations. The CVEs of kinds of zinc-blende materials such as diamond, cBN, ZnS etc. are systematically investigated and A₁ mode is found to play the dominant role in the crystallization. But for BC₂N-1 and BC₂N-2, negative results are obtained, showing that they are not stable at ambient pressures and BC₂N may not be described by 8-atom cell. This scheme is expected to clarify the most stable phase from those predicted by ab initio total energy, lattice vibration calculations.After the stable pressure range of B₃CN₄, impurity effects on its electronic structure are studied. Vacancy of C, C substuted by N, B substituted by Ga and interstitial Ga are all calculated though. The band distortion is strongly C content dependent, reflected in the optical response spectra. While C content goes down to 1/64, almost no optical response below ⁵eV is observed, while any of the other impurity systems has strong low frequency reflection/absorption just as metals. The interstitial dopant of Ga is found to produce deep impurity bands in the main gap, resulting in the blue-shift of metallic reflection edge.Additionally, the lattice dynamics and elastic properties of typical elastic crystal are studied in order to filter the abinitio calculated elastic constants and obtain the information of lattice stability. Since more than 3 independent elastic constants are needed to describe elastic crystals except cubic ones, it is difficult to confirm if the elastic constants are reasonable, or if the calculated crystal is stable. This may be handled out by solving eigenvalue problems of Christoffel dynamics matrices in a unit reciprocal sphere. A new method of calculating elastic constants is also proposed.