Effect of Point Defects on Electronic Properties and Structure of Talc (001) Surface by First Principles

The surface structure and electronic properties of Mg vacancy defects on talc (001) and impurity defects with Fe, Mn, Ni, Al, and Ca replacing Mg atoms were calculated by using density functional theory. The calculation results show that the order of impurity substitution energy is Mn < Ni < Al < Ca < Fe. This indicates that Fe impurity defects are most easily formed in talc crystals. The covalent bonding between Si atoms and reactive oxygen atoms adjacent to impurity atoms is weakened and the ionic property is enhanced. The addition of Fe, Mn, and Ni atoms makes the surface of talc change from an insulator to a semiconductor and enhances its electrical conductivity. The analysis of electron state density shows that surface states composed of impurity atoms 4S orbital appear near the Fermi level.

First-principles study of the structure, elastic constants, electronic and thermodynamic properties of C15 laves phase ZrMo2

The structure, elastic constants and electron state density of ZrMo2 in [Formula: see text]15 phase are investigated by pseudopotential plane-wave method based on density functional theory (DFT). The thermodynamic properties are studied with the quasi-harmonic Debye model. The calculated results are in good agreement with the previous experimental results and theoretical simulations. The calculated phonon spectra and elastic constants show that [Formula: see text]15 phase of ZrMo2 is mechanically stable. Through the analysis of [Formula: see text]/[Formula: see text] value and Poisson’s ratio, [Formula: see text]15 phase of ZrMo2 shows ductility at 0–150 GPa, and it increases with the increment of pressure. We further explore the mechanism of the metallic properties by analyzing the electronic density of states. In addition, Debye temperature, thermal expansion coefficient and heat capacity as a function of pressure and temperature is discussed, respectively.

Electrical conductivity of YBa2Cu3O7-δ single crystals under conditions of anionic ordering in Cu(1)O1-δ layers

The influence of thermocycling annealing processes on the oxygen ordering degree (order parameter) in YBa2Cu3O7-δ single crystals has been studied. It has been shown that an increase in the critical onset temperature of the transition to the superconducting state during thermocycling annealing procedures is consistent with the decrease of the σс/σаb parameter. This fact indicates a redistribution of the electronic density between the structurally inhomogeneous Cu(2)O2 and Cu(1)O1-δ planes, due to the formation of oxygen long-range order in the O(4)–Cu(1)–O(4) linear groups along the (b) crystal structure axis of the unit cell, and elimination of oxygen defects in the square nets of the Cu(2)O2 planes. The existence of the critical value of the conductivity anisotropy σс/σаb, below which its behavior does not correlate with the change of Тс, has been confirmed. In this case an increase in Тс and orthorhombic distortion of the crystal structure during isothermal annealing are caused by the amplification of the “interlayer” interaction between the Cu(2)О2 and Cu(1)О1-δ planes. As a result, the contribution of the Cu(1)О1-δ chain layers to the electron state density at the Fermi level increases. These layers can acquire superconducting properties due to tunneling of Cooper pairs from the Cu(2)О2 planes resulting in the formation of the induced superconductivity in these planes.

Особенности формирования электронной структуры при синтезе соединений Ti-=SUB=-2-=/SUB=-AlC, Ti-=SUB=-2-=/SUB=-AlN, Ti-=SUB=-2-=/SUB=-SiC и Ti-=SUB=-2-=/SUB=-SiN

The electronic structure and total energy of the Ti2AlC, Ti2AlN, Ti2SiC, and Ti2SiN compounds are investigated by methods of the density functional theory and pseudopotentials. Electron state density curves have been constructed for crystal systems and for systems differing in order. It has been shown that even in completely disordered systems there is a qualitative similarity of the electronic structure with an electronic structure of the respective crystalline compounds. This similarity is further enhanced as the degree of ordering increases. The total energy of the studied systems grows with increasing disorderance in about the same way for all studied systems except Ti2SiC. In the latter case, it turns out to be much more sensitive to the degree of disordering, which seems to be due to the greater role of the covalent component of interatomic bonds.