N.A. Lvova

Quantum chemical simulations of water adsorption on a diamond (100) surface with vacancy defects

Results from quantum chemical calculations of the structural, electronic, and energy characteristics of the chemisorption of water on a diamond C(100)-(2 × 1) surface with a vacancy defect are presented. The metastable state of the surface with an adsorbed H2O molecule and possible configurations of the surface with adsorbed -H and -OH water dissociation fragments are described. It is shown that the presence of a vacancy on the surface decreases the activation energy of the dissociative adsorption of a water molecule.

Quantum-Chemical Simulation of Interaction of Hydrogen with Diamond Nanoclusters

The quantum chemical calculations of vacancy defect stable configurations on the diamond surface C (100) –(2×1) are represented, differing in the geometry, electronic states and formation energy. The activation energies of chemisorption and C-H bond energy in mono- and dihydride states are defined. The possible desorption mechanisms of hydrogen in the defect area, depending on the surface hydrogenation level, are proposed. We found that depending on the defect state type, desorption mechanism, and surface coverage by hydrogen, the activation energy of molecular hydrogen desorption from the (100) diamond surface containing a vacancy defect has the value of ED = 1.9–5.2 eV.

Boron atoms in the subsurface layers of diamond: Quantum chemical modeling

Results from quantum-chemical modeling of the configurations of boron impurities and BV complexes of “boron + monovacancy” on diamond surface C(100)–(2 × 1) are presented with their positions varied in subsurface layers. The geometric, electronic, and energy characteristics of these configurations are calculated. It is shown that the most stable BV complexes are complex defects consisting of an impurity defect in the fourth layer and an intrinsic defect in the third layer. The bonding energy of a hydrogen atom and a surface containing the most stable of the studied defects is estimated.

Cluster structure and elastic properties of superhard and ultrahard fullerites

Velocities of the longitudinal and shear sound waves are measured in ultrahard fullerites created by static high-pressure-high-temperature treatment under P=13 GPa and T=1670–1870 K. The highest value of 26.0 km/s for the longitudinal waves is measured, that is about 40% more than in diamond. Bulk modulus of different ultrahard fullerites covers the range of about 600–1700 GPa. The highest hardness is about 30×103 kg/mm3. We ascribe these unique properties to formation of 20–30 atoms clusters by the walls of adjacent molecules under process of 3D-cross-linking. Most distinctly these clusters declare themselves in the cubic structure with the lattice parameter about 6 A and 32 atoms per unit cell.

Hemisorption of hydrogen on the diamond surface containing a "boron + vacancy" defect

Boron-doped diamond is a p-type semiconductor and due to its unique natural properties, it is a promising material for microelectronics [1]. Goss and coworkers studied boron aggregates in diamond [2]. According to ab initio calculations, the B4Vand BV-complexes correspond to the lowest energy of formation attributed to a single B atom. In [3], the authors study the electronic structure of a BV-center in diamond for its various charge states – BV, BV, BV−1 and BV−2 – using first-principles calculations. The charge state of the BV−1 complex defect is stable and is suitable for qubit realization. However, for the reconstructed C(100)–(2 × 1) surface, the BV-complexes remain insufficiently studied. Such a complex defect located on the surface or in the near-surface layers can lead to local surface restructuring, to the charge distribution between the atoms around the defect, and can influence the energy of particle adsorption. Atomic hydrogen takes part in the growth mechanisms of CVD diamonds [4], in diamond etching for use in microelectronics [5]. In this paper, we investigate the energy characteristics of hydrogen adsorption on a C(100)–(2 × 1) diamond surface with BV-complexes by quantum-chemistry methods.

The divacancy V2 and V - C = C - V configurations on the diamond surface: quantum-chemical simulation

diamond surface. We provide calculations for the geometric, electronic, and energy characteristics for these configurations. Energy characteristics of water and hydrogen molecule adsorption on the surface with divacancy defects are estimated. The presence of V2 and V ‐ C = C ‐ V divacancy defects are shown to change the mechanism and energy characteristics of molecular adsorption.

Structure and properties of metastable phases of m-Sb2Te3and m-Bi0.4Sb1.6Te3

The m-Sb2Te3 and m-Bi0.4Sb1.6Te3 metastable phases were found after high-pressure (4 GPa) and high-temperature (873 K) treatment of initial rhombohedral Sb2Te3 and Bi0.4Sb1.6Te3. These metastable phases crystallize in the same structure because they have almost identical diffraction pattern. The crystal structure of metastable phases, determined by the powder X-ray and electron diffraction methods, is monoclinic (C2/m). The cell dimensions of m-Sb2Te3 are: a=15.64(8) Å, b=4.282(8) Å, c=9.38(2) Å, β=89.70°(5). The reliability factors are: RBragg=0.12, RF=0.13, χ2=4.35. There are two different types of Sb atoms: with seven-coordinated by Te atoms for Sb1 and for Sb2 – eight-coordinated by Te atoms forming composite coordination polyhedra. A comparison with the structure of pressure-induced β-Sb2Te3-phase, observed in situ under high pressure, has been made. Pressure-induced β-Sb2Te3phase can be retained at ambient conditions as m-Sb2Te3. The annealing of m-Sb2Te3 and m-Bi0.4Sb1.6Te3 samples at 673 K during 2, 5 hours returns their structures to initial symmetry. This fact was supported by the exothermal peak found by differential scanning calorimetry. The ab initio study verified metallic character of quenched phases: the energy spectrum is consistent with the proposed monoclinic structure with short interlayer distances. The electrical resistivity and the Hall coefficient in the temperature range of T = 1.8−450 K have been measured. m-Sb2Te3 phase is superconductive at T < 2K.