Modeling of nonlinear processes of nucleation and growth of GaSxSe1–x(0 ≤ х≤ 1) solid solutions

The results on the study of modeling and physico-chemical study of the kinetics ofnucleation and growth of GaSxSe1–x(0 ≤ х≤ 1) solid solution. The nucleation heterogeneous process and growth of GaSxSe1–xcrystals have been studied and simulated taking into account nonlinear equations considering the kinetic behavior of crystallizing phases.GaSxSe1–xsingle and nanocrystals were grown from solution, melt, and by chemical transport reaction through steam. GaSxSe1–xcrystals were grownbychemical transport reaction in a two-tem-perature gradient furnace in a sealed quartz ampoule. Iodine was used as a transporting additive. Using the Fokker–Planck equation, the evolution of the distribution function of crystals of solid solutions of the GaS–GaSe system by size at the nucleation time is studied by a numerical method. For the convenience of comparing theory with experimental data, we used the GaS1–xSex(x= 0.7 molar fraction of GaSe) composition of the solid solution. The Monte Carlo method is used to approximate the time evolution of the nucleation of two types of particles for the GaS0.3Se0.7 solid solution, simulated by a constant nucleus size. The results of modeling non-linear crystallization processes are consistent with experimental data.

NEW POSSIBILITIES OF LOW-TEMPERATURE GENERATION OF ACTIVE CENTERS DURING HETEROGENEOUS-HOMOGENEOUS ACTIVATION OF PROPANE ON SILICA GEL SURFACE, MODIFIED WITH ZnO BY THE SOL-GEL AND CHEICAL TRANSPORT REACTION METHODS

By the example of heterogeneous catalytic oxidation of propane the process of peroxide radicals transfer from the surface of catalysts into the reactor volume is studied. For comparison ZnO/SiO2 contacts obtained by depositing the active phase on the silica surface by sol-gel and chemical transport reaction (CTR) were used. It was established that for the case both the temperature of the radicals transfer from the surface to the volume and the value of the activation energy are lower than in the case. It was shown that the modification of the silica gel surface with zinc oxide by the CTR method is more effective as compared with the sol-gel method. It is established that in both cases the active phase of the catalyst exhibits paramagnetic properties. On the basis of the given, semi-artificial kinetic method of radical detachment, the obligatory mechanism of activation failed.

X-ray diffraction and theoretical study of the transition 2H-3R polytypes in Nb1+xSe2 (0 < x < 0.1)

Single crystals of 2H and 3R niobium diselenide were grown by a chemical transport reaction. The current determinations by single crystals X-ray diffraction reveal a non-stoichiometric composition. The structures are built from Se—Nb—Se slabs with Nb in trigonal prismatic coordination whereas the extra or additional Nb atoms are located in the octahedral holes between the slabs giving rise to the formula 2H and 3R-Nb1+xSe2 with 0.07 < x < 0.118. In particular, vacancy and Nb-Nb interactions may play an important role on the non-stoichiometry and the stacking mode in NbSe2. By increasing the number of additional Nb atoms in the pure 2H-NbSe2, a transition 2H to 3R polytype should occur in order to minimize the Nblayer—Nbextra—Nblayer repulsions between these adjacent slabs. The theoretical study shows that both 2H and 3R-Nb1+xSe2 are thermodynamically stable in the range 0 < x < 0.1.

NONTRADITIONAL METHOD OF CHEMICAL MODIFICATION OF THE SURFACE OF POROUS SILICA

The main results obtained in the study of the possibility of low-temperature chemical modification of porous silica, applying various nanodispersed substances to their surface are given. It has been shown that the use of the chemical transport reaction (CTR) $ \mathrm{H_{2}O_{2}/ZnO} $ for this purpose with the participation of $ \mathrm{H_{2}O_{2}} $ vapors at room temperature makes it possible to apply a number of oxides, sulfides and metal halides to the surface of various sorbents. An important advantage of the proposed modification method is that changes in the original texture parameters of the substrate are minimized. Using the example of modifying the behavior of highly pure $ \mathrm{SiO_{2}} $ with zinc oxide, it was shown that the decomposition of gaseous intermediate products of CTR $ \mathrm{H_{2}O_{2}/ZnO} $ occurs mainly in pores with a diameter of more than $ 6~nm $.

Une surstructure de α-Ge, type diamant, induite par un dopage d'antimoine

Single crystals of antimony-doped germanium, Ge1–xSbx+0.01(x≃ 0.0625), were grown by chemical transport reaction. The alloy crystallizes as a superstructure of diamond-type α-Ge. All atoms in the asymmetric unit lie on special positions and are characterized by strong covalent bonds. The antimony atoms substitute for one germanium atom at full occupancy at Wyckoff position 4a(site symmetry -43m), and are also at an adjacent tetrahedral interstitial site with partially occupation (16%) at position 4c(or 4d) (site symmetry -43m). The structural model does not show close Sb...Sb contacts, and suggests that the interstitial antimony atoms move between the two adjacent tetrahedral sites.

Structure cristalline du composé Hg3-xSbx(S+Se)2+xI2-x(x≃ 0.1)

Single crystals of the mercury chalcohalide Hg3-xSbx(S+Se)2+xI2-x(x≃ 0.1) (mercury antimony sulfide selenide iodide), were grown by a chemical transport reaction. The structure contains three independentA(Hg/Sb) atoms; each atom is strongly covalently bonded with twoX(Se/S) atoms to form approximately linearX–A–Xunits. TheX–A–Xunits link to formA4X4rings, which are combined into infinite crankshaft-type bands running along the [100] direction. Four equatorialE(I/X= Se,S) atoms at relatively long distances complete the distorted octahedral coordination ofA(Hg/Sb). The crystal under investigation was twinned by non-merohedry with a refined twin domain fraction of 0.814 (6):0.186 (6). The structure is isotypic with Hg3Se2I2[Beck & Hedderich (2000).J. Solid State Chem.151, 73–76], but the current determination reveals a coupled substitution, with partial replacement of Hg+2by Sb+3, balanced by the equivalent substitution of I−1by S−2and Se−2. Bond-valence calculations are consistent with this relative substitution model.

Structure cristalline du composé intermétallique Ni18Ge12

Single crystals of octadecanickel dodecagermanide were grown by chemical transport reaction. The intermetallic compound crystallizes in a superstructure of the hexagonal NiAs type (B8 type). All atoms in the asymmetric unit lie on special positions except one Ni atom (two Ni atoms have site symmetry -6.. and another one has site symmetry .2. while the Ge atoms have site symmetries 32.,m.. and 3..). In the structure, the Ni atoms are arranged in 11- or 13-vertex polyhedra (CN = 11–13). The coordination polyhedra of the Ge atoms are bicapped square antiprisms (CN = 10) or 11-vertex polyhedra (CN = 11). The structure exhibits strong Ge...Ni interactions, but no close Ge...Ge contacts are observed. The Ni atoms with CN = 13 form infinite chains along [001] with an Ni—Ni distance of 2.491 (2) Å.

Cd4As2Br3

Single crystals of Cd4As2Br3(tetracadmium biarsenide tribromide) were grown by a chemical transport reaction. The structure is isotypic with the members of the cadmium and mercury pnictidohalides family with general formulaM4A2X3(M= Cd, Hg;A= P, As, Sb;X= Cl, Br, I) and contains two independent As atoms on special positions with site symmetry -3 and two independent Cd atoms, of which one is on a special position with site symmetry -3. The Cd4As2Br3structure consists of AsCd4tetrahedra sharing vertices with isolated As2Cd6octahedra that contain As–As dumbbells in the centre of the octahedron. The Br atoms are located in the voids of this three-dimensional arrangement and bridge the different polyhedra through Cd...Br contacts.