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The crystal structure of the silicon tetrafluoride molecular complex with 4-phenylpyridine SiF4·2ppy has been determined by X-ray diffraction analysis. Although 4-phenylpyridine has a higher proton affinity than pyridine, the Si–N bond length in crystalline SiF4·2Py is shorter by 0.03(2) A than in SiF4·2ppy. The energies of donor-acceptor Si–N bonds in SiF4·2Py (220.6 kJ/mol) and SiF4·2ppy (225.0 kJ/mol) calculated by the M06-2X/def2-TZVP method agree with a slightly higher proton affinity of 4-phenylpyridine.

The head and wake waves of filtration combustion of calcium hydride in a flow of a mixture of silicon tetrafluoride and hydrogen at silicon tetrafluoride weight fractions of 0.8, 0.85, 0.9, and 0.98 were studied. The spatial profiles of the combustion waves were constructed. With their aid, the thermal structure of the head and wake waves was found, and the width of each zone in the combustion wave for different compositions of the gas phase was determined. With increasing concentration of the reagent gas in the flow, the reaction zone width increases in the head wave and decreases in the wake wave. The structure of the head wave corresponds to the intermediate combustion wave, and the structure of the wake wave, to the normal combustion wave.

The dependence has been investigated of the maximum temperature in the reaction front, the linear velocity of the combustion wave of CaH2 in a mixture of gases SiF4 and H2 with the formation of SiH4, and the conversion of calcium hydride on the initial mass concentration of silicon tetrafluoride. The experimental data have been obtained in a vertical flow reactor with forced gas filtration in the range of SiF4 mass fractions of 0.7–0.98. The results of the experiment have been compared with the data of calculation according to the classical physical and mathematical model of the exothermic reaction wave propagation in a porous medium during gas blowing. The similarities and differences have been established between the experimental and calculated values of the maximum temperature in the reaction front, the linear velocity of the combustion wave propagation, and the conversion of the solid-phase reagent.

The impurity composition of natural and isotopically enriched silane obtained from magnesium silicide and silicon tetrafluoride is studied by gas chromatography–mass-spectrometry. The impurities characteristic of silane with natural and isotopically enriched composition are atmospheric gases; carbon dioxide; hydrides; saturated, unsaturated, and aromatic hydrocarbons; halogenated hydrocarbons; alkyl derivatives and silane homologues, siloxanes. The impurity content of high-purity silane does not exceed 10–7–10–5 mol % After synthesis, the concentrations of impurities in the silane and fractions isolated by its distillation purification lie in the range n × 10–6 − n × 10 mol % The limits of detection for impurities are 1 × 10–8–9 × 10–4 mol %

Thermodynamic calculations and experimental studies have demonstrated that the regeneration of fluorine from the fluorine-containing byproducts and wastes of contemporary industry is reasonable to be performed in the form of hydrogen fluoride via the processing of fluorine-containing compounds in a hydrogen-containing fuel–oxygen-containing oxidant flame. The considered sources of feedstocks for the synthesis of hydrogen fluoride are hydrofluoric acid, including acid of azeotropic composition, uranium hexafluoride, carbon tetrafluoride, nitrogen trifluoride, ammonium bifluoride, silicon tetrafluoride, hexafluorosilicic acid, and hexafluorosilicates.

The impurity composition of 28SiF4 has been studied for the first time by gas chromatography/mass spectrometry using gas-adsorption capillary columns. For identification and determination of impurity substances by gas chromatography/mass spectrometry, we have proposed using cryogenic preconcentration of the impurities from 28SiF4. The presence of C2–C4 hydrocarbon impurities and sulfur hexafluoride has been confirmed. We have identified 35 impurity substances in 28SiF4, including linear and branched C5–C8 saturated hydrocarbons, aromatic hydrocarbons, chlorine- and fluorine-containing hydrocarbons, and silicon- and sulfur-containing substances, of which 28 have been detected for the first time.

We have studied reaction between silicon tetrafluoride and calcium hydride in a vertical flow reactor, which proceeds as combustion in filtration mode. The principal kinetic characteristics of the reaction between SiF4 and CaH2 in a counterpropagating wave have been determined: reaction order n = 1, rate constant k = 0.38 ± 0.05 s–1, and activation energy Ea = 17 ± 2 kJ/mol.

Hexafluorosilicate (LH2)SiF6 and the cis-[SiF4(L)] chelate complex characterized by 19F NMR are products of reaction between hydrofluorosilicic acid and 1,10-phenanthroline (L). XRD findings show that the structure of (LH2)SiF6 is stabilized by NH···F hydrogen bonds (N···F 2.618(4), 2.676(4) A) and CH···F contacts. The relative resistance of the cis-[SiF4(L)] complex to hydrolysis is associated with the chelate effect.

The process of plasma-chemical conversion of silicon tetrafluoride in a radiofrequency (13.56 MHz) discharge has been studied. The dependence of the yield of silicon on the specific energy input and the H2/SiF4 molar ratio has been examined in the pressure range of 0.1–0.3 Tort. The maximum yield of silicon is 60%, and the optimal specific energy consumption is 44.6 MJ per mole of Si. A mechanism has been proposed for the plasma-assisted reduction of silicon tetrafluoride under the given experimental conditions.

Glasses in the PbF2-PbO-SiO2 system doped with 1 mol % of rare-earth elements (Nd3+, Er3+, or Yb3+) are synthesized and studied. The glasses were heat-treated in order to obtain glass ceramics with a fluoride crystalline phase. The changes in the structure and spectral optical properties of glass ceramics with respect to initial glasses were determined by using X-ray diffraction analysis and by studying the luminescent characteristics of dopant ions.

Systematic analysis of factors affecting the accuracy of DFT calculations of 29Si NMR chemical shifts in four-coordinate silicon compounds showed that the best agreement with the experimental values is attained using B1PW91 and PBE0 functionals in combination with the TZP basis set. In calculations of 29Si chemical shifts by quantum-chemical methods particular attention should be given to the contribution of relativistic spin-orbit interaction and conformational equilibrium.

Preforms with a fluorosilicate glass core produced by the modified chemical vapor deposition process with SiF4 as the fluorination agent were used to study the effect of initial vapor mixture composition on the refractive-index difference Δn between the fluorosilicate and silica glasses and the glass deposition rate. The results indicate that Δn can reach about −0.01. The marked reduction in the deposition rate of fluorosilicate glass with increasing SiF4 concentration seems to be due to chemical reactions that lead to the formation of stable gaseous chlorofluorosilanes.

Silicon tetrafluoride SiF4 reacted with aliphatic alcohols ROH to form liquid complex compounds SiF4 · 2ROH. The gas-liquid SiF4(g) -SiF4 · 2ROH(l) systems can be used for separating silicon isotopes. The hydrodynamics and mass transfer of the separation of silicon isotopes in a countercurrent column were studied using SiF4-SiF4 · 2ROH systems based on butanol-1, pentanol-1, and hexanol-1 at 293 K. Corrections were applied to the empirical coefficients in the equations of the dependence of the total holdup of a helical prismatic packing (HPP) and its specific pressure drop on a load in an isotope exchange column. The height equivalent to the theoretical plate (HETP) increased more than twofold in the butanol-1-hexanol-1 homologous series (the flow rate in the column was (3.4 ± 0.2) mmol SiF4/(cm2 min) for the working gas). It was shown using pentanol-1 as a complex-forming agent that the limiting stage of mass transfer during the separation of silicon isotopes was the isotope exchange between SiF4 and the SiF4 · 2ROH complex.

Possibility of performing a mechanically activated synthesis of pure monosilane by the reaction of calcium hydride with silicon tetrafluoride in a rotating flow-through reactor containing milling bodies was analyzed. A number of operation parameters of the grinding reactor were theoretically evaluated and experimentally determined.

Using high-resolution IR spectroscopy, we have compared the impurity compositions of monosilane (SiH4) fractions enriched in impurities in the process of cryofiltration and low-temperature distillation of monosilanes derived from silicon tetrafluoride (SiF4) and trichlorosilane (SiCl3H). The results demonstrate that the more volatile impurities present in both monosilanes are methane (CH4) and carbon dioxide (CO2), whereas the impurities specific to the fluoride-derived monosilane are SiF4, SiF3H, and SiF2H2. The less volatile impurities common to both monosilanes are ethane (C2H6), disiloxane (Si2H6O), and disilane (Si2H6); the impurities specific to the fluoride-derived monosilane are tetrafluoroethylene (C2F4) and monofluorosilane (SiFH3); and those specific to the chloride-derived monosilane are hydrogen chloride (HCl) and the chlorosilanes SiClH3, SiCl2H2, and SiCl3H.

The broadening and shifting of 12 saturated absorption resonances of the SiF4 gas induced by molecular collisions have been measured in the lasing region of a low-frequency CO2 laser at the P(30) line of the 9.4-μm band. Values of the collisional broadening are identical accurate to the measurement uncertainty. The collisional shift does not exceed 10% of the collisional broadening. This means that collisions with changes in the internal energy of molecules prevail over dephasing collisions.

Matrix isolation Fourier transform IR spectroscopy has been used for studying the products and mechanism of the silicon tetrafluoride reaction with water at various component ratios and reaction durations. Assignment of new bands in the spectrum confirms the earlier assumptions of first the formation of a molecular complex with water and later of trifluorosilanol, which finally condenses to give hexafluorodisiloxane.

The data on fabrication of silicon layers on various substrates by plasma enhanced chemical vapor deposition from the (silicon tetrafluoride)-hydrogen system are reported. The emission spectra of the plasma in the system are recorded. The samples were studied by the X-ray diffraction and secondary ion mass spectrometry techniques. The morphologic properties of the surface are examined, and the Raman spectra, the transmittance spectra in the infrared region, and photoluminescence spectra are recorded. The phase composition of the layers corresponds to nanocrystalline silicon, in which the dimensions of coherent-scattering grains vary with the conditions of the preparation process in the range from 3 to 9 nm. The layers exhibit intense photoluminescence at room temperature.

The synthesis of silane by reacting silicon tetrafluoride and calcium hydride is shown to proceed as a propagating wave. The main parameters of the process, namely, the propagation velocity, the reaction front temperature, and calcium conversion, are determined. The mathematical model of wave propagation for an exothermic reaction in a porous medium with purging a gaseous oxidant describes qualitatively the major laws of the process.

Using gas chromatography and high-resolution Fourier-transform IR spectroscopy, we have determined the concentrations of C1–C4 hydrocarbon impurities in isotopically unmodified silicon tetrafluoride before and after fine purification and in 28Si-enriched SiF4. The concentrations of C1–C4 hydrocarbon impurities in silicon tetrafluoride for SiH4 synthesis have been shown to correlate with those in the synthesized silane.

Known experimental and theoretical data used in the analysis of stereoisomeric structures of penta-and hexacoordinate silicon tetrafluoride complexes with the molecular N-and O-donor ligands are discussed.

The impurity compositions of silicon tetrafluoride and silane prepared from it have been determined by high-resolution Fourier-transform IR spectroscopy. In the spectra of SiF4 samples differing in purity, we have identified rovibrational bands arising from Si2F6O, SiF3OH, HF, SiF3H, SiF2H2, SiH3F, CH4, CO2, and CO impurities. Their detection limits lie in the range 9 x 10(-5) (CO2) to 3 x 10(-3) mol % (Si2F6O). In the spectra of SiH4 samples of different purity, we have detected CH4, CO2, SiF3H, SiF2H2, and SiF4 impurities. Their detection limits lie in the range 8 x 10(-5) (CO2) to 1 x 10(-3) mol % (SiF4).

The spectral characteristics of the SiF4 molecule in the range 3100700 cm1, including the absorption range of the band 3, are studied in the gas phase at P = 0.47 bar and in solutions in liquefied Ar and Kr. In the cryogenic solutions, the relative intensities of the vibrational bands, including the bands of the isotopically substituted molecules, are determined. The absorption coefficients of the combination bands 23, 3 + 1, 3 + 4, and 34 are measured in the solution in Kr. In the gas phase of the one-component system at an elevated pressure of SiF4, the integrated absorption coefficient of the absorption band 3 of the 28SiF4 molecule was measured to be A(3) = 700 ± 30 km/mol. Within the limits of experimental error, this absorption coefficient is consistent with estimates obtained from independent measurements and virtually coincides with the coefficient A(3) = 691 km/mol calculated in this study by the quantum-chemical method MP2(full) with the basis set cc-pVQZ. ©2005 Pleiades Publishing, Inc.

Hydrocarbon impurities in silicon tetrafluoride originate from the sodium hexafluorosilicate used as a precursor for the preparation of SiF4 through the thermal decomposition of Na2SiF6. We have studied the fraction of C1–C4 hydrocarbons released as a function of temperature during the thermal decomposition of sodium hexafluorosilicate. The lighter hydrocarbons—methane, ethane, and ethylene—are shown to be released at lower temperatures. One source of hydrocarbon impurities in sodium hexafluorosilicate is the sodium carbonate used to prepare it.

The possibility of the reduction of hexafluorodisiloxane by calcium hydride in the synthesis of silane from silicon tetrafluoride has been studied. This reaction is shown to be not decisive for oxygen contamination of silane. The most likely reason for the appearance of impurity Si2OH6 in “fluoride” silane is the Ca(OH)2-catalyzed reaction of silane with trace water. The concentration of impurity Si2OH6 in silane at the stage of synthesis may be efficiently decreased by the preliminary purging of calcium hydride with a hydrogen (grade A) flow.

The possibility of calculating the molecular structure, vibrational frequencies, and electrooptical parameters of the silane molecule SiH4 and the silicon tetrafluoride molecule SiF4 is analyzed by using the ab initio quantum-chemical method (the MP2 perturbation theory) and the density functional theory (the B3LYP functional) with a wide variation of basis sets up to the cc-pV5Z set. The results obtained are used as reference values for studing mixed fluorosilanes SiH4−x Fx (x=1–3) and for refining their geometry, the vibrational frequencies, and the absolute intensities of absorption bands. Based on the results of high-level quantum-chemical calculation corrected against the experimental data for SiH4 and SiF4, the structural parameters of mixed flouro-silanes are estimated and their vibrational frequencies are calculated and assigned for the first time. The quantum-chemical estimates of the absolute intensities of IR spectra are for the first time presented for all mixed fluorosilanes.

Layers of silicon with an isotope composition that differs from the native composition have been obtained by the method of plasma enhanced chemical vapor deposition from gaseous silicon tetrafluoride. The deposited layers possess an amorphous structure and high oxygen content. The Raman spectra and the spectra of optical transmission in the IR range are presented. The in-depth distribution of Si isotopes has been determined using the second ion mass spectrometry techniques.

The production of silicon layers using plasma enhanced chemical vapor deposition in the mixture of silicon tetrafluoride and hydrogen is reported. The samples have been analyzed by X-ray diffraction, Raman spectroscopy, and secondary ion mass spectrometry. The phase composition of the layers is nanocrystalline silicon with the crystalline-domain sizes from 3 to 9 nm in dependence of the conditions of the process. The samples are characterized by intense photoluminescence at room temperature.

The possibility of combined application of gas chromatography, high-resolution IR spectroscopy, and transient microwave gas spectroscopy (TMGS) for investigation of the impurity composition of silicon tetrafluoride is studied. Using high-resolution IR Fourier spectroscopy, the lines of a number of impurities are observed in the region from 4500 to 550 cm−1. The absorption lines of some of the main well-known impurities in silicon tetrafluoride in the 2-mm wavelength range are analyzed. The advantages of the TMGS method for investigating the multicomponent SiF4-impurities system are demonstrated. Using the TMGS method, the freons CHF3, CH2F2, and CH3F are experimentally detected in silicon tetrafluoride.