Chlorine Layers Research Articles

Atomic structure of saturated chlorine monolayer on Ag(111) surface

STM and LEED techniques were applied to study the structure of saturated chlorine monolayer formed on Ag(111) as a result of Cl 2 adsorption at room temperature. We present for the first time atomic resolution STM images of chlorinated Ag(111) surface obtained in ultra-high vacuum conditions. The structure of chlorine layer was identified by means of a Fourier analysis of the STM images as Ag(111)–(17 × 17)-Cl. The structure of saturated chlorine monolayer remains unchanged for Cl 2 exposures up to 100 000 L and coexists with AgCl islands on Ag(111) surface.

Complex Metal Halide Oxides (2). Layered Complex Metal Chloride Oxides Having the X1X2 Structure as Catalysts for the Oxidative Dehydrogenation of Ethane

Abstract SrBi3O4Cl3 with the X1X2 structure, in which metal–oxygen sheets were separated by single chlorine layers and double chlorine layers alternately, has been prepared and tested as a catalyst for the oxidative dehydrogenation of ethane. SrBi3O4Cl3 crystallized in space-group I4/mmm with lattice parameters of a = 3.9370(1), c = 27.0177(7) Å. The structure was refined by a Rietveld analysis, which showed that between two Bi(1) and Bi(2) sites, the former being of Bi in the metal–oxygen sheet faced the single chlorine sheet and the later faced the double chlorine sheet, strontium ions occupied the Bi(2) site only, and did not occupy the Bi(1) site. The SrBi3O4Cl3 catalyst showed a good catalytic performance for the oxidative dehydrogenation of ethane. Particularly, a high selectivity to ethene of more than 90% was obtained. Based upon an investigation of the potential role of chlorine during the cource of the reaction, it was apparently shown that a role of surface chlorine species was important, rather than gas-phase chlorine, for the reaction. It is considered that the high selectivity to ethene can be attributed to the fact that the chlorine radical mechanism operates on the catalysts, and that the surface chlorine blocks the formation of active-oxygen species to prevent destructive oxidation reactions.

Superconductivity at 25.5 K in electron-doped layered hafnium nitride

The electronic properties of crystals with a layered structure can be radically altered by the intercalation, between the layers, of guest species that act as electron donors or acceptors. Such studies have been performed extensively on graphite, transition-metal dichalcogenides and oxide bronzes1. Interest in redox intercalation reactions2 has increased recently because the high-transition-temperature (high-Tc) superconductors based on copper oxide also have layered structures, the superconductivity occurring within two-dimensional CuO2 planes separated by charge-reservoir oxide layers3. Similarly, in metal-doped fullerenes, which show relatively high transition temperatures, the electron donor atoms sit in the interstitial sites between adjacent fullerene balls4. In a previous study5, we described a layered nitride, β-ZrNCl, consisting of Zr–N double layers sandwiched between two close-packed chlorine layers. On lithium intercalation, the crystal changed from a semiconductor to a metal, and became a superconductor at 13 K. Here we report the properties of the isostructural compound β-HfNCl. After electron-doping the crystal by lithium intercalation, we observe superconductivity with a Tc of up to 25.5 K. This transition temperature is higher than that observed in any intermetallic compound, and suggests that layered nitride structures may offer transition temperatures comparable to those observed in layered copper oxide structures.

Reaction products in synchrotron radiation induced dry etching of Ga and Cu

Light induced dry etching using synchrotron radiation (SR) in the VUV range and chlorine as etching gas has been applied to metals (Cu) and semiconductors (GaAs). High quality replica of a mask are etched by irradiation with filtered radiation and the reaction products are analyzed by depth dependent Auger spectroscopy (ADP). For Cu, etching leads to thick CuClx product layers which originate from light induced reactions at the interface between chemisorbed chlorine and the product layer combined with a diffusion of Cu from the substrate to this interface which is attributed to an electrostatic potential perpendicular to the films (Cabrera–Mott mechanism). The anisotropy of etching is preserved despite a transport of Cu through a product layer of about 16 μm. The reaction products swell in volume by nearly a factor of two due to the Cl intake. For GaAs etched with Cl2 the product layer itself consists exclusively of Ga. The thickness of the product layer is restricted to about 1 μm, different to the Cu/Cl2 system. The reaction proceeds again at the interface between chemisorbed chlorine and product layer and is limited by the transport of As. This transport of As (alone or together with Ga) follows a Cabrera–Mott mechanism. During etching As reacts completely with the Cl and forms AsClx. All AsClx molecules are volatile and desorb while Ga sticks.

Tunable UV laser induced digital etching of GaAs: wavelength dependence of etch rate and surface processes

The wavelength dependence of etch rate in digital etching of GaAs employing Cl 2 UV laser system and its surface processes are discussed. The characteristic wavelength dependence on etch rate is divided into two regions, approximately 2 3 or 1 3 monolayer/cycle, which are identical to regions where Ga trichloride or Ga monochloride forms as the Ga-containing etching products, respectively. These results are well explained by a model involving the photodissociation of physisorbed chlorine layer or chemisorbed GaCl-like layer.

Cation Ordering in the Sillén X1-Type Oxychloride, BaBiO2Cl

The structure of BaBiO2Cl has been determined from single crystal X-ray diffraction data. It crystallizes in the orthorhombic space group Cmcm with a = 5.880(4), b = 12.945(18), c = 5.677(3) Å, V = 432.0(3) Å3, and Z = 4. BaBiO2Cl is related to a family of bismuth oxyhalides which can be viewed as separate oxygen and chlorine layers stacked along the b axis, with bismuth and barium cations located between the layers. The structure differs from other bismuth oxyhalides in that the barium and bismuth cations are found to be ordered in two distinct coordination environments. The cation ordering and fourfold coordination environment of bismuth is seen as a structural expression of the stereochemically active bismuth 6s lone pair of electrons.

Disorder in layered TiCl3. Symmetry of the coordination site

AbstractThe structural layers of TiCl3 consist of two monoatomic layers of chlorine atoms octahedrally coordinated to intercalating Ti atoms. Mechanical (grinding) and/or chemical treatment of the regular α‐ or γ‐forms may produce the disordered δ‐form, wherein shifts and rotations between neighboring layers give rise to the largest disorder compatible with close‐packing of the chlorine layers, accompanied by reduction of crystallite size both along the layers and across them. A Bernoullian statistical model fully describes the statistical structure of the polycrystalline samples in terms of 4 parameters, two of them relating with the stacking disorder, the other two with crystallite size distribution.The catalytic sites for isospecific α‐olefin polymerization lie on the side edges of the structural layers. It is shown that Ti atoms on these edges possess two isochiral free valencies, enabling them to exchange the roles of the (Ti‐coordinated olefin) and of the (Ti‐growing chain) bonds at each polymerization step with no change of the local chirality, in analogy with the metallocene‐based isospecific catalysts.

Substrate orientation dependence of self-limited atomic-layer etching of Si with chlorine adsorption and low-energy Ar + irradiation

The self-limited atomic-layer etching of Si(100), (111), (110) and (211) has been investigated by alternated chlorine adsorption and low-energy Ar + ion irradiation using an ultraclean electron cyclotron resonance apparatus. The etch rate per cycle is described by a Langmuir-type equation with the etch rate in saturation and the chlorine adsorption rate for each substrate orientation. There is a possibility that the etch rate per cycle in saturation is expressed by a well-regulated fractional number of the atomic-layer thickness. It has been found that the chlorine radicals are dominant adsorption species in the present experimental conditions, and that the sticking probabilities of chlorine radicals are almost independent of the substrate orientation. In XPS measurements, peaks of Si + have been observed, but neither of Si 2+ nor Si 3+ on the chlorine adsorbed surface and the adsorbed chlorine layer has been estimated to be as thin as about one monolayer. It has been proposed that the atomic-layer etch rate per cycle in saturation is explained by a simple equation considering the bond structure of each substrate surface.

Surface structure of the disordered Cl/Ti(0001) system determined by diffuse low-energy electron diffraction

Molecular chlorine adsorbs dissociatively on the clean Ti(0001) surface to form a disordered overlayer that only orders after high-temperature annealing. We have made measurements of the diffuse low-energy electron diffraction (DLEED) from a disordered chlorine layer at 20% of a monolayer coverage. After substraction of the DLEED intensities from the clean surface, the Pendry Y function was calculated using (78,82) and (90,94)-eV energy pairs. These experimental functions were then compared with calculated Y functions for several plausible adsorption sites on the Ti(0001) surface: on-top, bridge and the two types of threefold hollow sites (with or without an atom directly under the site). The calculations also allowed the Ti–Cl interlayer distance to vary, but fixed other structural parameters. Using the Pendry R factor (RP) the on-top and bridge sites are easily rejected, but the two types of threefold sites cannot be distinguished. The analysis produces a best fit for a threefold site with a Ti–Cl interlayer spacing of 2.48±0.07 Å with a value of Rp = 0.45. This structure has a Ti–Cl bond length of 2.96 Å that is in good agreement with a value of 2.94 Å predicted from solid-state chemistry considerations.

Structure of the intercalate NaWO 2Cl 2

NaWO 2Cl 2, F w = 309.74, is formed by the intercalation of sodium into WO 2Cl 2. The structure of the intercalate has been determined using a combination of constant wavelength and time-of flight powder neutron diffraction data. NaWO 2Cl 2 adopts orthorhombic symmetry, space group Ammm (No. 65), with a = 3.7943(3), b = 3.8006(4), and c = 15.739(1) Å; Z = 2; V = 226.96 Å 3; and D cal = 4.53 g cm −3. The structure consists of blocks, each containing a layer of both W and O atoms sandwiched between two layers of Cl atoms. The trilayer blocks [ClWO 2Cl] are stacked in the direction perpendicular to the layers, i.e., parallel to the c-axis, and are separated by Na + ions located in two sets of sites between adjacent chlorine layers. The basic structure is related to that of the parent material WO 2Cl 2 by a displacement of the [ClWO 2Cl] blocks in the basal plane.

Synthesis and Structure of the Centered Cluster Compound Ba3(Zr6Cl18Be)

AbstractReaction of powdered Zr with ZrCl4, BaCl2 and Be in suitable proportions in a Ta container at 800°C produces the title compound. Suitable monocrystals for X‐ray diffraction were obtained from reactions to which a comparable amount of Hg2Cl2 had been added. The structure of Ba3Zr6Cl18Be is a superstructure of the K2ZrCl6 · Zr6Cl18H type (R3c, Z = 6; a = 9.6852 (9) Å, c = 52.52 (1) Å; R, Rw = 2.7, 3.2% for 826 independent reflections, 2θ ≤ 50°). Trigonally compressed [Zr6(Be)Cl12i]Cl6a clusters are interconnected by six‐coordinate barium atoms that lie in Cla antiprisms (a twisted version of the ZrIV site) while (9 + 3)‐coordinate barium substitutes for potassium within chlorine layers. Distortions associated with the size and field of barium are responsible for the superstructure and for differences from other analogues.

Microindentation studies on BaFCl single crystals

The mechanical behaviour of BaFCI single crystals has been investigated by use of Vickers microindentation. The tests performed on (0 0 1) and (1 0 0) planes show an important hardness anisotropy, higher values being obtained on (1 0 0) planes and all being close to that of BaF2. A qualitative investigation of the impressions has established the occurrence of cleavage along {1 0 0} planes, whereas slip occurs much more easily along (0 0 1) planes. For tests performed on the (0 0 1) plane, a quantitative analysis of the indentation cracks yields a very low toughness value. On comparing the BaFCl and BaF2 structures, it is concluded that the chlorine layers introduce a marked anisotropy but do not have the expected weakening effect.

Adsorption of dichloromethane and its interaction with oxygen on the palladium(100) surface: effect of chlorine layers on oxygen chemisorption and oxidation of carbon residues

Dichloromethane, CH{sub 2}Cl{sub 2}, adsorbs dissociatively on a Pd(100) surface at room temperature with C, Cl, and H adsorbing into 4-fold sites while H tends to diffuse underneath the surface layer with increasing CH{sub 2}Cl{sub 2} exposures. Combined AES, TDS, LEED, and HREELS observations and Monte Carlo simulations suggest a model in which C and Cl adsorb at next nearest neighbor (NNN) sites having a Cl-C-Cl angle of 90{degree} and exclude adsorption on all the nearest neighbor (NN) and Cl-Cl NNN adsorption sites. Atomic Cl desorbs at 1,100 K with the activation energy of desorption of ca. 68 kcal/mol. Oxygen-precovered surface with p(2{times}2) structure completely inhibits CH{sub 2}Cl{sub 2} adsorption while CH{sub 2}Cl{sub 2}-precovered surface is very reactive toward oxygen in the temperature range 350-510 K. The selectivity of oxidation of carbon fragments to CO increases with CH{sub 2}Cl{sub 2} preexposures, i.e., Cl-C-Cl coverages. A Monte-Carlo simulation of the oxidation of surface carbon by surface oxygen atoms resulted in a semiquantitative account of the observed rates of production of CO and CO{sub 2} as a function of chlorine coverage.

Some topochemical reactions based on the mox lamellar oxychlorides of transition metals

Intercalation of molecules and alkali metals are possible in lamellar transition metal oxyhalides. Intercalations in such lattice depend on different factors, which are the rigidity of the lattice and the stability of the +3 oxidation state of the metal. This structure leads also to irreversible topochemical reactions involving the substitution of the chlorine atoms by organic molecules possessing one or two nucleophilic groups. In the first case simple grafting reactions occur giving new layers compounds as FeOOCH 3 or FeOOCOCH 3 by example. In the second one, if we choose rigid organic molecules we realize double grafting or pillaring reactions KRK + 2FeOCl → 2KCl + FeOROFe. A new and interesting type is based on the following process 2FeOCl + K 2Fe(CO) 4 → 2KCl + (FeO) 2Fe(CO) 4. Chlorine layers are removed as potassium halide and simultaneously tetrahedral anionic groups are added.

Effect of hydrogen chloride during oxidation of silicon on trap generation by avalanche electron injection

The effects of HCl on trap generation in silicon dioxide layers at high electric fields are reported by comparing 700-Å, 200-Å, 9% HCl, and 0% HCl dry oxides stressed at constant avalanche electron injection currents and voltages. Experimental data indicate that HCl decreases the hydrogenation rate of boron acceptor in the silicon surface layer due to a high-density chlorine layer at the oxide/silicon interface but HCl increases the density of the peaked interface trap at 0.3 eV above the silicon midgap. A new chlorine-related positively charged electron trap in the oxide is observed and isolated from the chlorine-independent negatively charged oxide hole trap. Chlorine also reduces the density of the smaller cross-section oxide electron trap which gives the turn-around phenomena.

Structural Studies of Antimony Pentahalide Graphite Intercalation Compounds Using 001 Reflections

AbstractX‐ray diffraction studies at room temperature are reported of stages 1, 2 and 3 SbClmF5‐m graphite intercalation compounds (m = 0, 2, 4, 5). The analysis of the 001 reflection inten‐sities shows the periodic stacking of carbon, chlorine and antimony layers in all compounds. The results are in good agreement with the structural model recently found in other metal halide graphites. But we have found that the antimony atoms are not located on the mirror plane (z = O) in the centre of the intercalate layer. In case of dilute SbCl5 graphites, the identity period along c axis Ic increases with SbCl5 concentration in the stage 2 com‐pound. Interlayer distances were determined in dependence on SbCl5 concentration. A molecular layer form consisting of SbX5 molecules which are linked by halogen bridges is suggested to fill the galleries in the graphite lattice.

Structural studies of graphite intercalation compounds of antimony chloridefluorides using (001) x-ray reflexions

X-ray diffraction studies were made for SbCl 5, SbCl 4F, SbCl 2F 3 and SbF 5 graphite intercalation compounds of stages 1, 2 and 3. The (001) reflex intensities were analyzed in terms of a centrosymmetric structural model showing periodic layer stacking, with chlorine layers contacting antimony as well as carbon layers. The results are in agreement with the structural model recently found in other metal halide graphites. Contrary to the classical model, a displacement (about 0.35 Ā) of the Sb atoms from the mirror plane in the centre of the intercalant layer was found. Thus in projection onto the c-axis different distances between antimony and halogen atoms result. The analyses of different stages of one compound lead to similar layer parameters. The positions of the five halogen atoms in each compound were refined using a unique z-coordinate. However, it seems reasonable that the halogen atoms are arranged in different z-coordinates. In case of dilute SbCl 5 graphites prepared in CCl 4 solution, the identity period along c-axis I c increases with SbCl 5 concentration in the stage 2 compound. X-ray analysis reveals that Sb atoms move towards the mirror plane, when the SbCl 5 concentration increases. In agreement to Moessbauer studies it has to be assumed that the gallery is filled by SbX 5 trigonal bipyramids, and the molecular axis is tilted with respect to the c-axis. The shifting of molecules towards the centre maybe due to the fact that polymeres linked by halogen bridges are formed.

Further studies on electronic guest/host interactions of FeCl 3-graphite from Mössbauer spectroscopy — A comparative investigation with FeOCl

Mössbauer spectra of first stage FeCl 3-graphite and of the inverse Fe(C 5H 5) 2-iron oxide chloride intercalation compounds have been measured between room temperature and 4 K. The spectra at 295 K consist of resonance lines which are characteristic of ferric ions only. In addition, the spectra showed high-spin Fe 2+-sits at low temperatures. This result may be accounted for by an incomplete redox reaction caused by partial transfer of the π-electrons of the graphite (host layer), or of the ferricinium ions (guest layers), to the adjacent layers. Contrary to what is found in the literature, at low temperatures only these delocated electrons are fixed in the d-orbitals of some Fe 3+ atoms resulting in Fe 2+-sites. This result suggests that a shielding effect of the chlorine layers exists in these intercalation compounds which hinders a complete electron transfer to the Fe 3+-centers at room temperature. The ratio Fe 2+/Fe 3+ = 1:3.6 at 4 K is in agreement with Hennig's formula Cl −FeCl 2·3FeCl 3(C n ) +.

Chlorine adsorption on silver surfaces

The properties and structure of adsorbed chlorine layers on clean silver surfaces have been studied. On the Ag(111) surface, isobars over the temperature range from ∠300 to ∠900 K have been obtained using Auger electron spectroscopy. A model dependent, initial heat of adsorption of ∠50 kcal/mol was obtained. Structural observations using LEED showed that an epitaxial layer of AgCl(111) was formed. Electron impact desorption measurements give a desorption cross section for chlorine. On the Ag(110) surface a set of isotherms was obtained and the model independent isosteric initial heat of adsorption was also calculated to be ∠50 kcal/mol.

The lithium and molecular intercalates of FeOCl

Lithium intercalation compounds of the lamellar iron oxychloride have been obtained by means of the butyl lithium technique. Chemical and geometrical considerations suggest that the Li+ ions are localized in octahedral sites. Molecular intercalates are formed with amines. Single crystal studies on FeOCl (C5H5N)0.33 show the existence of a superstructure along the a axis. It may be related to an alternate arrangement of the pyridine molecules between the chlorine layers of the host structure. Univalent groups such as OH−, NH−2, (CH3NH)− can also be substituted for the chlorine ions in the FeOCl structure.