Surface cosegregation on Fe-3%V-C, Fe-3%V-C,N and Fe-15%Cr-N (110) single crystals

Surface cosegregation has been studied on (110) oriented Fe-3%V-C, Fe-3%V-C,N and Fe-15%Cr-N single crystals applying AES and LEED. The surface compounds VC,V(C,N) and CrN are formed upon annealing in the temperature range from 450 to 750° C depending on the alloy. The stoichiometries of the binary surface compounds correspond to VC_1.2 and approximately CrN_0.8 as determined by quantitative evaluation of Auger peak height ratios. The composition of the V(C,N) surface compound varies between V(C_0.6N_0.6) at 450° C and V(C_0.2N_1.0) at 640° C. Three-dimensional precipitates are not formed as indicated by Ar⁺ depth profiling. After short annealing times streaking LEED patterns are observed indicating partial disorder in one direction of the direct space. Upon sufficient annihilation of surface defects sharp (4 × 1) patterns appear. A missing and added row model is proposed for the surface compounds on bcc(110) substrate surfaces.     

Facetting of Fe-15% Cr-N, Fe-3% V-C and Fe-3% V-C,N (111) surfaces

Summary Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and scanning electron microscopy (SEM) have been used to study the surface chemistry of crystallographically open bcc (111) surfaces of Fe-15% Cr-N, Fe-3% V-C and Fe-3% V-C,N alloys. The binary and ternary surface phases CrN, VC and V(C,N) were formed at temperatures ranging from 450 to 750°C depending on the alloy. On Fe-15% Cr-30ppmN (111) two-dimensional surface compounds CrN were formed at temperatures above 600°C according to the bulk phase diagram of the Fe-Cr-N system, whereas on Fe-15% Cr-N samples with nitrogen contents of more than about 100 ppm the precipitation of three-dimensional CrN took place at temperatures below 600°C. Optical and SEM micrographs as well as oxidation experiments at room temperature indicated that the substrate surfaces are inhomogeneously covered by the surface phases. Facetting of the bcc (111) surfaces induced both by cosegregation of the solutes and by surface precipitation was observed in real space (SEM) as well as in reciprocal space (LEED). It is shown that the surface phases are epitaxially arranged on (100) facets of the substrate.     

Cosegregation-induced formation of surface compounds on (110) and (111) oriented surfaces of bcc alloys with 3d and 4d metals

Cosegregation phenomena were studied on the (110) and (111) surfaces of Fe-3.5%Mo-N single crystals by means of Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). On both surface orientations cosegregation of molybdenum and nitrogen was found to cause the formation of two-dimensional MoN surface compounds which are stabilized by strong chemical interactions between the two solutes. The maximum surface coverages of the segregants, which were established at temperatures around 500°C, correspond to less than a full monolayer of the MoN surface compounds. LEED investigations revealed a complex diffraction pattern of the MoN covered (110) alloy surface, while a (1 × 1) structure was observed on the (111) surface. However, no facetting of either surfaces occurred. This finding is in sharp contrast to previous results obtained for ferritic alloys with various 3d metals such as Fe-15%Cr-N and Fe-3%V-C,N. It is concluded that the maximum MoN surface coverage is too small to induce the facetting of the bcc(110) and bcc(111) alloy surfaces.     

In situ surface analysis of annealed Fe-1.5%Mn and Fe-0.6%Mn low alloy steels

The oxidation of two binary alloys, Fe-1.5%Mn and Fe-0.6%Mn, was studied in terms of the annealing conditions and paying special attention to the selective oxidation of manganese. The crystallographic structure of the surfaces and their compositions were determined by RHEED and XPS by using in situ analytical devices attached to the annealing reactor to avoid surface contamination after the treatments. Additionally, ex situ investigations on the morphology and composition of the surfaces were performed by FEG-SEM, WDX, and GIXRD. The annealing processes were performed at 800 degrees C under N(2)-5%H(2) protective atmospheres at water vapor dew points between -10 and -30 degrees C. The oxides formed were identified and the resulting surface structures resolved.     

Influence of N, C and S on the initial stage of oxidation of Fe-20Cr single crystals

Fe-20Cr single crystals (100), (110) and (111) have been investigated by Low Energy Electron Diffraction (LEED) and Auger Electron Spectroscopy (AES). The samples have been oxidised between 500° C and 900° C inside the UHV chamber at oxygen partial pressures between 1×10^–8 mbar and 1×10^–6 mbar. Before the oxidation the samples have been precovered with N, C or S by segregation from the bulk. Oxidation experiments of samples without any impurities on the surface have been carried out for comparison. The oxidation at 500° C results in a preferential growth of Fe-oxide on the samples, with increasing temperature mainly Cr-oxide grows on the surfaces. On S covered surfaces the oxidation is strongly retarded. Presence of N on the surfaces does not change the oxidation behaviour of the samples in comparison to oxidation without N. C on the surfaces retards the oxidation of the surfaces in the very initial stage. During the oxidation the amount of C on the surfaces decreases and no differences to oxidation without C on the surfaces are observed at higher oxygen exposures (100 L). After oxidation no well ordered LEED patterns could be observed, Cr- and Fe-oxide grow in no ordered orientation on the surfaces.     

Single crystal RuO2 (110): Surface structure

The surface structure of RuO₂ (110) has been studied with LEED, AES and XPS. The “as-grown” surface shows no LEED patterns and both AES and XPS indicate that the surface is depleted in oxygen in high vacuum. After extensive annealing in an O₂ atmosphere reproducible LEED patterns characteristic of the (110) surface were obtained. For the well-ordered surface the oxygen XPS results revealed oxygen associated with the bulk RuO₂, the presence of RuO₃ and oxygen bound to surface atoms.     

Preparation and characterization of terraced surfaces of low-index faces of anatase, rutile, and brookite

Simple polishing and relatively low temperature annealing procedures for preparing atomically flat terraced surfaces of various single-crystal TiO2 polymorphs are described. Anatase (101), anatase (001), rutile (100), rutile (110), and brookite (111) surfaces could all be prepared with a terraced surface structure as revealed in AFM images. The rutile (100) and (110) and anatase (101) surfaces were also shown to produce acceptable LEED patterns immediately upon insertion into a UHV system without the usual sputter and anneal cycles.     

Formation and nitridation of vanadium–aluminum intermetallic compounds

V(5)Al(8) and V(3)Al intermetallics have been formed by interdiffusion, by annealing of sputtered V/Al-multilayers at 700 degrees C in vacuo; sapphire (102) was used as substrate. The V/Al intermetallics were nitridated in NH(3) at 900 degrees C for 1 min by RTP (rapid thermal processing). The samples were investigated with XRD (X-ray diffraction), SNMS (secondary neutral mass spectrometry), and AFM (atomic force microscopy). A 5-10 nm thick AlN film (001 textured) was formed by nitridation of V(5)Al(8) (110 textured) and 2-3% nitrogen was incorporated in the V(5)Al(8) bulk. Nitridation of V(3)Al resulted in the formation of VN and AlN. Direct nitridation of V/Al-multilayers showed that near the surface nitridation is faster than intermixing of the V and Al layers. The capability of VN as diffusion barrier for Al could also be shown.     

Synthesis of [(HIPTNCH2CH2)3N]Cr Compounds (HIPT = 3,5-(2,4,6-i-Pr3C6H2)2C6H3) and an evaluation of chromium for the reduction of dinitrogen to ammonia

Red-black [HIPTN3N]Cr (1) ([HIPTN3N]3- = [(HIPTNCH2CH2)3N]3- where HIPT = 3,5-(2,4,6-i-Pr3C6H2)2C6H3 = HexaIsoPropylTerphenyl) can be prepared from CrCl3, while green-black [HIPTN3N]Cr(THF) (2) can be prepared from CrCl3(THF)3. Reduction of {1|2} (which means either 1 or 2) with potassium graphite in ether at room temperature yields [HIPTN3N]CrK (3) as a yellow-orange powder. There is no evidence that dinitrogen is incorporated into 1, 2, or 3. Compounds that can be prepared readily from {1|2} include red [HIPTN3N]CrCO (4), blood-red [HIPTN3N]CrNO (6), and purple [HIPTN3N]CrCl (7, upon oxidation of {1|2} with AgCl). The dichroic (purple/green) Cr(VI) nitride, [HIPTN3N]CrN (8) was prepared from Bu4NN3 and 7. X-ray studies have been carried out on 4, 6, and 7, and on two co-crystallized compounds, 7 and [HIPTN3N]CrN3 (65:35) and [HIPTN3N]CrN3 and 8 (50:50). Exposure of a degassed solution of {1|2} to an atmosphere of ammonia does not yield "Cr(NH3)" as a stable and well-behaved species analogous to Mo(NH3). An attempt to reduce dinitrogen under conditions described for the catalytic reduction of dinitrogen by [HIPTN3N]Mo compounds with 8 yielded a substoichiometric amount (0.8 equiv) of ammonia, which suggests that some ammonia is formed from the nitride but none is formed from dinitrogen.     

Synthesis and Structures of Sb[N(H)(C(6)H(2)(t)Bu(3))](3) and Bi[N(H)(C(6)H(2)(t)Bu(3))](3): Implications for the Steric Limits of Supermesityl Substitution

Syntheses and isolations of the tris(amino)stibine and tris(amino)bismuthine E[N(H)(C(6)H(2)(t)Bu(3))](3) (E = Sb, Bi) from ECl(3) and LiN(H)(C(6)H(2)(t)Bu(3)) are described, together with spectroscopic and structural characterization [crystal data for C(54)H(90)N(3)Sb, M = 903.04, space group P&onemacr;, a = 11.491(5) ?, b = 24.652(7) ?, c = 10.002(5) ?, alpha = 98.38(3) degrees, beta = 96.44(5) degrees, gamma = 77.25(3) degrees, V = 2724(2) ?(3), D(c) = 1.101 Mg/m(3), Z = 2, R = 0.0547; crystal data for C(54)H(90)BiN(3), M = 990.27, space group P&onemacr;, a = 11.511(5) ?, b = 24.785(15) ?, c = 9.981(5) ?, alpha = 98.06(5) degrees, beta = 96.50(4) degrees, gamma = 77.40(5) degrees, V = 2742(2) ?(3), D(c) = 1.200 Mg/m(3), Z = 2, R = 0.0619]. The compounds bear the "bulky" 2,4,6-tri-tert-butylphenyl substituent (known as supermesityl or Mes), and their formation is considered in the context of the same reactions for PCl(3) and AsCl(3), which have been previously shown to produce the aminoiminopnictine structures [N(H)(C(6)H(2)(t)Bu(3))]P=N(C(6)H(2)(t)Bu(3)) and [N(H)(C(6)H(2)(t)Bu(3))]As=N(C(6)H(2)(t)Bu(3)). The observations establish the limits of the steric control by the supermesityl substituent and provide qualitative support for the thermodynamic significance of substituent steric strain.     

Ethene adsorption, dehydrogenation and reaction with Pd(110): Pd as a carbon 'sponge'

The interaction of ethene with the Pd(110) surface has been investigated, mainly with a view to understanding the dehydrogenation reactions of the molecule and mainly using a molecular beam reactor. Ethene adsorbs with a high probability over the temperature range 130 to 800 K with the low-coverage sticking probability dropping from 0.8 at 130 K to 0.35 at 800 K. The adsorption is of the precursor type, with a weakly held form of ethene being the intermediate between the gas phase and strong chemisorption. Dehydrogenation begins at approximately 300 K and is fast above 350 K. If adsorption is carried out at temperatures up to approximately 380 K, adsorption saturates after about 0.25 monolayer have adsorbed, but above approximately 450 K, adsorption continues at a high rate with continuous hydrogen evolution and C deposition onto the surface. It appears that, in the intermediate temperature range, the carbonaceous species formed is located in the top layer and thus interferes with adsorption, whereas the C goes subsurface above 450 K, the adsorption is almost unaffected, and the C signal is significantly attenuated in XPS. However, the deposited carbon can easily be removed again by reaction with oxygen, thus implying that the carbon remains in the selvedge, that is, in the immediate subsurface region probably consisting of a few atomic layers. No well-ordered structures are identified in either LEED or STM, though some evidence of a c(2 x 2) structure can be seen. The Pd surface, at least above 450 K, appears to act as a "sponge" for carbon atoms, and this effect is also seen for the adsorption of other hydrocarbons such as acetaldehyde and acetic acid.     

MA(delta)Sb(2-delta) (M = Zr, Hf; A = Si, Ge): a new series of ternary antimonides and not "beta-ZrSb2"

The ternary antimonides ZrSi(delta)Sb(2-delta), HfGe(delta)Sb(2-delta), and ZrGe(delta)Sb(2-delta) were prepared by annealing of the elements in stoichiometric ratios below 800 degrees C. ZrSi(delta)Sb(2-delta) was earlier erroneously described as the binary "beta-ZrSb(2)", which does not exist as such, because the incorporation of tetrel atoms is necessary for the formation of this structure. ZrSi(delta)Sb(2-delta) has a small yet significant phase width with at least 0.066(7) < or = delta < or = 0.115(3), whereas the Ge analogues exist with larger tetrel concentration, i.e., ZrGe(0.211(5))Sb(1.789) and HfGe(0.205(6))Sb(1.795). The whole series of title compounds crystallizes in the Co(2)Si type (space group Pnma), with lattice dimensions of, e.g., for ZrGe(0.211(5))Sb(1.789), a = 730.4(1) pm, b = 395.13(6) pm, c = 957.6(2) pm, V = 0.27635(7) nm(3), Z = 4. The anionic substructure comprises infinite ribbons formed by the atom sites Q1 and Sb2, with Q1 being mixed occupied by Si or Ge and Sb atoms. These ribbons exhibit Q1-Q1 single bonds and Q1-Sb2 "half" bonds. Assuming the validity of the 8 - N rule, one can assign seven valence-electrons to Sb2 but only five to Q1, which might explain the preference of the tetrel atoms for the latter site.     

Low energy electron diffraction and cyclic voltammetry studies of flame-annealed platinum single crystals

Low energy electron diffraction (LEED) and cyclic voltammetry were used to examine the surface structure of flame-annealed platinum (100), (110), and (111) electrodes. Well-defined LEED patterns were obtained for these surfaces but only after heat treatment in vacuo at 400°C in order to remove surface contaminants. The cyclic voltammograms correspond to those expected for these low index platinum surfaces from prior studies.     

Photoelectron spectra of the decomposition of ethylene on (110) tungsten

Photoelectron spectra, LEED patterns, and work function changes were obtained for ethylene adsorbed on (110) tungsten at room temperature, and with subsequent heat treatment. For saturated adsorption of C₂H₄ on (110) W at room temperature, features in the photoelectron spectrum were observed which are believed to be due to the C, HCC, and Cmetal bonds in an adsorbed species of the form C₂H₂. The work function decreased by 1.2 eV at saturation, but LEED showed no change from the clean surface pattern. Upon heating to ≈ 500 K, where hydrogen is known to desorb, the CH bond was broken, whereas the CC and Cmetal bonds remained. The work function increased, from saturation, by ≈ 0.6 eV and the LEED pattern exhibited a large diffuse background with no new spots. Upon heating to ≈ 1100 K the CC bond broke and the LEED pattern ordered into the characteristics carbon contamination pattern.     

Three new organically templated 1D, 2D, and 3D vanadates: synthesis, crystal structures, and characterizations

Three new vanadate compounds of the formulas (C(2)N(2)H(10))VO(OH)(4) (I), (NH(4))(3)(C(3)N(2)H(5))V(4)O(10) (II), and V(OH)(3).0.97H(2)O (III) have been synthesized by a solvothermal method and characterized by IR spectroscopy, elemental analysis, and thermogravimetric analysis. The crystal structures of the above three vanadates have been established by single-crystal X-ray diffraction. Compound I crystallizes as tetragonal, space group P4/mmm, with a = 9.0465(11) A, c = 3.9897(10) A, V = 326.51(10) A(3), and Z = 2. Compound II crystallizes as orthorhombic, space group Immm, with a = 3.6012(10) A, b = 11.312(4) A, c = 15.050(4) A, V = 613.1(3) A3, and Z = 2. Compound III crystallizes as cubic, space group Fd3m, with a = 10.4252(17) A, V = 1133.1(3) A(3), and Z = 16. Structural analyses reveal a one-dimensional beeline-chained structure, which consists of VO(6) octahedra in I. Compound II possesses a two-dimensional V-O-layered structure formed by VO(5) square pyramids; protonated imidazole and remaining NH(4+) cations are inserted between the layers. The three-dimensional open framework of III with the pyrochlore type consists of V(12) and V(4) secondary building units by using VO(6) octahedra as building units.     

Synthesis of new [8Fe-7S] clusters: a topological link between the core structures of P-cluster, FeMo-co, and FeFe-co of nitrogenases

A coordinatively unsaturated dinuclear iron(II) complex of bulky thiolates, [(TipS)Fe(micro-SDmp)]2 (1; Tip = 2,4,6-(i)Pr(3)C(6)H(2), Dmp = 2,6-(mesityl)(2)C(6)H(3)), was synthesized from stepwise reactions of Fe{N(SiMe(3))2}2 with 1 equiv of HSDmp and then with 1 equiv of HSTip. Complex 1 was found to react with elemental sulfur (S8) in toluene to generate a new class of [8Fe-7S] cluster, [(DmpS)Fe(4)S(3)]2(micro-SDmp)2(micro-STip)(micro(6)-S) (2). The cluster 2 was also produced from one-pot reactions of Fe{N(SiMe(3))2}2 + HSDmp + HSTip + S8 (8:6:10:7/8) and Fe3{N(SiMe(3))2}2(micro-STip)4 + HSDmp + S8 (8/3:16/3:7/8), where another [8Fe-7S] cluster, [(TipS)Fe(4)S(3)]2(micro-SDmp)2{micro-N(SiMe(3))2}(micro(6)-S) (3), was also found as a minor byproduct. In either of the clusters, two Fe(4)S(3) incomplete cubane units are connected by three anionic ligands, namely three thiolate S atoms for 2 or two thiolate S atoms and one amide N atom for 3, and one hexa-coordinate S atom resides at the center of the [8Fe-7S] core. They have a common Fe(II)(5)Fe(III)3 oxidation states, and an S = 1/2 ground spin state was indicated by rhombic EPR signals at 10 K with g = 2.19, 2.07, and 1.96 for 2 and g = 2.13, 2.06, and 1.93 for 3. The structural relevance of clusters 2 and 3 to P-cluster, FeMo-co, and FeFe-co of nitrogenases is discussed.     

铈及混合稀土对Fe-36Ni低膨胀合金凝固组织的影响

研究了Ce及混合稀土对Fe-36Ni低膨胀合金凝固组织的影响,结果表明:加Ce或者混合稀土处理后,合金中分别形成了大量高熔点的Ce2O3颗粒和(Ce,La)2O2S复合第二相颗粒,错配度理论计算表明,Ce2O3,Ce2O2S和La2O2S的(0001)面与Fe-36Ni的(100)面分别具有6.21%,5.77%和5.42%的较低错配度,因此Ce2O3和(Ce,La)2O2S可以作为有效的非均质形核核心,使凝固组织的等轴晶比例增加,等轴晶尺寸减小,合金凝固组织得到显著细化。     

Rapid solid-state synthesis of tantalum, chromium, and molybdenum nitrides

Solid-state metathesis (exchange) reactions can be used to synthesize many different transition-metal nitrides under ambient conditions including TiN, ZrN, and NbN. Typical metathesis reactions reach temperatures of greater than 1300 degrees C in a fraction of a second to produce these refractory materials in highly crystalline form. Likely due to the large amount of heat produced in these solid-state reactions, some transition-metal nitrides such as TaN, CrN, and gamma-Mo(2)N cannot easily be synthesized under ambient conditions. Here metathesis reactions are demonstrated to produce the cubic nitrides TaN, CrN, and gamma-Mo(2)N when sufficient pressure is applied before the reaction is initiated. By pressing a pellet of TaCl(5) and Li(3)N with an embedded iron wire, crystalline cubic TaN forms under 45 kbar of pressure after a small current is used to initiate the chemical reaction. Crystalline cubic CrN is synthesized from CrCl(3) and Li(3)N initiated under 49 kbar of pressure. Crystalline gamma-Mo(2)N is produced from MoCl(5) and Ca(3)N(2) (since MoCl(5) and Li(3)N self-detonate) initiated under 57 kbar of pressure. The addition of ammonium chloride to these metathesis reactions drastically lowers the pressure requirements for the synthesis of these cubic nitrides. For example, when 3 mol of NH(4)Cl is added to CrCl(3) and Li(3)N, crystalline CrN forms when the reaction is initiated with a resistively heated wire under ambient conditions. Cubic gamma-Mo(2)N also forms at ambient pressure when 3 mol of NH(4)Cl is added to the reactants MoCl(5) and Ca(3)N(2) and ignited with a resistively heated wire. A potential advantage of synthesizing gamma-Mo(2)N under ambient conditions is the possibility of forming high-surface-area materials, which could prove useful for catalysis. Nitrogen adsorption (BET) indicates a surface area of up to 30 m(2)/g using a Langmuir model for gamma-Mo(2)N produced by a metathesis reaction at ambient pressure. The enhanced surface area is confirmed using scanning electron microscopy.     

AES- and LEED-investigations of the oxidation behaviour of Fe-19,5 Cr-4,5 Al single crystals

Summary The oxidation behaviour of Fe-19,5 Cr-4,5 Al single crystals (110), (111) has been investigated to obtain reference data for further oxidation experiments on materials of the same composition but doped with yttrium. The oxidation of annealed specimens showing Al surface segregation and unannealed (without Al-enrichment) samples was carried out at different temperatures in an UHV-chamber at an oxygen pressure of about 10^–7 mbar. During oxidation Auger spectra (20...70 eV) were taken and LEED-patterns were photographed before and after oxidation if detectable. It has been shown that the type of oxide formed was dependent on the chemical composition of the surface and the oxidation temperature.This poster was awarded the First Prize in Poster Section B by the Deutscher Arbeitskreis für Spektroskopie (DASP)     

Induction of homochirality in achiral enantiomorphous monolayers

We report the induction of homochirality in enantiomorphous layers of achiral succinic acid on a Cu(110) surface after doping with tartaric acid (TA) enantiomers. Succinic acid becomes chiral upon adsorption due to symmetry-breaking interactions with the Cu(110) surface. The doubly deprotonated bisuccinate forms mirror domains on the surface, which leads to a superposition of (11,-90) and (90,-11) patterns observed by low-energy electron diffraction (LEED). On average, however, the surface layer is racemic. An amount of 2 mol % of (R,R)- or (S,S)-tartaric acid in the monolayer, corresponding to an absolute coverage of 0.001 tartaric acid molecule per surface copper atom, is sufficient to make the LEED spots of one enantiomorphous lattice disappear. After thermally induced desorption of TA, the succinic acid lattice turns racemic again. In analogy to the "sergeants-and-soldiers" principle described for helical polymers, this effect is explained by a lateral cooperative interaction within the two-dimensional lattice.