Mössbauer study of IF5-intercalated graphite

Graphite intercalation compounds (GIC) with IF₅ were investigated at 4.2 K by Mössbauer spectroscopy with the 57.6-keV gamma resonance of ¹²⁷I. The identical hyperfine parameters (isomer shift, axially-symmetric electric field gradient tensor) observed for three differently prepared samples (powdered and highly-oriented pyrolytic graphite (HOPG)) reveal the presence of square-pyramidal IF₅ intercalated molecules, as in the pure molecular solid. Apart from absorber thickness effects, no changes in the intrinsic spectral shapes of the resonance spectra were found for different orientations of the γ-ray direction relative to the graphite $\text{c}$ axis. This means that the four-fold molecular axis of IF₅ must be tilted by an angle close to 54.7° relative to $\text{c}$; such is the case if one of the triangular bases of IF₅, formed by three fluorine atoms, is oriented parallel to the carbon planes. The observed anisotropy of the recoil-free fraction shows that the IF₅ molecules are much more strongly bound in the $\text{c}$ direction as compared to the direction parallel to the graphite planes.     

An ESCA study of the Fe2+/Fe3+ ratio in passive films on iron-chromium alloys

X-ray photoelectron spectra of Fe $\text{2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ electrons are measured for passive films on iron and FeCr alloys passivated for 1 h in 1M H₂SO₄ at + 100 and + 500 mV (s.c.e.). When chromium content of FeCr alloys increased to ca. 12 at.%, binding energy of Fe $\text{2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ electrons from oxidized state shifted abruptly from ca. 710·3 to ca. 709·6 eV and no more change in binding energy was observed at high chromium content. This abrupt shift of binding energy is in accord with the fact that the Fe²⁺/Fe³⁺ ratio showed sharp increase at ca. 12 at.% chromium. This phenomenon can be attributed to the inhibition of oxidation of iron due to the change in film structure and properties.     

Intercalation of graphite by silicon tetrafluoride and fluorine to yield a second-stage salt C24SiF5

Powdered graphite (either finely broken HOPG or Union Carbide SP1) interacts with 1:1 SiF₄/F₂ mixture (～50 atmospheres) at 40°, to yield a second-stage compound (a₀ = 2.46(1), c₀ = 11.29(1) Å) of composition C～₂₄SiF₅. This product interacts with excess PF₅ with quantitative displacement of SiF₄ to yield a first-stage fluorophosphate (a₀ = 2.46(1); $\text{c}{}_0\text{= 7.65(1)}\text{A}\text{?}\text{)}$ according to the equation: C₂₄SiF₅ + 2PF₅ → C₂₄PF₆. PF₅ + SiF₄. The first-stage fluorophosphate falls to a second-stage salt (a₀ = 2.46(1); $\text{c}{}_0\text{= 10.87}\text{A}\text{?}\text{)}$ under vacuum: $\text{C}{}_{24}\text{PF}{}_6\text{.}\text{PF}{}_5\text{20°}\text{→}\text{C}{}_{24}\text{PF}{}_6\text{+}\text{PF}{}_5$. This is identical to that prepared directly: $\text{24}\text{C}\text{+}\text{PF}{}_5\text{+}\text{1}\text{2}\text{F}{}_2\text{→}\text{C}{}_{24}\text{PF}{}_6$. Extensive reduction of C₂₄PF₆, at ≈20° : 2C₂₄PF₆ + PF₃ → 48C + 3PF₅ to graphite of crystallinity comparable to that of the starting graphite establishes that the initial C₂₄SiF₅ does not involve extensive fluorination of the graphite. The ease of displacement of SiF₄ by superior fluoroacids indicates that C₂₄SiF₅ can be a valuable precursor for other graphite salts. Ready formation of a first-stage fluoroborate, $\text{C}{}_7\text{BF}{}_4\text{, I}{}_{\mathrm{<mtext>c</mtext>}}\text{= 7.70(1)}\text{A}\text{?}$ contrasts with the failure to prepare similarly a first-stage SiF₅^? salt.     

Novel graphite salts and their electrical conductivities

A set of novel first stage graphite salts of general formula C₈⁺MF₆^? has been prepared (M = Os, Ir, As). Single crystal X-ray diffraction studies indicate that these salts are hexagonal with a ≈ 4.9 and $\text{c ≈ 8.1}\text{A}\text{?}$. The unit cell volume indicates that the anions are closely packed in the galleries. Platinum hexafluoride, which is the most powerful oxidizer of the third transition series, forms a first stage compound, which analytical, structural, and magnetic studies establish as C₁₂²⁺PtF₆^2?. In this salt the anions are not close packed, but the electron withdrawal from the graphite planes is greater than for the C₈⁺MF₆^? series. The variation in the electrical conductivity (in the a–b plane), as a function of composition, has been investigated with the OsF₆, IrF₆, PtF₆ and AsF₅ intercalates. For OsF₆ and IrF₆, the conductance per plane of graphite is found to be a maximum at approximately C₂₄MF₆ (second stage); the conductivity being an order of magnitude greater than that of the parent material. Intercalation beyond C₂₄MF₆ leads to a marked decrease in conductivity. C₈MF₆ is comparable in conductivity with the parent graphite. This behavior contrasts with the graphite/AsF₅ system in which a steady increase in conductance per graphite plane with increasing AsF₅ content is observed. For the PtF₆ system, the second as well as the first stage materials are poorly conducting.     

Optical absorption and luminescence in poly(4,4′-diphenylenediphenylvinylene)

Poly(4,4′-diphenylenediphenylvinylene), PDPV, is a soluble conjugated polymer that shows a degree of conjugation similar to that in poly-(paraphenylene). The optical properties of thin films exposed to AsF₅ show the appearance of features below the $\text{π-π}{}^1$ gap at 3 eV that can be interpreted in a model of dopant-induced polaron and bipolaron defects. When excited above the $\text{π-π}{}^1$ gap, PDPV shows a strong luminescence peaked at 2.4 eV. The Stokes' shift of 1 eV can be accounted for by radiative decay from a photogenerated polaron-exciton defect.     

Structure and electrochemical properties of intercalation compounds in the system silver-titanium disulfide,Agx TiS2

In the system AgTiS₂ three structurally related phases exist, a dilute gas phase and second- and first-stage intercalates. Single crystals of Ag_0.35TiS₂, grown by vapour transport, were studied by X-ray diffraction. A superstructure due to three-dimensional order of silver, a √ 3 × a √ 3 × 2c, space group $\text{P}\text{3}\text{1}\text{c}$, is present below T_c = 301 K. The transition to the disordered substructure, $\text{a = 3.428, c = 6.398}\text{A}\text{?}$, space group group $\text{P}\text{3}\text{m}\text{1}$, is of second-order nature.Kinetic parameters of the mobile ions (silver) were measured at temperatures above 450 K on powder compacts in an electrochemical cell using small signal a.c. frequency response and the galvanostic intermittent titration technique (GITT). The first- and second-stage phases show fast ionic conduction of silver; the chemical diffusion coefficient

is about 10^?5 cm² s^?1A at 460 K and the activation energy is about 0.26 eV. The free enthalpy of intercalation is small, ?2.9 kJ mol^?1 for stage II Ag_0.19TiS₂ and ?4.6 kJ mol^?1 for stage I Ag_0.42TiS₂ (at 480 K). The temperature coefficient of the e.m.f. is positive. The intercalated phases show metallic type electronic conduction.     

Electrochemical monitoring of atmospheric corrosion phenomena

An atmospheric corrosion monitor (ACM) which consists of $\text{Cu}\text{Zn}$ or Cu/steel couples, has been used to study various aspects of atmospheric corrosion. Calibration of ACM's is carried out under 1 ml of distilled water. Measurements under 10^?510^?1N KCl show that the conductivity of the electrolyte is not an important parameter in determining the amount of current flow. A detailed study was related to the effect of salt particles on atmospheric corrosion. While no current flow and no corrosion occurred on clean surfaces up to r.h. ≈ 95%, large increases of the galvanic current were observed when salt particles were placed on the ACM surface provided that the relative humidity in the test cell was higher than the r.h. value of a saturated solution of the salt particle applied. The ACM has also been used to monitor changes in the composition of gaseous atmospheres (air, N₂, N₂ plus SO₂). Outdoor exposure of the $\text{Cu}\text{Zn}$ and Cu/steel ACM suggests that this instrument can be used not only to monitor time-of-wetness, but also the corrosivity of a test environment.     

Chemical and microstructural organization of graphite intercalated with SbCl5 and SbF5 from 121Sb-Mössbauer spectroscopy

Differently prepared stage-one and stage-two graphite intercalation compounds (GICs) with SbCl₅ and SbF₅ were investigated with the 37.1 keV ¹²¹Sb-Mössbauer resonance and by transmission electron microscopy (TEM). The TEM observation parallel to the graphite c-axis gave insight into the inplane arrangement of various samples. An island structure was clearly imaged in non-stoichiometric samples. The ¹²¹Sb resonance spectra revealed the presence of both Sb⁵⁺ and Sb³⁺ intercalant species, with their relative abundances depending on the preparation conditions. In SbCl₅-graphite, the Sb³⁺ species were identified as neutral SbCl₃ molecules, with their trifold molecular symmetry axes oriented almost parallel to the carbon-layer planes. In undersaturated stage-two SbCl₅-GICs, Sb⁵⁺ and Sb³⁺ intercalant species in Sb⁵⁺:Sb³⁺ ratios varying from 1.4 to 2.2. were found; the former were identified as SbCl₆⁻-like. In saturated stage-one SbCl₅-graphite, the intercalant species were predominantly SbCl₅-like, with only small amounts of SbCl₃. Comparative studies of stage-one and stage-two SbF₅-GICs revealed the dominant presence of Sb⁵⁺ components in these systems, with Sb⁵⁺:Sb³⁺ ratios varying from ⋍4 to 17; two kinds of Sb³⁺ species were identified on the basis of Mössbauer spectra. One species, molecular SbF₃, was shown not to be in an intercalated state, while the other one was intercalated and tentatively assigned to SbF₄⁻ or SbF₅²⁻ molecules. It clearly emerged for all investigated systems that the Sb⁵⁺:Sb³⁺ ratio of intercalated species depended critically on the preparation conditions and hence was not a direct measure of the intercalation-induced charge transfer.     

19F NMR of antimony and arsenic pentafluoride-graphite derivatives

¹⁹F NMR results are reported on first stage SbF₅-GIC and second stage AsF₅-GIC as a function of temperature. Above a “cutting temperature”, we postulate the existence of two fluorine population at least, which we have tried to identify on our spectra according to a pentafluoride oxidation reaction. The results are correlated with a charge transfer coefficient evolving with temperature, and are discussed in comparison with other experimental results.     

Passivation of copper: Role of some anions in the mechanism of film formation and breakdown

The effect of SO^2?₄, Cl^? and I^? ions on the anodic currents of the successive passivating processes that take place in alkaline phosphate solutions, on the above has been studied using potentiodynamic, galvanodynamic and open circuit potential measurements. Additions of I^? resulted in the largest current increase followed by Cl^?. Additions of SO₄^2? showed an initial anodic current decrease, a trend that was reversed with the progress of cycle number. While the current increase, of Cu(1) oxide, with increasing halide additions, follows closely:

$\text{δ}{}_{\mathrm{i}}\text{=}\text{(}\text{A}\text{[X-1])}\text{(}\text{B}\text{+m[X-])}$

that of Cu(II) oxide process obeyed

$\text{δ}{}_{\mathrm{i}}\text{=}\text{A}\text{[X-]}$

where Δi is the increase in peak current and [X^?] the halide concentration added. A mechanism involving adsorption of the halide ions on the electrode surface and/or exchange with OH^? attached to the soluble metal ion, is discussed.     

四方钨青铜(TTB)相Nb18W16O93材料的锂离子扩散路径(英文)

使用改进的固态烧结方法(1000℃,36h)成功合成四方钨青铜(TTB)相Nb₁₈W₁₆O₉₃,并通过XRD,SEM和XPS对其进行表征与分析。GITT结果表明,Nb₁₈W₁₆O₉₃(10^-12cm²/s)的锂离子扩散系数高于传统的Ti基负极。使用密度泛函理论模拟计算揭示锂离子的扩散机制。TTB相有3种扩散路径,其中扩散能垒最小的层间扩散(0.46 eV)比其他典型负极(例如,石墨0.56 eV)更具有优势,使TTB相Nb₁₈W₁₆O₉₃成为潜在的高特定功率阳极材料。     

Polarized resonance Raman spectroscopy of fully-oriented crystalline trans-(CH)x

Fully-oriented non-fibrous crystalline trans-(CH)_x films prepared by stress-orientation and conversion of poly-7,8-bis-(trifluoromethyl)-tricyclo-{4.2.2.0.}-deca-3,7,9,-triene, poly-(BTFM-TCDT), are investigated by polarized resonance Raman spectroscopy. With an excitation wavelength of 647.1 nm and parallel polarization (parallel to the c-axis, i.e., the stretching direction), two strong Raman bands are observed at 1078 cm^?1 and 1469 cm^?1, whereas for perpendicular polarization, no Raman scattered light can be detected. This polarization behaviour proves the perfect alignment of long unperturbed conjugated sequences parallel to the c-axis.Upon excitation with blue laser light ($\text{λ}{}_{\mathrm{<mtext>l</mtext>}}\text{= 457.9}\text{nm}$), for which preferentially short conjugated sequences come into resonance, an increase of the depolarization ratio compared to the red excitation is observed.These results are discussed in terms of conjugation length distributions and resonance conditions with respect to the polarized optical absorption spectra.     

ESR study of dilute intercalation compounds: C/F,C/K/F and C/Li/F

The intercalation of fluorine into HOPG is a relatively slow process and enables one to study quasiequilibrium states during the intercalation process. A very interesting ESR spectrum is observed at the initial intercalation process. The room temperature spectrum consists of several new lines at g = 2.002 ± 0.002 (line ‘a’) and g = 2.015 ± 0.002 (line ‘b’). Line ‘b’ and possibly line ‘a’ (at the very beginning of the interacalation process) are due to spins in highly disordered domains. Line ‘b’ disappears as the intercalation process proceeds, supporting a recent Raman study on the same system which suggested that disordered domains are transformed into ordered domains as the intercalant density increases. We argue that both lines ‘b’ and ‘a’ (in the initial state of intercalation) are probably hyperfine split spectra due to interaction with the fluorine nuclei ($\text{I =}\text{1}\text{2}$), although we cannot rule out the possibility that line ‘a’ is associated with spins in ordered domains. At low temperatures lines ‘a’ and ‘b’ merge into a single line (line ‘d’) at g = 2.008 ± 0.003. This collapse is associated with changes in the signals' peak-to-peak heights. We suggest the formation of intercalant clusters and strain field as the origins of these effects. The intercalation of F₂ into the donor GIC C/K and C/Li gives very similar ESR properties. At low temperatures a new line appears at g = 2.035 ± 0.002 (line ‘e’) in all the three systems. This line is believed to be associated with a ‘line dislocation’ or ‘domain wall’.     

Low-energy ion beam treatment of α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>(0001) and improvement of photoluminescence of ZnO thin films

A low energy N₂ ^? ion beam impinged on a α-Al₂O₃(0001) single crystal surface in the range of fluence 5×10¹⁵/cm²?1×10¹⁸/cm² at room temperature. After ion bombardment, chemical bonding on the modified sapphire surface was investigated by x-ray photoelectron spectroscopy. Below a fluence of 1×10¹⁵/cm², only a non-bonded N1s peak at the binding energy 398.7 eV was found, but further irradiation up to 2×10¹⁷/cm² induced Al?O?N bonding at around 403 eV. The occurrence of Al?N bonding was identified at ion fluence higher than 5×10¹⁷/cm² at 396.6 eV. II–VI ZnO thin films were grown on an untreated/ion-beam-induced sapphire surface by pulsed laser deposition (PLD) for the investigation of the modified-substrate effect on photoluminescence. The ZnO films grown on modified sapphire containing Al?O?N bonding only, and both Al?O?N and Al?N bonding showed a significant reduction of the peak related to deep-level defects in photoluminescence. These results are explained in terms of the formation of Al?N?O and Al?O?N layers and relaxation of the interfacial strain between Al₂O₃ and ZnO.