Investigation on the electric properties in molten quaternary systems by MD simulation

For the pyrochemical reprocessing of spent metallic fuels in molten salt, it is of importance to estimate the enrichment degree of Cs. The molecular dynamics simulation has been carried out on molten quaternary systems (Li, Na, K, Cs)Cl at 625K and (Li, Na, K, Cs)F at 727K for the various compositions in order to investigate the electric properties, i.e., the electric conductivity, self-diffusion coefficient, self-exchange velocity and the relative differences in the internal cation mobilities of Cs in molten LiCl-NaCl-KCl eutectic mixtures and in the FLINAK melts. These results allow us to conclude that the electric conductivities, self-diffusion coefficients and self-exchange velocities of Li⁺, Na⁺, K⁺ and Cs⁺ with reference to Cl⁻ and F⁻ have almost similar tendencies for each composition. We found it possible to enrich at up to χ_Cs = 0.38 in molten LiCl-NaCl-KCl eutectic as well as LiCl-KCl system and up to χ_Cs = 0.42 in the FLINAK melts as well as in molten FLINA system. In addition, the sequence of the simulated electric conductivity in molten quaternary alkali chloride and fluoride systems was in a fair agreement with that of the current simulated self-exchange velocities and self-diffusion coefficients.     

Electron emission from MOS electron emitters with clean and cesium covered gold surface

MOS (metal-oxide-semiconductor) electron emitters consisting of a Si substrate, a SiO₂ tunnel barrier and a Ti (1 nm)/Au(7 nm) top-electrode, with an active area of 1 cm² have been produced and studied with surface science techniques under UHV (ultra high vacuum) conditions and their emission characteristics have been investigated. It is known, that deposition of an alkali metal on the emitting surface lowers the work function and increases the emission efficiency. For increasing Cs coverages the surface has been characterized by X-ray Photoelectron Spectroscopy (XPS), Ion Scattering Spectroscopy (ISS) and work function measurements. Energy spectra of electron emission from the devices under an applied bias voltage have been recorded for the clean Au surface and for two Cs coverages and simultaneous work function curves have been obtained. The electron emission onset is seen to appear at the surface work function. A method for cleaning the ex situ deposited Au top electrodes to a degree satisfactory to surface science studies has been developed, and a threshold for oxide damage by low-energy ion exposure between 0.5 and 1 keV has been determined.     

Interaction of alkali metals with the Fe3O4(1 1 1) Surface

The adsorption and interaction of alkali metals (Li, Na, K, Rb and Cs) with the Fe_tet1-terminated Fe₃O₄(1 1 1) surface have been computed at the level of density functional theory. At low coverage, adsorption of alkali metals on site-1 (O_a-O_c-O_d) is energetically more favorable than on site-2 (O_a-O_c-O_d). Li has the strongest adsorption energy, followed by K, Rb, Cs and Na. The computed net charges show that the alkali metals can donate electrons to surface Fe and O atoms in the order of Li < Na < K ≈ Rb ≈ Cs. It is also noted that increasing the coverage does not significantly improve the promoting effect of alkali metals. In addition, alkali metals can move facilely on the Fe₃O₄(1 1 1) surface.     

Electrical resistivity of intercalated molybdenum disulfide

The electrical resistivity of hexagonal synthetic single crystals and natural crystals of MoS₂ intercalated with alkali metals (K, Rb and Cs) has been measured. The average room-temperature values is 0.004 ω-cm, and the best single crystal data on K_0.4MoS₂ showed at T^1.5 temperature behaviour characteristic of acoustical phonon scattering for a quasi-isotropic metal. The metallic behaviour and scattering mechanism are discussed.     

XPS and UPS study of Na deposition on thin film V2O5

The deposition of Na on thin film V₂O₅ has been study by using photoelectron spectroscopy. Vanadium ions are strongly reduced due to the deposition of Na. Three kinds of Na species were observed on the surface: the first is assigned to intercalated Na; the second is contributed to Na₂O₂; the third is appointed to metallic Na. The formation of Na₂O₂ leads to arise an emission line at about 10.3 eV in the valence band spectra. The metallic Na will further react with the oxides substrate and form Na₂O₂ on the surface in UHV chamber.     

Mössbauer effect of iron-57 intercalated in layered compound 2H−Fe0.5TaSe2

⁵⁷Fe Mössbauer effect study has been performed on intercalated compound 2H?Fe_0.5TaSe₂ over a temperature range between 14K and 500K. The highly concentrated intercalation samples 2H?Fe_xTaSe₂ with Fe atoms (x up to 2) are prepared by the electrochemical method for the first time. The Mössbauer measurement shows that iron is in Fe³⁺ high spin state and with the same probability occupies both octahedral and tetrahedral interstices of the Van der Waals gap. The fact that effective masses of iron ions in both sites are close to 57 amu and characteristic temperatures (Θ_M =130K, 107K respectively) are found to be relatively small is attributed to the weak forces on Fe³⁺.     

Raman spectroscopy of intercalated layered structure compounds

The Raman spectra of intercalated 1TTaS₂ and 2HTaSe₂ have been measured for the first time. Normal symmetry allowed optic phonons and charge density wave induced modes are observed in both systems. The spectra of the ethylenediamine intercalated compounds are qualitatively similar to those of the pure materials with small quantitative shifts in frequency and changes in the relative intensities of the CDW induced modes. The qualitative similarities in the spectra of the pure and the intercalated compounds reflect the two dimensional character of these materials and show that the dynamic properties of the charge density wave states are largely determined by intralayer effects.     

Influence of the Structure of the MEu(PO3)4 Crystals on Their Luminescent Properties

The influence of the M⁺ cations of alkali metals on the luminescent properties of the Meu(PO₃)₄ crystals, where M represents Li, Na, K, Rb, and Cs, is considered. The nonlinear dependences of the spectral position of the barycenters in the J components of the ⁷ F term of Eu³⁺ on the ionic radius R(M⁺), which are individual in the series of crystals of chain and cyclic structure, have been revealed. It is shown that the liability of the ligands of the europium complex in MeEu(PO₃)₄ to the polarizing action of the cations of alkali metals is manifested as an intermediate dependence of the luminescence lifetime of Eu³⁺ on R(M⁺).     

In-situ photoelectron spectroscopy study of a TiS2 thin film cathode in an operating Na intercalation electrochemical cell

TiS₂ thin films were prepared and intercalated in UHV either chemically or electrochemically and investigated by photoelectron spectroscopy. The chemical reaction was induced by Na evaporation. For the electrochemical reaction, the film was deposited on a Na solid electrolyte and the voltage between the TiS₂ and a graphite layer on the back of the plate was controlled during PES investigations. With both methods the same effects on the substrate are observed. The Na Auger peak appears and increases at a kinetic energy typical for intercalated Na. Due to the electron transfer to the conduction band an increase of the electron density at the Fermi level is clearly observed. The progressive filling of higher energy states shifts the Fermi level as reference for the PES spectra, and as a result the S 2p core levels and valence bands are shifted to higher binding energies. A shoulder appears at the lower binding energy side of the Ti 2p peak, indicating a higher negative charge density on the Ti atoms. It is also shown how the in-situ electrochemical intercalation allows experimentally to extrapolate the variation of the ionic contribution to the battery voltage. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Texture control and its impact on the properties of SrBi2Ta2O9 thin films prepared by pulsed laser deposition

SrBi₂Ta₂O₉ (SBT) ferroelectric thin films with different preferred orientations were deposited by pulsed laser deposition (PLD). Several methods have been developed to control the preferred orientation of SBT thin films. For SBT films deposited directly on Pt/TiO₂/SiO₂/Si substrates and in situ crystallized at the deposition temperature, the substrate temperature has a significant impact on the orientation and the remnant polarization (Pr) of the films; a higher substrate temperature benefits the formation of (115) texture and larger grain size. The films deposited on Pt/TiO₂/SiO₂/Si substrates at 830 °C are (115)-oriented and exhibit 2Pr of 6 μC/cm². (115)- and (200)-predominant films can be formed by using a La_0.85Sr_0.15CoO₃ (LSCO) buffer layer or by annealing amorphous SBT films deposited on Pt/TiO₂/SiO₂/Si substrates at 450 °C using rapid thermal annealing (RTA). These films exhibit good electric properties; 2Pr of the films are up to 12 μC/cm² and 17 μC/cm², respectively. The much larger 2Pr of the films deposited on the LSCO buffer layer and of the films obtained by RTA than 2Pr of the films deposited on Pt/TiO₂/SiO₂/Si substrates at 830 °C is attributed to a stronger (200) texture. Received: 30 January 2001 / Accepted: 30 May 2001 / Published online: 25 July 2001     

Effect of alkali-metal doping on photoluminescence of CdS films

It is established that doping of CdS polycrystalline films with alkaline metals (Li, Na, K, Cs) results in an increase of luminescence intensity by 3–5 times compared with pure films. This increase is accounted for by the placement of alkaline-metal ions in V _Cd ²⁻ cation vacancies, which are nonradiative recombination centers in these films. From the dependences of the luminsecence intensity of the doped films on the synthesis conditions (deposition temperature, concentration of doping impurities, type of doping metal), the parameters that ensure the maximum luminescence intensity of the films are determined as T_dep ≈ 450°C and C_Me = 1·10⁻⁵ at %. The luminescence intensity decreases by 1–3% upon exposure of the films to UV light (λ_max = 365 nm, I = 10²¹ quanta·sec·cm⁻¹) for several hours. This is indicative of the stability of these films against UV radiation. __________ Translated from Zhurnal Prikladnoi Spektroskopii Vol. 74, No. 3, pp. 362–366, May–June, 2007.     

Synthesis and characterization of SWNT-heavy alkali metal intercalation compounds,effect of host SWNTs materials

Singlewall carbon nanotubes (SWNTs) produced by electric-arc and laser ablation methods were characterized by X-ray diffraction before and after the reaction with alkali metals (M=K, Rb, and Cs). Reaction with annealed SWNTs gave MC₈ composition at saturation. The alkali metal lattice showed short range order incommensurate with graphene cylinders of SWNTs. X-ray diffractogram simulations have enabled the study of the influence of SWNTs structure on that of intercalation compounds. Chemically-purified bundles, constituted of open SWNTs, can be intercalated inside and between the tubes forming disordered structures. Annealed or pristine bundles were intercalated only between the tubes leading to short or long range ordered structure depending on host crystallinity and alkali metal (K, Rb or Cs). The expansion of the 2D SWNTs lattice after intercalation is comparable to graphite intercalation compounds. Some 2D arrangements of SWNTs and K atoms are proposed and discussed to reproduce XRD results. ¹³C NMR and ESR studies of annealed doped SWNTs emphasize the fact that the intercalation compounds of SWNTs are metallic.     

Al2O3 thin films by plasma-enhanced chemical vapour deposition using trimethyl-amine alane (TMAA) as the Al precursor

2 O₃ thin films by plasma-enhanced chemical vapour deposition (PECVD) using trimethyl-amine alane (TMAA) as the Al precursor. The thin films were deposited on both Si and quartz silica (SiO₂) substrates. Deposition rates were typically 60 Å min^-1 keeping the TMAA temperature constant at 45 °C. The deposited Al₂O₃ thin films were stoichiometric alumina with low carbon contamination (0.7–1.3 At%). The refractive index ranged from 1.54 to 1.62 depending on the deposition conditions. The deposition rate was studied as a function of both the RF power and the substrate temperature. The structure and the surface of the deposited Al₂O₃ thin films were studied using X-ray diffraction, atomic force microscopy (AFM) and scanning electron microscopy (SEM). Received: 20 May 1997/Accepted: 12 June 1997     

Adsorption of Cs and O2 on MoS2

Adsorption of Cs on basal planes of MoS₂ has been studied with LEED, Auger and work function measurements. LEED observations show that in the 200–300 K range Cs is adsorbed as amorphous layers on MoS₂. Correlation of Auger and work function measurements indicates that the work function, sticking coefficient and the maximum density of Cs that can be deposited on the MoS₂ surface depend strongly on substrate temperature. Cesium is deposited on MoS₂ in two adsorption states. Although MoS₂ is extremely inert to O₂ adsorption, the presence of Cs causes a drastic increase in the adsorption of oxygen which in turn increases the amount of Cs that can be deposited on the surface. Lastly, it has been found that part of the Cs adatoms are diffused into the bulk of MoS₂.     

LiMn2O4 films grown by pulsed-laser deposition

LiMn₂O₄ films have been deposited onto silicon wafer by pulsed-laser deposition (PLD) technique in order to test their reliability as cathode materials in rechargeable lithium microbatteries. The film formation has been studied as a function of the preparation conditions, i.e., composition of the target, substrate temperature, and oxygen partial pressure in the deposition chamber. Depending on the conditions of deposition, Mn₂O₃ was present as an impurity phase. When deposited onto silicon substrate maintained at 300 °C in an oxygen pressure of 100 mTorr from the target LiMn₂O₄+15 % Li₂O, the PLD films are well-textured with crystallite size of 300 nm. It is found that such a film crystallizes in the spinel structure (Fd3m symmetry) as evidenced by x-ray diffraction and Raman scattering measurements. Surface morphologies of layers were investigated by SEM. The cells Li//LiMn₂O₄ have been tested by cyclic voltammetry and galvanostatic charge-discharge techniques in the range 3.0–4.2 V. The voltage profiles show the two expected steps for Li_xMn₂O₄ with a specific capacity as high as 120 mC/cm² μm. The chemical diffusion coefficients for the Li_xMn₂O₄ thin films appear to be in the range of 10⁻¹¹-10⁻¹² cm²/s. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Surface science investigations of Cu intercalation in 1T TaSe2 and TiSe2 and its deintercalation by adsorbed Br2

The intercalation of Cu adsorbed onto 1T TaSe₂ and TiSe₂ (0001) van der Waals planes as well its deintercalation by adsorbed Br₂ is studied by synchrotron induced photoelectron spectroscopy and low energy electron diffraction. Cu intercalation into 1T TaSe₂ leads to a change in lattice distortion (charge density waves) as is evident from a transition of a commensurate √13x√13 to a 3×3 superstructure and changes in Ta 4f core line and valence band spectra. For 1T TiSe₂ intercalation follows closely the rigid band model. After adsorption of Br₂ at 100 K and annealing to room temperature a CuBr overlayer is detected on both samples. The substrate spectra indicate the deintercalation of near surface Cu. The experimental results suggest that the diffusion of Cu proceeds normal to the van der Waals plane. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Semiempirical pseudopotential surfaces for chemical reactions: The (AB)+ X - dialkali halide systems

A generalization of the Roach-Child semiempirical pseudopotential calculation for K + NaCl to several analogous dialkali halide systems has been used to elucidate the chemical interactions governing the reaction dynamics. The Li + LiF ground-state potential surface, which exhibits a ～ 20 kcal/mole basin for isosceles Li₂F, is qualitatively similar to one obtained in a recent configurational interaction calculation. It is shown that regions of the Na₂Cl ground-state surface corresponding to Na₂ ⁺ interacting with Cl^- can be described in terms of an ion-pair Rittner potential model similar to that employed for the alkali halides. Chemical trends in the triangular complex well depths satisfactorily account for the experimentally observed transition between the collision complex mechanism (Rb + KCl) and the osculating complex model (Li + KBr) for the alkali-alkali halide exchange reactions at thermal energies. For collinear configurations with the alkalis on opposite ends, avoided intersections between the lowest two potential surfaces are characterized in terms of diabatic surfaces computed from truncated basis sets. Crossings of these surfaces account for the vibrational-electronic energy transfer between alkali atoms and vibrationally excited alkali halides. The ionic X ^- + A ₂ ⁺ potential surfaces are used to predict the product electronic excitation and partitioning of exoergicity in reactions of halogen atoms with alkali dimer molecules.     

Doubly charged clusters - neutral atom charge transfer: the role of the Coulombic repulsion

We have studied experimentally the collisional charge transfer between a neutral atom and a multicharged metal-atom cluster. The charge transfer cross section measured for Na ₃₁ ^{+ +} + Cs is in the range of 400 ?². The time-of-flight mass analysis of the singly charged collision products demonstrates that an energy of about 0.5 eV is deposited in the cluster fragment during the charge transfer collision. This effect can be interpreted as a charge transfer to an excited state of the metal cluster. The measured cross section for Na ₃₁ ^{+ +} + Cs is larger than the one for Na ₃₁ ⁺ + Cs collisions. This difference between these two systems is due to the existence, for the first one, of a Coulombic repulsion term in the collision output channel. Received 24 October 2000     

Electronic states of CsLi and CsLi<Superscript>+</Superscript> molecules

Configuration interaction calculations have been carried out on electronic states of the CsLi molecule and the CsLi⁺ cation. Adiabatic potential energy, spectroscopic constants, dipole moments, and vibrational levels are presented for the lowest states of ^1,3Σ⁺, ^1,3Π, and ^1,3Δ symmetries of the alkali dimer CsLi molecule dissociating into Cs (6s, 6p, 5d, 7s, and 7p) + Li (2s, 2p, 3s, 3p, and 3d) as well as for the lowest ²Σ⁺, ²Π, and ²Δ electronic states of the CsLi⁺ cation dissociating into Li (2s, 2p, 3s, 3p, and 3d) + Cs⁺ and Li⁺ + Cs (6s, 6p, 5d, 7s, and 7p). The results of the present many-electron configuration interaction calculations on the cation support the previous core-polarization effective potential calculations. The present calculations on the CsLi molecule are complementary to previous theoretical work on this system, including recently observed electronic states that had not been calculated previously. We have used an ab initio approach involving a nonempirical pseudopotential for the Li (1s²) and Cs cores and a core-valence correlation correction. A very good agreement of data from spectroscopic constants for some of the lowest states of the CsLi and CsLi⁺ molecules with those available in recent theoretical works has been obtained. The existence of numerous avoided crossings between electronic states of ²Σ⁺ and ²Π symmetries is related to a charge transfer process between the two ionic CsLi⁺ and LiCs⁺ systems.