Hall coefficients for A15 compounds: Nb-Sn and V-Si

The Hall coefficients, R_H, for A15 structure single crystal V₃Si and polycrystal V25.25 at.% Si and Nb 26 at.% Sn have been measured as functions of temperature. The data between the superconducting transition temperature, T_c, and 80 K for the Nb-Sn show a small dip centred arounf 30 K which accompanies a cubic-to-tetragonal lattice distortion commencing at about 45 K. R_H for the VSi samples, which are believed to be non-transforming, is temperature independent in the range 17–40 K and equal to 2.0 ± 0.2×10^?10m³C^?1. These result are discussed in relation to the effect of the tetragonal distortion on the band structures of these compounds.     

Glass formation and vibrational properties in the (Ge,Sn) system

The glass forming tendency and infrared reflectance spectra are investigated in the (Ge, Sn) system. It is found that the glass formation of the SnSe system is much more difficult than that of the Ge system. This is mainly ascribed to the ionicity difference between GeSe and SnSe bonds. Infrared spectra in the glassy Ge_1-xSe_x system at the composition 0.67<x<1.0 indicate the presence of clusters within which the atomic connectivity is similar as in the glassy GeSe₂. The vibrational modes caused by the $\text{SnSe}{}_{\mathrm{<mtext>4</mtext><mtext>2</mtext>}}$ tetrahedron are found in the glassy (Ge, Sn)-Se system. It is suggested that tin atoms are mainly incorporated into the clusters. Importance of the medium range order is discussed in the formation of the glasses.     

Theoretical prediction of the structural,elastic, electronic and thermodynamic properties of V3M (M = Si,Ge and Sn) compounds

Density functional theory (DFT), is used in our calculations to study the V₃M (M = Si, Ge and Sn) compounds, we are found that V₃Sn compound is mechanically unstable because of a negative C₄₄ = −19.41 GPa. For each of these compounds considered, the lowest energy structure is found to have the lowest N(E_f) value. Also there is a strong interaction between V and V, the interaction between M (M = Si, Ge, Sn) and V (M and M) is negative, not including Si [Sn]. In phonon density of states PDOS, the element contributions varies from lighter (high frequency) to heaviest (low frequency).     

Superconducting and normal state properties of tantalumcarbonitrides under hydrostatic pressure

We have investigated the formation and the normal- and superconducting properties of TaC_xN_y. By analogy to NbCN the superconducting transition temperature of the nitrogen-rich samples increases with pressure and decreases for carbon-rich samples. This is correlated with a different resistivity vs temperature behaviour.     

Dependence of the superconducting transition temperature on electron per atom ratio and lattice deformation in noble metal alloys

Superconducting transition temperature measurements of the α-phase alloys CuSi, CuGe, AgGe, AgAl, AuGe, and AuZn are reported. It is found that for alloys with the same electron per atom ratio, T_c values are highest when the pure metal lattice is deformed the least by the solute.     

High pressure induced shifts in the superconducting transition temperature of sputtered films

Both negative and positive shifts in the superconducting transition temperature with negligibly small changes in the resistance ratio were observed in the NbGe and V₃Si films, respectively, under pressure up to ～ 22 kbar. The results are discussed along with the roles of defects recently proposed in these materials.     

High electronic specific heat of some cubic UX3 intermetallic compounds

The low temperature specific heat of cubic UX₃ intermetallic compounds with X = Al, Ga, In, Si, Ge and Sn have been measured. High values for the coefficient of the electronic specific heat have been found, ranging from 14 to 169 mJ/mol K².     

Blue photoluminescence from SiGeSiO2 co-sputtered films

A blue photoluminescence band centered at 440 nm was observed from SiGeSiO₂ co-sputtered films at room temperature. This band gains intensity after the film was annealed at a temperature around 900°C in N₂ atmosphere. From analysis of photoluminescence excitation, Raman and X-ray photoelectron spectra, it turns out that the luminescence is probably from some interfacial state between Si_1−xGe_x nanoparticles and the SiO₂ matrix.     

Heats of formation of interstitial compounds and their alloys

The bonding of compounds of the form RY in the NaCl structure is predominantly ionic, as shown by spectroscopic studies of charge transfer. A formula for the heats of formation of these compounds (R=Ti, Zr, Hf, V, Nb, Ta and Y=C, N, O) is constructed based on a modification of Pauling's electronegativity table. The formula yields good agreement with experiment, and is refined to yield more accurate values of the R electronegativities. The heat of formation of NbN is anomalously small, and this may be correlated with its high superconducting transition temperature. Miscibility gaps in the alloy systems R′_xR″_1?xC are discussed.     

XPS valence band spectra of Nb3Sn,Nb and Sn

XPS valence band spectra of Nb₃Sn, Nb and Sn have been obtained under high resolution. The 4d band structure in the Nb₃Sn spectrum is similar to that seen in Nb metal.     

The BM5Se9 phases: Mössbauer studies of the superconductors and the ferromagnets

Syntheses performed in the A₃B/MSe₂ (A = Sb, Ga, Sn) (B = Nb, V, Ta) (M = V, Nb, Ta) systems lead to the formation of B(M/A)₅Se₉ phases which could be ferromagnets or superconductors. It has been shown that the presence of V modifies the superconducting exchanges and allows ferromagnetic couplings. Mössbauer spectroscopy of both ¹²¹Sb and ¹¹⁹Sn gives local electronic structure of the SbNb₅Se₉ superconductor and the Sn_0.94Ga_0.06V_0.30Nb_4.70Se₉ ferromagnet. For the latter compound, a complete study as a function of temperature is carried out showing that the Sn environment is not modified when the ferromagnetic transition occurs. The Debye temperature is calculated and compared to those obtained for other selenide compounds.     

He(I) and He(II) photoelectron spectra of some organo-mercury compounds with carbon π-systems

The UV photoelectron spectra of a series of aromatic mercury derivatives {(C₆H₅)₂ Hg, (pCH₃C₆H₄)₂Hg, C₆H₅HgCl, pCH₃C₆H₄HgCl} and of a series of alkynyl mercury derivatives {(CH₃C)₂Hg, ((CCH₃)₃CCC)₂Hg, (ClCC)₂Hg} are reported and discussed. Particular attention is devoted to possible interactions between mercury orbitals and π systems of the ligands. In this respect differences are observed between the two series: in the alkynyl derivatives ligand-field effects are evident on the 5d mercury orbitals, whilst they are not observable in the aromatic compounds.     

Bond activation sequence observed in the chemisorption and surface reaction of ethanol on Ni(111)

The mechanism of ethanol decomposition on the Ni(111) surface has been investigated between 155 and 500 K. The sequence of bond scission steps which occur as ethanol undergoes dissociative reactions on this surface has been deduced using deuterium and ¹³C isotopic labels. Bond activation occurs in the order (1) OH, (2) CH₂ (methylene CH), (3) CC, (4) CH₃ (methyl CH). The products observed are CH₃CHO(g), CH₄(g), CO(g), H₂(g) and surface carbon, C(a). The latter species exhibits a carbidic AES lineshape in the temperature range 450 to 670 K, at which temperature it dissolves into the Ni bulk. Acetaldehyde, CH₃CHO, and methane, CH₄, desorb with the same threshold temperature (260–265 K), and the formation of both of these products is controlled by scission of the methylene CH bond (CH₂ group). The CH₃ group is cleaved from the intermediate surface CH₃CHO species to form CH₃(ads). H₂ exhibits a broad, doublet desorption peak from 300 to 450 K. The carbonoxygen bond in ethanol remains intact and CO ultimately desorbs in a single desorption limited process (T_p = 430 K). A small fraction of CO(a) species undergo exchange with the carbidic surface carbon in a minor process observed above 440 K.     

Structure and optical properties of GeGaS films deposited by thermal evaporation

Amorphous GeGaS thin films have been successfully deposited onto glass slides at room temperature by the thermal evaporation technique. The structural units of the films were studied using Raman spectroscopy. In addition to the basic structural units of GeS₄ tetrahedra, there are some SS and GeGe homopolar bonds which exist in the films. The increase in Ge atoms leads to the replacement of SS bonds by GeGe bonds, and the isolated Ge(Ga)S₄ tetrahedra units transform into corner-sharing or edge-sharing Ge(Ga)S₄ tetrahedra units in the films. The refractive index and optical band gap were derived from transmission spectra of films. The values of optical band gap decrease while the refractive indices increase with increasing Ge content. Composition dependence of optical band gap and refractive index has also been interpreted in terms of the variation in the structure of films based on Raman spectra.     

Ab initio study of some fundamental properties of the M3X (M=Cr,V; X=Si,Ge) compounds

M₃X (M=Cr, V; X=Si, Ge) compounds are studied using first-principles calculations based on the Density Functional Theory (DFT). It is found that the bulk of Cr₃X (X=Si, Ge) compounds are comparable to those of Al₂O₃, the nearest-neighbor distance D_M−M and D_M−X in these compounds increase and the bulk modulus decrease, there is a strong interaction between M and M (M=Cr the interaction is stronger). Also the interaction between M (M=Cr, V) and X (X=Ge) is negative, an anti-bonding-type interaction is dominant between these atoms.     

Scanning tunneling microscopy observation of initial growth of Sn and Ge1−xSnx layers on Ge(0 0 1) substrates

We have investigated the initial growth of Sn and Ge_1−xSn_x layers on Ge(0 0 1) surface by using scanning tunneling microscopy. After the growth of a 0.035 ML-thick Sn layer at room temperature, Sn clusters lining vertically to a dimer row was observed. In the case of the 0.035-0.018 ML-thick Sn growth at 250 °C, the characteristic surface reconstruction with the step-edge undulation like a comb was observed. In the growth of a Ge_0.994Sn_0.006 layer at 250 °C, the multilayer polynuclear growth with a lot of two-dimensional small domain was observed. These surface reconstructions should be accounted for by the large compressive stress induced in the surface layer due to the incorporation of Sn atoms.     

Effect of operating temperatures on the coefficient of performance of active carbon-methanol systems

Experimental and predicted results of an active carbon (AC35)-methanol pair as a function of operating temperatures are discussed.Experimental COP's higher than 0.5 are obtained when the evaporating temperature lift is less than 25°C. The COP is seen to be very sensitive to the evaporating and adsorbing temperatures. The regenerating temperature is of the order of 100°C which makes that pair a good candidate for solar cooling applications.A comparison between the results obtained with that pair and predicted performances of three other pairs (NH₃H₂O, ZeoliteH₂O and another active carbon-methanol pair) shows that:

• 1.(1) two adsorber cycles give higher COP's than the NH₃H₂H₂O continuous cycle;
• 2.(2) intermittent adsorptive cycles give higher COP's than the NH₃H₂O continuous cycle as long as the evaporating temperature lift is less than 50°C;
• 3.(3) among the solid adsorbents, the active carbon-methanol pairs seem to be the most promising for cooling applications. The choice of the active carbon depends on the evaporating temperature lift: AC35 is very well adapted for temperature lifts larger than 30°C; ACLH could be better adapted to smaller evaporating temperature lifts, as suggested by Passos et al.

     

Structural instability in new magnetic heusler compounds

The crystal structure and magnetic properties have been determined for a new series of compounds of the form Rh₂TSn. For T = Mn, Ni, or Cu, the room temperature structure is the fully ordered cubic Heusler structure. For T = V, Cr, Fe, or Co, a new structure is observed which is an exceptionally large tetragonal distortion of the Heusler structure (c/a = 1.18?1.27). The appearance of this tetragonal distortion is attributed to an electronic instability of the band Jahn-Teller type.     

Structural phase transition in mixed IV–VI compounds

On the basis of a linear chain model and CPA, structural phase transition in mixed IV–VI compound A_χB_1?χTe (where A and B stand for Pb, Sn or Ge) is discussed. It is suggested that observed non-linear nature of T_c?χ relation may be attributed to the change in the electronic state with χ.     

The photoelectron spectra of the tetrafluoro and tetramethyl compounds of the group IV elements

The photoelectron spectra of CF₄, SiF₄, GeF₄, C(CH₃)₄, Si(CH₃)₄, Ge(CH₃)₄, Sn(CH₃)₄, and Pb(CH₃)₄ h