LMTO Band Structure Calculations of ThCr2Si2-Type Transition Metal Compounds

The electronic structures of ThCr₂Si₂-type compounds were studied by means of self-consistent LMTO band structure calculations. Different bonding interactions in SrRh₂P₂are analyzed and their dependence on the electron count are discussed in terms of the formal substitution of elements. The overall bonding situation can be characterized as an interplay between covalent, metallic, and ionic interactions, although metal–metal bonding plays an important role. Particularly, the evolution of the interlayer bonding between the nonmetal atoms by changing the transition metal is examined in more detail. It turns out that the shortening of the interlayer bonds by filling thedshell of the transition metal is due to an increasing occupation of nonmetal bonding states which are pushed up to the vicinity of the Fermi level by antibonding metal–nonmetal interactions. The band structures of superconducting LuNi₂B₂C and nonsuperconducting SrRh₂P₂are compared and their similarities are pointed out. A van Hove singularity, generated by metal–metal interaction, coincides with the Fermi level in LuNi₂B₂C and lies about 0.2eV higher in SrRh₂P₂. By doping, it should be possible to induce superconductivity in SrRh₂P₂and related compounds.     

Preparation and structure of (NbSe4)3.33 I. [I3Nb10Se40]

The structural determination of a novel linear chain compound (NbSe₄)_3.33 I is reported. The structure was solved by means of Patterson and Fourier syntheses and refined toR = 0.030, R_W = 0.040, for 566 independent reflexions [I ≥ 3σ (I)]. The structure is built up of infinite [NbSe₄] chains along the c axis (tetragonal symmetry). Iodine atoms are located within the channels between these chains. The iodine atoms do not show the same distribution within all the channels as is the case for related compounds such as (NbSe₄)₃I, (TaSe₄)₂I. This new compound undergoes a phase transition at 285 K associated with a change density wave (CDW) origin.     

Scanning tunneling and atomic force microscopy study of the misfit layer compounds (LaS)1.14(NbS2)n (n=1, 2) and [(Pb,Sb)S)]1.14NbS2

The misfit layer compounds (LaS)_1.14(NbS₂)_n (n=1, 2) and [(Pb,Sb)S]_1.14NbS₂ were examined by scanning tunneling microscopy (STM) and atomic force microscopy (AFM). In these compounds the NaCl-type double MS (M=La, Pb, Sb) layers (Q layers) alternate with the NbS₂ layers (H layers) made up of NbS₆ trigonal prisms. It was possible to record AFM and STM images for only the H layers for (LaS)_1.14(NbS₂)_n, but for both the H- and Q-layers for [(Pb,Sb)S]_1.14NbS₂. Partial and total electron density plots of the H and Q layers were calculated to interpret the observed STM and AFM images. The bright spots in the STM and AFM images of the H layer correspond to S atoms, and those of the Q layer to Pb and Sb atoms. The STM images for the Q layers of [(Pb,Sb)S]_1.14NbS₂ suggest that a short-range ordering of the Pb and Sb atoms occurs in the (Pb,Sb)S sheets of the Q layer.     

Thermodynamics of viscous flow and elasticity of glass forming liquids in the glass transition range

The elastic moduli of glasses from different chemical systems, including oxide, chalcogenide, oxynitride, and metallic, were investigated through the glass transition (T(g)), typically from 0.4 to 1.3 T(g). These data were used to interpret the temperature sensitivity of the shear viscosity coefficient obtained on the same materials. The relevant Gibbs free activation energy was estimated from the apparent heat of flow by means of the temperature dependence of the shear elastic modulus. The activation entropy associated with the viscous flow was also derived and was found to correlate with the fragile versus strong character of the glass forming liquids. Finally, the physicochemistry of the flow process was described on the basis of the glass network de-structuration which shows up through the temperature dependence of Poisson's ratio, and an expression for the shear viscosity coefficient is proposed which is chiefly based on the high temperature elastic behavior.     

Charge density waves in layer structures: A NMR study on a 2HNbSe2 single crystal

We have studied by pulsed NMR a single crystal of 2HNbSe₂ at ambient pressure and under hydrostatic pressure of 21 kbar in the temperature range 4.2–273 K. Our results are consistent with the onset of incommensurate charge density waves (ICDW) at T_CDW = 33 K atP = 1 bar and 26 K at P = 21 kbar. Below T_CDW, the lineshapes of the (m → m ? 1) transitions agree with a local distribution of Knight shift and electric field gradients respecting the symmetry of a triple ICDW, while above T_CDW, pre-transitional broadening is observed. The product T₁T = 500 ± 100 msk was found constant in the temperature range 4.2–77 K and pressure independant.     

The Na-TiS2 system: A critical discussion of the phase boundaries,structural and NMR studies

We present new evaluations of phase boundaries in the Na_xTiS₂ system from electrochemical intercalation and from X-ray and NMR measurements in samples intercalated using the liquid ammonia technique. After a critical discussion of the influence of the method of intercalation on the phase limits we present an extensive NMR study of the system. ²³Na quadrupolar coupling determinations support the fact that the low concentration (x < 0.25) phase II is a stage 2 phase. ²³Na Knight shift results show that the transition from the Ib trigonal prismatic phase to the Ia trigonal antiprismatic phase with increasing Na concentration takes place with a change in the electronic band structure. The stability of this phase is discussed in terms of the balance between elastic and electronic energies.     

Surface damage of soda-lime-silica glasses: indentation scratch behavior

Contact mechanics problems are of fundamental interest both to understand the process of surface damage and matter removal in brittle materials, and to develop a method to evaluate their scratch resistance. In order to get insight into these problems in the case of soda-lime-silica glasses, a classical indentation apparatus was used, and an original scratch experimental setup was designed, allowing for a monotonic loading (or unloading) of the indenter combined with a controlled sliding of the specimen beneath the indenter. The influences of the normal load, the moisture level and the glass composition have been studied, and clear relationships were established between the glass compositions and the indentation-scratching behavior. The indentation and scratching characteristics such as the critical-crack-initiation loads and the transition loads between the different scratch regimes were correlated and interpreted in the light of the brittleness index and structural considerations.