Jean-Pierre Gaspard

Tight-binding potential for atomistic simulations of carbon interacting with transition metals: Application to the Ni-C system

We present a tight-binding potential for transition metals, carbon, and transition-metal carbides, which has been optimized through a systematic fitting procedure. A minimal basis, including the s and p electrons of carbon and the d electrons of the transition metal, is used to obtain a transferable tight-binding model of the carbon-carbon, metal-metal, and metal-carbon interactions applicable to binary systems. The Ni-C system is more specifically discussed. The successful validation of the potential for different atomic configurations indicates a good transferability of the model and makes it a good choice for atomistic simulations sampling a large configuration space. This approach appears to be very efficient to describe interactions in systems containing carbon and transition-metal elements. By way of example, we present results concerning the epitaxial growth of graphene sheets on (111) Ni surfaces, as well as the catalytic nucleation of carbon nanotubes.

Peierls instabilities in covalent structures. I. Electronic structure, cohesion and the Z=8-N rule

The vast majority of the molecular, crystalline or liquid structures of groups V, VI and VII of the periodic table and their compounds obey the octet rule (Z = 8 - N). The structure and stability of those structures are discussed in a simple tight-binding approximation. In this framework we show that the Peierls electronic instability of a simple cubic structure leads to the octet rule. This instability does not rely upon the periodicity and consequently may occur in crystalline, amorphous or liquid matter. In a general discussion on the stability of covalent structures, we show that the existence of a Peierls distortion is governed by the balance between the attractive (band) term and the empirical repulsive term. As the hardness of the latter increases when going down the periodic table, this explains why the Peierls distortion is stronger for the light elements.

Local orders in II–VI liquid compounds

Abstract The structure of liquid CdTe, HgTe and ZnTe is studied by neutron scattering. Upon melting, drastically different behaviors are found: CdTe and ZnTe keep their fourfold coordination in the melt and remain semiconductors, whereas HgTe becomes sixfold coordinated and metallic. This dissimilarity is discussed in terms of the electronegativity difference and of the repulsive potential hardness. Entropie aspects are also discussed.

Monte Carlo calculation of the configurational entropy and combinatory factor on binary alloys

Abstract A method is presented for the calculation of free energy and entropy of regular solutions. It is based on a calculation of the configurational density of states (combinatory factor), from Monte Carlo computations. Application to the planar square lattice shows an excellent agreement with exact results. Results on a bcc lattice are presented.

Structure of liquid II–IV compounds: CdTe

Abstract The structure factor of CdTe is studied by neutron diffraction just above its melting point (1100°C). Short wavelength neutrons (λ = 0.35 A ) have been used in order to avoid the strong absorption of Cd around λ = 0.7 A . The structure factor and the pair correlation function differ substantially from those of group IV semiconductors (Si, Ge) or III–V compounds (GaAs, InSb) in the liquid state. The first peak of the pair correlation function coincides with the nearest neighbour distance in crystalline CdTe and the coordination number lies between 3 and 4. We suggest that the structure of liquid CdTe is similar to the structure of the continuous random network (Polk or Connell-Temkin model). This is consistant with the increase of the electrical resistivity with temperature.

Structural study of coordination defects in amorphous GaAs by a combination of EXAFS and X-ray anomalous scattering experiments

The short range order around Ga and As sites in a-GaAs films has been investigated by EXAFS and anomalous X-ray scattering. The three partial pair distribution functions indicate a strong chemical ordering. The presence of three-fold coordinated As is shown to play an important role in the relaxation of the network.

Local order of the high-pressure metallic phase of liquid selenium: a diffraction study

Liquid selenium undergoes a phase transition towards a metallic phase when sufficient pressure is applied. We performed an x-ray diffraction experiment at the ID30 beamline of the ESRF at the wavelength of 0.149 A to investigate the local order of the metallic liquid. The diffraction pattern of liquid Se has been recorded with the large-volume Paris-Edinburgh cell between (20 °C, 3 GPa) and (1650 °C, 4.1 GPa). The local order of the metallic liquid selenium is found to be close to that of liquid tellurium. In particular, the coordination number Z increases with temperature from 2.6 to 3.0.

On the structure of liquid tellurium

Abstract The atomic structure and the bonding mechanism in liquid tellurium are investigated by a tight binding Monte Carlo technique. The chain structure is preserved but a third covalent bond emerges with an intermediate interatomic separation (3.15 A). In addition a bond length alternation inside the chain appears in agreement with recent EXAFS experiments and the valence angles are significantly reduced. The electronic structure is studied and particular attention is paid to the modifications of lone pair interactions in the liquid structure.

Structure of high temperature fluid selenium

Monte Carlo simulations based on a semi empirical tight binding model including dispersion forces were performed to study liquid selenium at temperatures between 600 and 2000 K. The atomic structures obtained are in agreement with the X-ray scattering and extended X-ray absorption fine structure (EXAFS) data in a range of temperatures and densities. A correlation between the conductivity of high temperature fluid selenium and the degree of branching and breaking of the selenium chains is observed.

Evolution of the Peierls distortion in liquid AsxSb1−x compounds

Abstract The local order in the liquid binary alloy As x Sb 1− x (0⩽ x ⩽1) is studied by short-wavelength ( λ =0.7 A) neutron scattering. The coordination number ( Z ) and the interatomic distances ( d ) are analyzed in terms of the As concentration: Z increases continuously, from a value of 3.5 for pure arsenic to a value of 6.3 for pure antimony, whereas d shows a departure from Vegards rule. The interatomic distance increases in the range 0.15⩽ x ⩽1.00 and is almost constant in the range 0.00⩽ x ⩽0.15. A simple tight-binding model is developed, which indicates that the relevant parameter is the hardness of the core repulsion between the atoms.