C. Bergman

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.

Local order and phase separation in sulphur–tellurium melts: a neutron scattering study

Abstract A neutron scattering investigation of the local order in liquid sulphur–tellurium was performed for alloys at xs=0.2, 0.3, 0.4, 0.6 and 0.8, at three temperatures. A detailed analysis of the structure in terms of partial coordination number is performed by means of a modelling of the structure factors at large momentum-transfer range. Local order changes are essentially caused by tellurium whose coordination number decrease from 2.6 to 1.8 in a 20% composition range. The change in the local atomic environment is shown to be responsible for the observed miscibility gap that occurs around xs=0.4.

A Peierls distortion in the liquid state: Local order of liquid As

Abstract Local order of liquid arsenic was investigated up to 1100°C by neutron scattering. The coordination number, equal to 3 just above the triple point, exhibits no change with temperature. A Monte Carlo simulation using the directional properties of the p-bonds shows that Peierls-like distortion may occur in the liquid state.

Structure of liquid potassium-antimony alloys : neutron scattering experiments and ab initio molecular dynamics calculations

Neutron scattering measurements have been performed on liquid potassium-antimony alloys at x Sb = 0.15, 0.25, 0.4 and 0.5 in the temperature range 820 < T(K) < 1100 depending on the composition. These compositions are chosen according to the specific features of the phase diagram. The total structure factors are determined and they exhibit a well-defined prepeak at q = 1.15 A for all the alloys except the K-rich one. The pair correlation functions are then obtained. The experimental results show a tendency for segregation at x Sb = 0.15. Ab initio density-functional molecular dynamics (MD) studies have been performed for the liquid alloys with x Sb = 0.25 and 0.5 and for the crystalline intermetallic compounds K 3 Sb, K 5 Sb 4 and KSb. Theory and experiment agree on the existence of a salt-like order at the octet composition, changing continuously to a polyanionic order with chain-like Sb-clusters at equiatomic compositions.

Gallium-arsenic phase diagram: Arsenic-rich liquids determination by neutron diffraction

Abstract An original association of the classical level arm rule and the neutron diffraction technique is presented in order to improve the knowledge of the gallium — arsenic phase diagram in the arsenic-rich part. Neutron diffraction measurements have been performed on Ga 0.3 As 0.7 alloy in the temperature range 1053 ⩽ T K ⩽ 1473 , using a constant specific volume V/m = 0.353 cm3/g. The evolution, versus temperature T, of the intensities of GaAs Braggs peaks allowed us to determine the liquidus of the As-rich part by using the level arm rule. The arsenic molar fractions, x(As), are 0.950, 0.873, 0.756 and 0.702 for T = 1083, 1218, 1373 and 1443 K respectively. The eutectic liquid is located at T = 1074 ± 6 K and 0.95

Atomic and electronic structure of liquid As

Abstract We investigated the atomic structure of liquid arsenic using neutron scattering ( λ=0.715 A ) in a temperature domain ranging from the triple point (T=817°C, p=37 atm) up to T =1100° C ( p ≊100 atm ) . The interatomic distance was found identical to that of the crystalline phases ( d =2.50 A ) and the coordination number Z is strictly equal to 3. This show that the Peierls distorsion is still present in the liquid state, even at the highest temperature of our measurements (1100°C), so that the well-known Z=8-N rule still applies in the liquid state. Quantum Monte-Carlo simulations of the liquid state have been performed using a tight binding description of the cohesive energy due to p electrons and a Born-Mayer repulsive pair potential. At constant volume, a Peierls-like distortion is present at any temperature but its amplitude decreases with temperature. By opposition, a semiconductor-metal transition is predicted when the volume decreases.

Liquid arsenic : A peierls distorsion in the liquid - experimental results and computer simulation

Abstract A neutron diffraction experiment has been performed in liquid arsenic at T=825°C (p=47 atm) just above the triple point (T=817°C, p=36 atm) at the Orphee reactor (Saclay) using λ=0.704 A wavelength. We have found a nearest neighbor distance (d1=2.50 A) and a coordination number (Z=3.0) undistinguishably close to the values of the crystalline phases or to those of the amorphous one. These features suggest that the atomic arrangement in the liquid is reminiscent of the structure of the crystal. We suggest a model of corrugated planes of trivalent atoms. In addition, a Monte Carlo simulation of the liquid structure taking into account the directional properties of the p bonds shows that Peierls-like distortion may occur in the absence of translational invariance. Such a model is consistent with the temperature increase of the electrical conductivity of liquid arsenic.

Origin of the looped two-melt phase in the liquid S-Te system

Recently established new thermodynamic relations among experimentally available data are used to evaluate the contribution of the structural change in the liquid S-Te system to the Darken stability. The analysis of the Darken stability shows that the unusual looped two-melt phase separation that takes place in a small composition interval well above the liquidus line is a consequence of the change in the local order of the liquid. Delicate balance between the entropy term and the contribution from the structural change realizes this unique two-melt phase separation. This local order change that is expected in a small composition range has actually been observed by neutron diffraction measurements.