R. Bellissent

Short-range order in undercooled and stable melts forming quasicrystals and approximants and its influence on nucleation

An icosahedral short-range order has been predicted to prevail in undercooled metallic melts. For melts of pure metals, this hypothesis was recently experimentally confirmed by diffraction experiments. This paper presents results of neutron scattering experiments on the short-range order of stable and undercooled liquids of alloys forming quasicrystals and polytetrahedral crystals. The studies indicate that an icosahedral topological short-range order prevails in all of the investigated melts. The influence of this icosahedral short-range order on the nucleation behaviour of solid phases from the undercooled liquid is discussed.

Short-range order in undercooled melts forming quasicrystals and approximants

Abstract For the first time, the short-range order of deeply undercooled liquids of alloys forming quasicrystalline and polytetrahedral phases (Al13Fe4 and Al74Co26) is investigated by combining the containerless processing technique of electromagnetic levitation with elastic neutron scattering. The experimentally determined structure factor, S(Q), was simulated by assuming that clusters with different structures exist in the liquid. The best fit of the experimental data is obtained under the assumption of an icosahedral short-range order. This supports the nearly 50 year-old prediction of Frank that an icosahedral short-range order prevails in undercooled metallic liquids.

Medium range order investigation in a-Si:H by small angle neutron scattering

Abstract Small angle neutron scattering (SANS) provides us with a very efficient method to study the way in which hydrogen is linked to the a-Si structure by using isotropic substitution of hydrogen by deuterium. The very different scattering lengths of H and D for thermal neutrons lead to very different scattered intensities versus isotopic concentration. From an investigation on sputtered samples, it seems possible to point out : 1. i) the evidence of structural heterogeneities with a mean radius of gyration of about 125 A in pure or hydrogenated a Si, 2. ii) the very strong enhancement or small angle scattered intensities versus hydrogen concentration. These results seem to indicate the presence of low density regions in a-Si and the location of hydrogen in these regions in a-Si:H.

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.

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.

A high temperature neutron diffraction study of a titanosilicate glass

Abstract Neutron diffraction experiments were performed on a K 2 O·TiO 2 ·2SiO 2 glass, from room temperature, through the glass transition region ( T g =885 K ) and up to the liquid state (1360 K), in order to investigate the structural modifications that occur between the glass and the melt. Si remains in tetrahedral coordination in the liquid with a small expansion of the Si–O bond (0.01 A at 1360 K). The Ti–O correlation has been extracted from the data to isolate the short Ti–O distance at 1.68 A from the Si–O peak. Titanium does not undergo a significant coordination change and a short Ti–O distance can be detected in the glassy and liquid states. The diffraction data indicate an expansion of the long Ti–O distances (∼0.03 A at 1360 K) and of the K–O distances (∼0.1 A at 1360 K). Structural changes at the medium range distance are visible and may be responsible for the anomalous thermodynamic properties of these melts.

Structural properties of sulphur-enriched GeSx binary glasses by neutron scattering

Abstract This paper deals with a neutron scattering structural study of binary GeS0 x glasses. Two compositions (GeS 6 and GeS 8.5 ) obtained from different quenching-rates qr (qr ≈ 100, 10 and 1°C s −1 ), were analyzed. The pair correlation function, g ( r ), obtained by Fourier transformation of the total structure factors, S ( q ), exhibit three peaks located around 2.2, 3.5 and 4.4 A. The first peak is due to the SGe and SS bonds while the second one is mainly constituted by the SGeS (3.6 A), SSS (3.3 A) and SSGe bridge-interactions. The third peak corresponds to the third- and fourth-neighbour S···S correlation of S 8 rings as observed in a neutron liquid sulphur study. This third peak has allowed us to describe the presence of S 8 rings in the glassy samples. The proportion of S 8 rings is small in the water-quenched GeS 6 glass (qr ≈ 100°C s −1 ) but increases when the quenching rate used to synthesize these glasses decreases and when the concentration is enriched in sulphur.

Neutron scattering structural study of AlCuFe quasicrystals using double isotopic substitution

Abstract Neutron diffraction data with double isotopes substitution using two isotopes each of Fe and Cu has been obtained on six samples of quasicrystalline AlCuFe with a 6D face-centered icosahedral symmetry. Calculations of heteroatomic and homoatomic pair correlation functions for this structure have been made. We find that there is a localized position of copper on a 6D body center site which gives a clear but still qualitative picture of the structure with iron, copper, and aluminum chemically ordered and occupying successive concentric ‘shells’ in the large atom surfaces centered at the two major atomic positions, n1 = [0, 0, 0, 0, 0, 0,] and n2 = [1, 0, 0, 0, 0, 0,], (indexed with respect to a primitive unit cell) in this 6D NaCl type structure. There is an indication that some of the aluminum is mixed with the iron (or is encapsulated in a small volume inside an iron surface).

aSi:H short range order by neutron scattering

Abstract A neutron scattering study of aSi:H, has been undertaken in order to point out the part played by hydrogen in the structural properties. The isotopic substitution method provide us with the three partial structure factors from which have been deduced the three corresponding pair partial correlation functions. SiH and SiH2 units have been directly put into light. These hydrogenated units have not produced noticeable distortion of the tetrahedral local order.

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.