Philip S. Salmon

Neutron and x-ray diffraction studies of liquids and glasses

The techniques of neutron diffraction and x-ray diffraction, as applied to structural studies of liquids and glasses, are reviewed. Emphasis is placed on the explanation and discussion of the experimental techniques and data analysis methods, as illustrated by the results of representative experiments. The disordered, isotropic nature of the structure of liquids and glasses leads to special considerations and certain difficulties when neutron and x-ray diffraction techniques are applied, especially when used in combination on the same system. Recent progress in experimental technique, as well as in data analysis and computer simulation, has motivated the writing of this review.

Topological versus chemical ordering in network glasses at intermediate and extended length scales

Atomic ordering in network glasses on length scales longer than nearest-neighbour length scales has long been a source of controversy. Detailed experimental information is therefore necessary to understand both the network properties and the fundamentals of glass formation. Here we address the problem by investigating topological and chemical ordering in structurally disordered AX2 systems by applying the method of isotopic substitution in neutron diffraction to glassy ZnCl2. This system may be regarded as a prototypical ionic network forming glass, provided that ion polarization effects are taken into account, and has thus been the focus of much attention. By experiment, we show that both the topological and chemical ordering are described by two length scales at distances greater than nearest-neighbour length scales. One of these is associated with the intermediate range, as manifested by the appearance in the measured diffraction patterns of a first sharp diffraction peak at 1.09(3) Å-1; the other is associated with an extended range, which shows ordering in the glass out to 62(4) Å. We also find that these general features are characteristic of glassy GeSe2, a prototypical covalently bonded network material. The results therefore offer structural insight into those length scales that determine many important aspects of supercooled liquid and glass phenomenology.

Real Space Manifestation of the First Sharp Diffraction Peak in the Structure Factor of Liquid and Glassy Materials

The problem of those discernible features of the intermediate range order (IRO) which can be attributed to the first sharp diffraction peak (FSDP) observed in the structure factor of many liquid and glassy materials is approached by treating this peak as a distinct feature. It is found, by considering the measured partial structure factors, Sαβ(k), for molten ZnCl2, GeSe2, MgCl2, NiBr2 and Nil2 and the measured total structure factors, F(k), for glassy SiO2, PS4 and liquid CCl4, that the propensity of the FSDP to have a prominent effect on the underlying features of the IRO depends noticeably on the system type. Specifically, the FSDP confers a marked oscillatory character of periodicity 2π/k1 (where k1 is the FSDP position) on the IRO when the local structural units, which give rise to the density fluctuations on the IRO scale, exist as stable entities for a timescale τ ≫ 5 × 10-12 s. The FSDP therefore accounts for the discernible features of the underlying IRO for the viscous glass forming liquids ZnCl2 and GeSe2, for the glasses SiO2 and PS4, and for the molecular liquid CCl4. The influence of the FSDP on the IRO is less pronounced for molten MgCl2 and is negligible for molten NiBr2 and Nil2, both of which have a high cation mobility which leads to a relative instability of the Ni2+ centred structural units. The effect on the FSDP of temperature and pressure are briefly considered as are the development of the FSDP in molten ZnX2 (when X is changed from Cl to I to Br) and the minimum size of r-space model which is required if the FSDP is to be accurately predicted.

Structure of molten lanthanum and cerium tri-halides by the method of isomorphic substitution in neutron diffraction

The total structure factors of the molten trivalent metal halides MX3, where M3+ denotes La3+ or Ce3+ and X- denotes C1-, Br- or I-, have been measured by using neutron diffraction. Difference function methods were then applied on assuming that the LaX3 and CeX3 melts for a given halide ion are isomorphic. The results which follow from this assumption show that the first sharp diffraction peak in the measured total structure factors arises from cation correlations and its movement to lower scattering vector values with increasing anion size is consistent with an enhanced separation in real space of cation centred polyhedra. On melting the MX3 salts exhibit a decrease in the coordination number of both the cations and anions. In the liquid state the M-X coordination environment is asymmetric with M-Cl, M-Br and M-I nearest-neighbour distances of 2.93(2) A, 3.01(2) A, 3.18(2) A and M-Cl, M-Br and M-I coordination numbers of 8.2(2), 7.4(2), 6.7(2) respectively. The Cl-Cl, Br-Br and I-I nearest-neighbour distances are 3.58(3) A, 3.76(3) A, 4.13(2) A respectively and there is a significant penetration of the X-X partial pair distribution function into the first peak of the M-X partial pair distribution function for all three anions. The Cl-Cl, Br-Br and I-I coordination numbers are 9.2(2), 8.7(2) and 8.2(2) respectively if the M-M coordination number is two.

Structure of glassy and liquid GeSe2

The partial structure factors of bulk-quenched glassy GeSe 2 were measured by using the method of isotopic substitution in neutron diffraction to enable the first detailed comparison at the partial pair distribution function level of a covalently bonded network system in both its glassy and liquid phases. The results show that the basic building block of the glass is the Ge(Se 1/2 ) 4 tetrahedron in which 34(5)% of the Ge atoms reside in edge-sharing configurations. The intrinsic chemical order of the glass is, however, broken with a maximum of 25(5 )% Ge and 20(5)% Se being involved in homopolar bonds at distances of 2.42(2) and 2.32(2) A, respectively, which is consistent with the existence of these features in the liquid phase of GeSe 2 . Like for the liquid, concentration fluctuations in the glass are found to extend over distances characteristic of the intermediate-range atomic ordering as manifested by the appearance of a first sharp diffraction peak at 1.00(2) A -1 in the Bhatia-Thornton concentration-concentration partial structure factor. A comparison is made between the measured partial structure factors and recent first principles molecular dynamics simulations for the glassy and liquid phases. It is found that the most significant disagreement between experiment and simulation occurs with respect to the Ge-Ge correlations and that the simulated results for the glass are too liquid-like, reflecting the use of a quench time greatly in excess of that achieved experimentally.

Structure of molten MCl3 systems from a polarizable ion simulation model

Computer simulations of a range of molten salts of stoichiometry MX3 using a polarizable, formal charge ionic interaction model are described. The systems studied — LaCl3, TbCl3, and YCl3 — span a range of cation sizes and the interaction model is a “generic” one, in the sense that the cation size is the only parameter in the interaction potential which distinguishes one system from another. The liquid structures predicted from the simulations are compared with recently obtained neutron diffraction data. Excellent agreement is found, except that the first coordination shell seems to be too tightly bound in the computer simulations. The cation in LaCl3 is found to be 7–8 fold coordinate, and the coordination number drops to 6 for the smallest cation considered (Y3+), so that the coordination number in these systems does not change substantially on melting, in contrast to earlier reports. The polarization effects promote a significant degree of edge-sharing between these coordination polyhedra relative to p...

A neutron diffraction study on the structure of liquid germanium

The static structure factor S(k) and pair distribution function g(r) of liquid germanium at 1000 degrees C were measured using neutron diffraction. A wider k-range was covered that in previous diffraction experiments on liquid Ge and particular attention was given to the data analysis procedure used to extra S(k) from the measured intensities. The g(r) has a first peak at 2.66+or-0.02 AA and gives an averaged coordination number of 5.6. The results are discussed with reference to the previous diffraction studies on liquid Ge and systematic differences between the neutron and X-ray results are observed.

The structure of Cu2+ aqueous solutions

The neutron first-order difference method has been used to determine the Cu2+-D2O and Cl--D2O coordination in 4.32 molal (mol kg-1) CuCl2 and the Cu2+-D2O coordination in 2.00 molal Cu(ClO4)2. In the copper chloride solution the Cu2+ near-neighbour distances are given by rCuO=1.96+or-0.03 AA and rCuD=2.54+or-0.03 AA and the hydration number is nO=3.4+or-0.2. The Cl- coordination is described by distances rClD(1)=2.27+or-0.03 AA and rCuO=3.25+or-0.05 AA and a hydration number nD=3.3+or-0.4. In the case of the perchlorate solution a Cu2+ near-neighbour distance of rCuO=1.96+or-0.03 AA is found with nO=4.1+or-0.3. The observed structure is discussed with reference to the ion-water dynamics and is contrasted with that of Ni2+ aqueous solutions.

The dynamics of water molecules in ionic solution. II. Quasi-elastic neutron scattering and tracer diffusion studies of the proton and ion dynamics in concentrated Ni2+, Cu2+ and Nd3+ aqueous solutions

For pt.I see ibid., vol.20, p.1573 (1987). The technique of high-resolution incoherent quasielastic neutron scattering (QENS) is used to measure the translational diffusive motion of water protons in concentrated NiX2, CuX2 and NdX3 aqueous solutions in which X=Cl-, ClO4- and/or NO3-. The technique of tracer diffusion is also used in order to measure the cation selfdiffusion coefficient in concentrated NiX2 and CuX2 aqueous solutions in which X=Cl- and ClO4-. It is shown that at room temperature ( approximately=25 degrees C) the cation-water proton binding times are described by the limits tau 1(Ni2+)>or approximately=5*10-9 s, tau 1(Cu2+) 10-10 s and that for the Ni2+ solutions, which are in slow exchange, the diffusion coefficient of protons bound to the cation is measurable using the QENS method.

Microvoids in chalcogenide glasses studied by positron annihilation

Abstract Microvoids have been detected in glassy As2Se3 by positron lifetime spectroscopy. The positron lifetime in perfect and defected crystalline As2Se3 was calculated and the results correlated with the size of the (open volume) defects. This correlation suggests that the observed mean lifetime τ = 345(2) ps for the glassy material corresponds to microvoids of on average about three missing atoms (i.e., 2.5–3 A radius). The lower limit of the microvoid concentration per atom is estimated to lie between 0.1 and 100 ppm (corresponding to a volume fraction 10−5−10−2%), well below the detection threshold of small-angle X-ray scattering. The microvoids are stable on heating to 260°C, i.e., after annealing at temperatures 80°C above the glass transition temperature. The present results are in conflict with earlier suggestions that positrons are trapped only in the vicinity of point-like negatively charged defects in amorphous As2Se3. Frequent observation of long positron lifetimes leads to the conclusion that microvoids are a feature common to a wide range of chalcogenide glasses. The consequence of these results for the recent model of Elliott, in which the first sharp diffraction peak in the total structure factor of covalently bonded glasses is attributed to the chemical ordering of microscopic voids around cation-centred units, is discussed.