Jonathan C. Wasse

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...

The structure of polaronic electron cavities in lithium–ammonia solutions

Neutron diffraction has been used in conjunction with isotopic substitution of deuterium for hydrogen to study the structure of lithium-ammonia solutions, at concentrations spanning the metal-nonmetal transition. Detailed analysis and visualization of our experimental data has been carried out via iterative refinement of a three-dimensional molecular model, allowing us to obtain unique insight into the formation of polaronic electron cavities in the solutions. At low electron concentrations the solutions are nonmetallic, and the ammonia molecules are orientated around cavity centres to form Bjerrum-type defects. As the electron content is increased, the solutions become metallic, and we find evidence of percolation channels through the solvent. The dissociated electrons therefore play an active role in determining the structure of these solutions, and serve to disrupt the hydrogen bonding present in liquid ammonia.

The structure of saturated lithium– and potassium–ammonia solutions as studied by using neutron diffraction

The technique of isotopic substitution in neutron diffraction has been used to measure the structure of saturated solutions of lithium and potassium in ammonia. Isotopic substitution of *N by 15N, combined with difference analysis, shows that K+ acts as a “structure breaking” ion while Li+ acts as a “structure making” ion. As a consequence, it is only for the latter that we observe intermediate range order, in the form of a pre-peak at ∼1 A−1 in the k-space data. From our analysis, we also find that lithium is tetrahedrally coordinated by ammonia molecules, at both 230 and 100 K. Potassium, on the other hand, is octahedrally coordinated at 160 K.

X-ray diffraction studies of solutions of lithium in ammonia: The structure of the metal-nonmetal transition

The structure of solutions of lithium in ammonia has been studied at 0, 2, 8, and 22 mol % metal (MPM) and 200 K by wide-angle x-ray diffraction. The principal diffraction peak shifts from 2.14(2) A−1 at 0 MPM to 1.93(3) A−1 at 22 MPM, reflecting the 30% decrease in overall density as the solution expands to accommodate the excess electrons. We find that the solvent is significantly perturbed over both the short- and intermediate-length scales. The nearest neighbor (N–N) coordination number decreases from 11.8(10) at 0 MPM to 7.6(10) at 22 MPM. In addition, electrostriction around the fourfold coordinated lithium ions causes N–N correlations to become progressively shorter as concentration is increased. At 22 MPM a strong diffraction prepeak is located at 1.05(3) A−1. Upon dilution to 2 MPM, our experiments find that this feature shifts to 1.29(5) A−1. We conclude that the prepeak observed in our experiments is a signature of polaronic solvent cavities of approximate radius 2.6 A. The first solvation shel...

The structure of lithium–ammonia and sodium–ammonia solutions by neutron diffraction

The microscopic structures of lithium–ammonia and sodium–ammonia solutions have been measured by the technique of isotopic labeling in neutron diffraction, at and above the metal–nonmetal transition that occurs in the range 2–8 mole percent metal (MPM). Substitution of *Li by 6Li has been used to obtain the lithium-centered first-order difference function at 8 MPM and 230 K. This function shows us that the lithium cations are strongly solvated by 4 ammonia molecules. Substitution of *N by 15N has then been used to probe the nitrogen-centered structure in lithium–ammonia solutions at 4, 8, and 12.5 MPM and sodium–ammonia at 12.5 MPM. These functions give us new insight into both the disruption of hydrogen bonding as alkali metal is added to ammonia, and the solvation structure of the sodium cations. The former manifests itself through a progressive loss of the hydrogen-bonded N–D peak at ∼2.4 A. The latter appears as an N–Na shoulder at ∼2.5 A, and shows us that sodium is solvated by ∼5.5 ammonia molecules...

Structure of molten trivalent metal bromides studied by using neutron diffraction: the systems DyBr3, YBr3, HoBr3 and ErBr3

The structure of the salts MBr3, where M 3+ denotes Dy 3+ ,Y 3+ ,H o 3+ or Er 3+ ,w as investigated by using neutron diffraction. On heating DyBr3, YBr3 and HoBr3, a phase transition from the FeCl3-type crystal structure to possibly the YCl3-type crystal structure was observed. The liquids were studied at the total-structure-factor level and difference function methods were also applied to DyBr3 and YBr3 by assuming isomorphous structures. The melts are found to comprise distorted MBr 3− 6 octahedra with M-Br distances comparable to the sum of the ionic radii and there is evidence for a substantial number of edge-sharing configurations. The octahedra pack to give intermediate-range ionic ordering as manifested by the appearance of a first sharp diffraction peak at 0.79(2)-0.87(2) A −1 which is associated with cation correlations

Structure of glassy AsTe: the effect of adding a small quantity of Cu or Ag

The structure of the glasses Cu0.1AsTe and Ag0.1AsTe is studied by using the method of isotopic substitution in neutron diffraction. It is found that the addition of metal atoms M (=Cu or Ag) does not significantly affect the structure of glassy AsTe on either the short- or intermediate-range length scales. Cu and Ag both take coordination numbers in excess of four in an As-Te network that is compact by comparison with its As-S and As-Se counterparts. In high-M-content M-As-Te glasses, Cu and Ag again take coordination numbers greater than four and the absence of significant ionic conductivity for the Ag-based materials is attributed to a compact As-Te network that restricts pathways along which silver ions can move.

Proton dynamics in lithium-ammonia solutions and expanded metals.

Quasielastic neutron scattering has been used to study proton dynamics in the system lithium-ammonia at concentrations of 0, 4, 12, and 20 mole percent metal (MPM) in both the liquid and solid (expanded metal) phases. At 230 K, in the homogenous liquid state, we find that the proton self-diffusion coefficient first increases with metal concentration, from 5.6x10(-5) cm2 s(-1) in pure ammonia to 7.8x10(-5) cm2 s(-1) at 12 MPM. At higher concentrations we note a small decrease to a value of 7.0x10(-5) cm2 s(-1) at 20 MPM (saturation). These results are consistent with NMR data, and can be explained in terms of the competing influences of the electron and ion solvation. At saturation, the solution freezes to form a series of expanded metal compounds of composition Li(NH3)4. Above the melting point, at 100 K, we are able to fit our data to a jump-diffusion model, with a mean jump length (l) of 2.1 A and residence time (tau) of 3.1 ps. This model gives a diffusion coefficient of 2.3x10(-5) cm2 s(-1). In solid phase I (cubic, stable from 88.8 to 82.2 K) we find that the protons are still undergoing this jump diffusion, with l=2.0 A and tau=3.9 ps giving a diffusion coefficient of 1.8x10(-5) cm2 s(-1). Such motion gives way to purely localized rotation in solid phases IIa (from 82.2 to 69 K) and IIb (stable from 69 to 25 K). We find rotational correlation times (tau(rot)) of the order of 2.0 and 7.3 ps in phases IIa and IIb, respectively. These values can be compared with a rotational mode in solid ammonia with tau(rot) approximately 2.4 ps at 150 K.

The structure of calcium-ammonia solutions by neutron diffraction.

The microscopic structures of calcium-ammonia solutions have been established by using neutron diffraction. Total structure factors measured at 230 K reveal immediately the evolution of an uncommonly intense diffraction prepeak in the metallic solutions. As concentration is increased from 4 mole percent metal to 10 mole percent metal (i.e., saturation), this feature intensifies and shifts from 0.6 to 0.9 A(-1). It is therefore evidence of well developed intermediate-range ordering among the solvated cations, and is a microstructural signature of the observed strong phase separation of metallic (concentrated) and nonmetallic (dilute) solutions. The technique of isotopic labelling of *N by 15N was then used in conjunction with difference analysis to focus on the solvent structure in metallic solutions at 4 and 10 mole percent metal. These nitrogen-centered functions are analyzed in conjunction with classical Monte Carlo computer simulation techniques, to provide us with detailed insight into the calcium solvation and the extent of hydrogen bonding. We find that calcium is solvated by approximately 6-7 ammonia molecules, with a Ca-N distance of around 2.45 A. There is evidence of hydrogen bonding among the solvent molecules, even in the saturated 10 mole percent metal solution.

The structure of liquid methylamine and solutions of lithium in methylamine

The technique of hydrogen/deuterium isotopic substitution in neutron diffraction has been used to measure the intra- and intermolecular correlations in liquid methylamine, and 2 and 8 MPM (mole percent metal) lithium methylamine solutions. We find that pure methylamine forms only one strong hydrogen bond per molecule, with evidence for weaker orientations towards the methyl group. As one introduces lithium metal, the intramolecular structure of the solvent is unaltered. However, intermolecular hydrogen bonding is progressively disrupted as the concentration of (solvated) cations and excess electrons increases. Comparison of the total structure factors for 0, 2, 8 and 20 MPM lithium methylamine solutions shows that the greatest shift in the position of the principal peak occurs between 8 and 20 MPM. This can be correlated to the electron delocalization associated with the non-metal-metal transition.