James D. Wicks

Intermediate-range structure of fast-ion-conducting AgI-doped molybdate and tungstate glasses

The structures of two fast-ion-conducting molecular glasses, and , have been investigated by neutron and x-ray diffraction. Both molybdate and tungstate glasses show a first sharp diffraction peak (FSDP) at very low Q-values ( and respectively) in the neutron data. However, only weak peaks are observed at similar Q-values in the x-ray data. The diffraction results have been used to model the structures using the reverse Monte Carlo (RMC) method. The RMC results show that all partial structure factors contribute to the FSDP except , , and (or ). This implies that the origin of the FSDP is somewhat different from that for the borate and phosphate glasses where and (the corresponding centres of the covalently bonded oxide groups) contribute strongly to the prepeak. The result excludes any simple explanation of the FSDP in terms of AgI clustering.

RMC: modelling neutron diffraction, X-ray diffraction and EXAFS data simultaneously for amorphous materials

Abstract The simultaneous modelling of neutron diffraction, X-ray diffraction and extended X-ray absorption fine structure (EXAFS) data for a disordered material has been developed with the reverse Monte Carlo (RMC) method. The details of this technique are described and its power is highlighted by modelling four sets of experimental data simultaneously for the multi-component, amorphous fast-ion conductor (AgI)x(AgPO3)1 - x. From the resultant structural models, it is proposed that the first sharp diffraction peak observed in the diffraction data at Q ⋍ 0.7 A −1 is related to the density distribution of the phosphate network; pathways through the structure which favour Ag+ ion conduction are highlighted and a structural explanation for the materials glass formation limit at x ≈ 0.58 is proposed.

Investigation of the ternary phase diagram of mechanically alloyed FeCuAg

The structural and magnetic properties of mechanically alloyed Fe - Cu - Ag at room temperature have been investigated using Mossbauer spectroscopy, x-ray diffraction and differential scanning calorimetry. The elements are naturally immiscible, but through prolonged and energetic ball milling (70 hours at 600 rpm) one can make metastable alloys, the structure of which depends on the elemental composition. In the binary Cu - Ag and Fe - Cu systems, crystalline single-phase solid solutions result, whereas in Fe - Ag the alloying is limited, with the product a mixture of elemental particles. In the ternary system it is possible to produce copper- and silver-rich single-phase fcc alloys, but not the equivalent bcc iron-based structure. As the proportions of the three elements become more equal, the resulting structure becomes highly disordered or amorphous. The composition range of this amorphous phase is different to that observed in sputtered Fe - Cu - Ag systems.

A new approach to modelling tetrahedral amorphous carbon

We have generated a new model for the structure of tetrahedral amorphous carbon using a modified reverse Monte Carlo modelling method. The novel feature of this approach is the definition of three different types of carbon atom, corresponding to tetrahedral, planar and linear bonding conformations. The particular strengths of the method are the large model size (3000 atoms), that all the possible arrangements of and bonds are allowed, and that no interatomic potential is required. For the first time we have determined the distribution of bonded sites within the predominantly disordered tetrahedral structure, and we find that they form polymer-like chains and small clusters which connect the bonded regions.

A new model for tetrahedral amorphous carbon (ta-C)

Abstract We have developed the reverse Monte Carlo (RMC) modeling method to generate two physically acceptable models for tetrahedral amorphous carbon (ta-C). The models have been produced by fitting to experimental neutron diffraction data and by applying various constraints consistent with chemical and physical knowledge of the material. In particular, three different carbon atom types (tetrahedral, planar and linear) have been defined and a new approach to applying coordination number constraints has been developed. The models were generated from an initial random distribution of 3000 atoms and from these we have determined ring number and coordination number distributions that are statistically averaged over the whole structure. We are able to look in detail at the distribution of different types of carbon bonding sites within the box and at the effects of incorporating 5 at.% H into the structure. The new models show that atoms with sp2 bonds tend to cluster and form polymer-like chains, which interlink the regions of sp3 or diamond-like bonding. Also, the relative flexibility of these polymeric chains results in more porous, less dense regions of the network. A comparison with other models for ta-C is also made.

Critical narrowing of molten 7Li0.62Na0.38 alloy

Abstract A molten 7Li0.62Na0.38 alloy has been studied at temperatures above the critical point for phase separation using the IN5 spectrometer at ILL (Grenoble, France). Different incident wavelengths were used allowing investigation with a very high resolution over a wide range of Q. Clear quasi-elastic broadening is seen and the phenomenon of critical narrowing is observed without ambiguity at T = 629 K (T − Tc ≈ 50 K).

Structural and magnetic anisotropy in amorphous alloy ribbons

We report x-ray, EXAFS and neutron diffraction experiments on stress-relieved (SR) and field-annealed (FA) ribbons of using optimized geometries to measure the structural and magnetic anisotropy to unprecedented accuracy. A peak of magnitude of the main peak height is found in the pattern from three independent neutron experiments on the FA ribbons, at . We establish that this peak is of magnetic origin, arising from moments that to some extent lie perpendicularly to the anneal field direction, and is associated with local structure. We show that this anisotropy is consistent with a recently proposed disturbed exchange model, suggesting that the result represents the first direct observation of the existence of antiferromagnetically coupled moments within an amorphous ferromagnetic alloy.

Modelling the structure and ionic conduction of amorphous (AgI)x(AgPO3)1−x with the RMC method

For the first time neutron diffraction, X-ray diffraction and EXAFS data have been combined simultaneously using the RMC method to model the fast-ion conducting glass, (AgI)x(AgPO3)1−x. This material is of considerable technological and scientific interest due to its high ionic conductivity at ambient temperature. We present some details of the RMC technique and highlight some of the structural information obtained from our models. The origin of the “first sharp diffraction peak” in the neutron diffraction data is explained, about which there has been considerable speculation. Diffusion pathways for ionic conduction are observed. A simple analysis of available free volume shows that a percolation transition in the ionic conductivity occurs between x=0.2 and 0.3, in agreement with a prediction based on conductivity measurements. This study highlights the considerable power that these developments of the RMC method have for the structural modelling of complex amorphous materials.