Disordered materials often provide mechanisms for improved diffusion and transport, which is desirable for many sustainable-energy materials. Pair-distribution function (PDF) analysis is becoming increasingly important in studying these materials at it provides local structure, interatomic distances, bond-angles and coordination num­bers in disordered materials such as glassy and amorphous materials. The essential difference between conventional ND and PDF (linked to the neutron total-scattering experiment) is that while in ND only the Bragg peaks are considered, neutron total­scattering means that also the diffuse, weaker, scattering present between the Bragg peaks is analysed, where deviations from the average can be measured. It is this “extra” scattering that provides information about the structure on a local scale and is therefore of high importance for structural studies when the material is not fully periodic.

The PDF, or, G(r), is obtained from the structure factor, S(Q), via a Fourier transformation,

(1.1)

It is the analysis of this quantity that gives information about the local structure of the material. The Fourier transform requires data over a large Q range to avoid truncation effects, so the technique typically uses rather short-wavelength neutrons, either at the hot-end of reactor sources or at spallation sources.

Precombustion CO2 capture in natural-gas plants predominantly involves the sep­aration of CO2 from H2 at high pressures, resulting in a pure H2 stream which is used in energy generation [12]. This process has a component with a higher con­centration of CO2, existing at elevated pressures, resulting in the relatively-low energy penalty for carbon capture of 10-16 % [13]. Additionally, due to the large differences in the polarisability and quadrupole moment between CO2 and H2, the two gases are more easily separated via chemical methods than other gases such as CO2 and N2 [61]. Porous materials with a high density of localized charge, such as achieved through open-metal sites, are particularly promising for this type of separation. Additionally, variances in gas properties such as diffusion rate may also be exploited by adsorbents to increase selectivity. Aside from selectivity for CO2, the working capacity of the adsorbent is another major factor in determining the effectiveness of candidate materials for precombustion capture processes. However, these factors are generally inversely related as a material with high selectivity will generally suffer low regenerability as the guest molecules are strongly bound to the adsorbent and are difficult to remove via a mild pressure-swing approach.

Whilst little work exploring H2 separation specifically from H2/CO2 mixtures has been performed using neutron scattering, more work using neutron scattering to study H2 confined in porous materials has been published than for any other guest molecule. This is a direct consequence of the ease of structural characterization of H (as D) using neutron diffraction as well as the unique information that can be gained for H2 using neutron spectroscopy [62]. This work is extensive and covered in publications concerning H2 storage [63, 64], where the interaction of H2 (D2) with Zn4O(bdc)3 (also known as MOF-5) [65, 66], Cu3(btc)2 [67-71], Mg2(dobdc) and Fe2(dobdc) as well as its oxidized analogue [72], Zn2(dobdc) [73], Al2(OH)2(bptc) [74], Zn(mIm)2 where mIm = 2-methylimidazolate and also known as ZIF-8 [75], Cu3[Co(CN)6]2 [76], as well as many carbonaceous materials and zeolites, have all been elucidated using neutrons. Such work is the subject of Chap. 8.

The polymer-based concept for building OPVs presents a good balance between technical complexities and production costs as compared to the high-vacuum deposited small molecule and Gratzel-device concepts; presented earlier. However, the conjugated or resonating double-bonds of conducting polymers, which are polar in nature, are very sensitive to the shorter wavelength of the solar spectrum and these bonds can be broken if the energy of the solar light lies within a certain ultraviolet (UV) range. Therefore, new types of organic systems are required to overcome this limitation, and make efficient use of a wide part of the solar spec­trum. Discotic liquid crystals (DLCs) are expected to fill this role as an alternative to the conducting polymers. The name “discotic” describes materials formed from disk-like molecules (discogens) which possess a central planar aromatic core to which peripheral aliphatic chains are attached. Thermal fluctuations of the chains, or tails, are sufficient to suppress inhomogeneously-distributed structural traps, giving rise to a “liquid-like” dynamic disorder of the tails. The cores can be formed by triphenylene, coronene or phthalocyanine molecules (Fig. 6.6).

When Chandrasekhar, observed the thermotropic DLCs for the first time in 1977 [12] he explained that the term “liquid crystal” signifies a state of aggregation that is intermediate between the crystalline solid and the amorphous liquid [13]. Materials that form liquid-crystalline phases are called mesogens and different phases can be distinguished within the liquid-crystalline state. These are nematic, smectic, cho­lesteric, and discotic phases. Figure 6.7 depicts schematically the nematic and columnar phases. The nematic phase has a one-dimensional orientational ordering and is subject to positional disorder. The columnar hexagonal phase however, exhibits both orientational and positional ordering, where the molecular core-on-

Fig. 6.6 Molecular schematics of triphenylene (left), coronene (middle) and phthalocyanine (right)

core stacking forms a hexagonal array of columns. The conductivity in this phase is due to the stacking between neighbouring disc-shaped molecules as a consequence of the n-n overlap originating from the delocalized п-orbitals above and below each aromatic core and interactions between the aliphatic chains. These chain-chain interactions ensure self-assembly of the discotic phase rather than a herringbone arrangement that characterizes systems with short chains. The n-n overlap, which defines the co-facial distance/separation between two discs in a stack, provides a one-dimensional pathway for charge migration (conductivity) along the column direction (stacking axis) and is normally thought to be the only part of the charge — transport system. The distance between the columns on the other hand is controlled by the length of the aliphatic chains forming the molecular tails. The highly anisotropic character of the electrical conductivity having its axial component greater than the in-plane makes these molecular wires attractive, offering features such as charge transfer for molecular-conducting devices including photovoltaic applications. Further, the columnar hexagonal positional ordering can be enhanced towards a helical type ordering (Fig. 6.7) which improves further the conductivity.

There are very few works on atomic-hydrogen diffusion on catalysts by QENS, the H species resulting from H2 dissociation have been predominantly studied by INS. When hydrogen is bonded to metal atoms, the heavy atoms can be considered as fixed during the local modes of hydrogen. A consequence is that the mean-square amplitudes for the hydrogen atoms will be generally small so that the sample tem­perature will be of small influence on the INS intensities. Another consequence is that the nondegenerate modes of hydrogen will have nearly the same intensity and an E mode will be twice as intense as an A mode. The INS spectra can thus be fitted and the relative intensities of the bands yield the populations of the various sites.

INS studies of the adsorption of H2 on various materials, to observe rotational transitions, and of various hydrogen species present on metal and sulfide catalysts were previously reviewed [4, 5]. Only recent work will be considered here.

Zirconium and its alloys are extensively used in nuclear applications due to the combination of a low neutron-absorption cross section, good mechanical properties under high stress and temperature conditions, low hydrogen/deuterium uptake and corrosion resistance. The main uses are for some structural components (particu­larly the calandria in CANada Deuterium Uranium (CANDU)-type reactors) and fuel cladding. It is anticipated to be used in the primary containment-vessel of high temperature D2O inside the core of the fourth generation supercritical-water-cooled reactor (SCWR). Its behaviour during manufacturing and in service as well as under accident scenarios is therefore of great importance and a topic of extensive research.

Pure zirconium has a hexagonal closed-packed (hcp) crystal structure up to 866 °C and transforms to a body-centred cubic (bcc) crystal structure at higher temperatures. Their complex deformation-mechanisms have been investigated with neutron diffraction where different crystallographic planes have different elastic constants and post-yield behaviour, leading to strain partitioning. The main alloying elements for nuclear applications are niobium and tin, the latter forming together with other minor elements the so-called Zircaloys (*Zircaloy is a trademark of Westinghouse Electric Company, Pittsburgh, PA.).

Neutron diffraction has been applied to zirconium and its alloys to characterize welds, as an in situ mechanical test technique to characterize deformation modes to allow predictive modelling of deformation, to investigate the development of tex­ture under temperature and stress, and to characterize the phase transformations including texture-variant selection during the hcp/bcc phase transformation. The following sections describe some of these experiments.

Zircaloys are also prone to brittle-hydride formation, particularly in welds. Hydrides can also form in parent plate if unfavourable textures are present due to a particular manufacturing route. Thus, many investigations have been performed on hydrides, such as imaging their location (radiography and prompt gamma), and assessing their susceptibility to hydride formation (residual stress and texture). Hydrogen accumulation or “pick-up” can also occur in the Zircaloy cladding of nuclear fuel and can cause embrittlement. In this case, the hydrogen content can be spatially visualized and quantified by neutron radiography. Blistering of Zircaloy fuel-cladding has also been investigated using neutron radiography, as X-rays are not effective in practice due to the high background, which includes gamma radi­ation from decay products.

The work of material scientists in the discovery, understanding, and development of Li-ion batteries largely depends on the techniques available to observe the relevant processes on the appropriate time and length scales. This chapter aims at demon­strating the role and use of different neutron-scattering techniques in gaining insight into Li-ion battery electrode and electrolyte function. This is not an exhaustive review of the neutron-scattering work on Li-ion battery materials or on the materials themselves, but an attempt to demonstrate the range of possibilities of neutron scattering in Li-ion battery materials research.

The role of neutron scattering in battery research is mainly based on the sen­sitivity of neutrons to Li compared to X-rays and electrons. The coherent neutron­scattering cross section of Li often allows the determination of the Li positions, atomic displacement parameters (ADPs), and occupancies using diffraction where X-rays do not, making it possible to elucidate Li-ion insertion/extraction mecha­nisms. The relatively-large coherent neutron-scattering cross section of Li provides sufficient contrast that Li distributions can be studied using imaging and, in thin — films, reflectometry. The incoherent neutron-scattering cross section of Li allows examination of Li mobility. Measuring Li diffusion directly is difficult as Li has only a moderate incoherent neutron-scattering cross section, so each material has to be considered individually to determine if the neutron-scattering signal originates from Li. Often measuring host material dynamics, e. g. anion and hydrogen group dynamics or lattice vibrations using quasi-elastic neutron scattering (QENS) and inelastic neutron scattering (INS), can provide information on Li dynamics. The diffusion pathway can be additionally corroborated or even independently obtained using the abovementioned large coherent cross-section of Li by considering the anisotropic contribution in the displacement parameter because of the deviation from harmonicity due to thermal motion at elevated temperatures. The relatively large neutron absorption cross-section of Li enables neutron depth profiling to determine Li distributions in materials. In these neutron-based techniques used to study Li-ion battery materials, the effective cross-sections of samples under study can be tuned through the isotopic composition. Naturally occurring Li is composed of 7.5 % 6Li and 92.5 % 7Li. The larger coherent neutron-scattering cross section and lower incoherent neutron-scattering and absorption cross-section of 7Li make it possible to improve data quality by tuning compositionally (isotopically) samples according to the neutron investigation technique being used.

Anita Das, Deanna M. D’Alessandro and Vanessa K. Peterson

Abstract Solid porous materials represent one of the most promising technologies for separating and storing gases of importance in the generation and use of energy. Understanding the fundamental interaction of guest molecules such as carbon dioxide in porous hosts is crucial for progressing materials towards industrial use in post and pre combustion carbon-capture processes, as well as in natural-gas sweetening. Neutron scattering has played a significant role already in providing an understanding of the working mechanisms of these materials, which are still in their infancy for such applications. This chapter gives examples of insights into the working mechanisms of porous solid adsorbents gained by neutron scattering, such as the nature of the interaction of carbon dioxide and other guest molecules with the host as well as the host response. The synthesis of many of these porous hosts affords significant molecular-level engineering of solid architectures and chemical functionalities that in turn control gas selectivity. When directed by the insights gained through neutron-scattering measurements, these materials are leading toward ideal gas separation and storage properties.

CuBIIICV^CVI = Se, S) compound semiconductors are part of the chalcopyrite (ch) family and are located in the middle of the ternary system Cu-BIII-CVI, on the pseudo-binary section Cu2CVI—B3IIC2V (see Fig. 5.5). The band gap (Eg) ranges from 1.0 to 1.5 eV for a single junction thin-film solar cell. The CuBIIICVI compounds crystallize in the chalcopyrite-type crystal structure, named after the mineral CuFeS2. This tetragonal crystal structure (space group /42d) consists of two specific cation-

sites. The monovalent cations are sited on the 4a (0 0 0) and the trivalent cations (In3 +, Ga3+) on 4b (0 0 lA) position. All cations are tetrahedrally coordinated by the anions (8d (x %%)) and vice versa.

A closer examination of the pseudo-binary tie-line reveals a stability of the ch — phase over a defined compositional-range. That means, the ch-phase accepts a deviation from ideal stoichiometry (CuBIIICVI) by maintaining the crystal structure, and without the formation of any secondary phase. The compound Cu1-yInySe05+y is single phase in the region of 0.513 < y < 0.543 and contains within this area only the chalcopyrite-type phase [15]. The common highly-efficient Cu(In, Ga)Se2 thin — film solar devices all exhibit an overall off-stoichiometric composition, due to the multi-stage process applied to grow these absorbers. Such deviations from stoi­chiometry always cause structural inhomogeneities and charge mismatches, which influence the material properties. One effect is the generation of point defects, which influences the electronic and optical properties of the compound semicon­ductor. In general 12 intrinsic point-defects can exist within the ch-type crystal structure.

• 3 vacancies: on the two cation and one anion sites (VCu, V™, V?)

• 6 anti-site defects: B&, CuB, , CuVI, B” VI, CVI III

• 3 interstitial defects: Cuj, BfI, CVI

These intrinsic point-defects cause different defect levels in the energy gap of the semiconductor (see Table 5.1) and therefore influence the electronic and optical properties, sensitively. Consequently, it is of great importance to know where the atoms are.

In addition to the generation of point defects, the anion position (x(CVI)) of chalcopyrite crystallites is also affected by off stoichiometry. A change of the anion position is proposed to be directly correlated with a change in Eg. The x-parameter controls the position of the valence-band maximum and conduction-band

minimum, and therefore Eg. Current studies have shown that Eg decreases for the Cu-poor composition in CuInSe2 caused by a change in x(Se), which is weakly dependent on the concentration of copper vacancies (VCu) [17]. The interplay between the crystal structure and the optical and electronic properties is a funda­mental problem, which has to be understood when tailoring high efficiency thin — film devices with a compound semiconductor as absorber layer. For instance, an uncontrolled change in Eg within an absorber layer is undesirable because it is less optimal for absorption of the incoming sunlight.

It is difficult to identify and quantify very small changes in the crystal structure, such as point defects or changes in atomic positions by imaging techniques. Therefore, it is preferred to study such effects by diffraction methods. The method of Rietveld refinement is applied to refine the crystal structure using an X-ray or neutron powder diffraction pattern of the off-stoichiometric compound in detail. This method provides information about the cation distribution and the position of the atoms within the structure with high accuracy. In the following section we discuss the reasons for the preferred use of neutrons in the description of structural changes in detail, and how point defects in compound semiconductors can be identified.

The structure and kinetics at, and close to, interfaces is of importance in many sustainable-energy devices (e. g. electrolytes and electrodes, see Chap. 7), but these properties are difficult to establish. Within certain constraints, the neutron reflection (NR) experiment can establish the scattering characteristics beneath a surface by measuring the reflected intensity as a function of angle. Above a critical angle (representing Q), total reflection occurs, but below this each layer interface pro­duces an oscillating reflected amplitude with period AQ = 2n/T, where T is the thickness of the layer. However, the measured reflected intensity is the total from all interfaces present, and because phase information is lost, the usual way forward is
to fit the measured signal with models. In practice it is the variation of sample composition within the depth of the sample that is of interest, and this is charac­terized as the scattering-length density (SLD) profile, which is the sum over the number density of each isotope at a given depth, z, times its bound coherent neutron-scattering length. The essential advantages of neutron measurement of reflection is the variation of scattering length with isotopic nuclei, which allows contrast variations, measurement of buried layers, and favours the light elements in the presence of heavier ones found in energy materials.

The main constraints in NR are the comparatively large and atomically-flat surface that is required, and establishing suitable models for analyzing the results.

Oxyfuel combustion involves the combustion of carbon-based fuels in a pure O2 stream, however, the limiting factor in the industrial implementation of these methods is the large amount of pure O2 that is required to be generated from air (O2/N2 separation). Microporous solids that are able to efficiently perform this separation have the potential to significantly reduce the large energy-costs currently associated with oxyfuel combustion. Small-pore zeolites have been employed for O2/N2 separations by exploitation of the difference in the kinetic diameter between the two gases through physical separation involving molecular sieving. The chemical tunability of the pore space of framework materials, however, facilitates the separation of O2 and N2 by taking advantage of the electronic differences between the two gases. In particular, MOFs containing electron-rich redox-active sites, such as Cr3(btc)2 [77] and Fe2(dobdc) [78], have been shown to reversibly bind O2 selectively over N2 via electron transfer from the metal centre to the O2.

The Fe2(dobdc) material binds O2 preferentially over N2 at 298 K with an irreversible capacity of 9.3 wt%, corresponding to the adsorption of one O2 per two Fe centres [78]. Remarkably, at 211 K the O2 uptake is fully reversible and the capacity increases to 18.2 wt%, corresponding to the adsorption of one O2 per Fe centre. Mossbauer and infrared spectroscopy measurements indicated partial charge-transfer from the FeII to the O2 at low temperature and complete charge — transfer to form FeIn and O22- at room temperature. NPD data (4 K) confirm this interpretation, revealing O2 bound to Fe in a symmetric side-on mode with an O2 intranuclear separation of 1.25(1) A at low temperature and of 1.6(1) A in a slipped side-on mode when oxidized at room temperature (Fig. 3.7).

Similar work reported highly selective and reversible O2 binding in Cr3(btc)2 [77], with infrared and X-ray absorption spectra suggesting the formation of an O2 adduct with partial charge-transfer from the CrII centres exposed on the surface of the framework. NPD data confirm this mechanism of O2 binding and indicate a lengthening of the Cr-Cr distance within the “paddle-wheel” units of the frame­work from 2.06(2) to 2.8(1) A.

Selectivity for O2 over N2 was also achieved in polymer/selective-flake nano­composite membranes fabricated with a polyimide and a porous layered alumino — phosphate. Using SANS to probe the large-scale structure of the O2/N2 host material, the substantially improved selectivities of O2 over N2 was shown to occur within only 10 wt% of the AlPO layers [79].