In the long term, H2 is envisaged as a potential energy carrier. However, one of the issues for portable applications of this energy vector relies on its economic and safe pressure storage under the conditions of transport. Although the targets set for 2015 by the U. S. department of energy (DOE) are difficult to reach, several options are extensively investigated. Compressed or liquefied H2 is not suitable for mobile applications, because of low volumetric energy density and safety problems. A promising way for mobile applications is solid state storage. One can differentiate physisorption in porous materials, including zeolites, MOFs and different types of carbons, and chemisorption resulting in the formation of hydrides. INS has been used to characterize various hydrides, starting from transition-metal hydrides, up to com­plex hydrides composed with light elements (lithium, boron, sodium or aluminium),

which provide high gravimetric H2 density. Catalysts play a role in some cases: for alanates, doping with a titanium catalyst increases the rates of adsorption and desorption of H2 [9], forborohydrides, hydrogen release through a hydrolysis reaction can be controlled catalytically [10].

Since transport can be a limiting step, the use of nanoparticles is an option to improve the kinetics. Raman scattering and INS techniques have been used to find out if the infiltration process of a carbon matrix with NaAlH4 creates new chemical species (e. g., Na3AlH6) and if the nanoparticles of NaAlH4 have a physical state different from the bulk or not [11]. The influence of hydrogen desorption — absorption cycling was also tested. The INS spectra of the melt-infiltrated com­posite of NaAlH4 and active carbon fibers (ACF-25) are compared to the corre­sponding spectroscopic data taken from bulk NaAlH4 in Fig. 2.3.

The comparison between spectra (a) and (b) in Fig. 2.3 indicates that INS is not sensitive to the desorption-absorption cycle. On the other hand, the comparison with bulk NaAlH4 shows a broadening of the peaks with some energy shifts, typical of crystal-size effects. The extra intensity observed between 130 and 200 meV was attributed to the presence of a small amount of Na3AlH6, also observed in the Raman spectrum [11].

In this section we describe several applications of neutrons to investigate the crys­tallography and phase composition of uranium, particularly in fuels. Metallic uranium has an orthorhombic crystal structure, leading to anisotropic single-crystal properties such as a negative thermal-expansion along the crystallographic b axis. This leads to substantial integrity problems when the material is heated. Therefore, the vast majority of nuclear fuels in power reactors consist of cubic uranium-oxide whereas in research reactors metallic uranium-molybdenum alloys are also used, with the molybdenum stabilizing the cubic gamma-structure. During operation, i. e. during heating and irradiation, atoms rearrange and phase transformations may occur, with the phases in a spent fuel having different material properties to those of a fresh fuel. For example, thermal gradients of several hundred degrees exist between the centre and outside of a fuel rod, leading to a spatial distribution of crystallographic phases over a distance on the order of a centimeter. The identification of the new phases, determination of their formation conditions and kinetics, as well as establishing their properties, are of paramount importance for new and existing fuel types. Similar considerations apply to structural materials, e. g. cladding or pressure-tubing materials in accident scenarios, and actinide-bearing minerals for mining and waste deposition [45].

In fuels, the elements of interest are high Z-number (uranium and other actinides) and low Z-number (oxygen, nitrogen, carbon). Neutron diffraction is better at deter­mining these structures while X-ray diffraction is biased towards the heavy atoms.

There are three structure models proposed for cubic UC2 [46-48] a non — quenchable phase existing between * 1,823 and *2,104 °C [49]. The three crystal structures differ in the arrangement of the carbon atoms. Simulated X-ray diffraction patterns are similar due to the bias towards the uranium lattice. Simulated neutron diffraction patterns can discriminate between the three different structures (Fig. 4.12) due to the sensitivity to the carbon atoms. Experimental neutron-dif­fraction data matches well with the structure proposed by Bowman [48].

This example illustrates the great advantages neutron diffraction offers over X-ray diffraction for crystal-structure investigations of nuclear materials and in particular nuclear fuels.

The smaller low Z-number elements are typically the mobile species and their rearrangement as a function of temperature leads to phase transitions, and neutron diffraction may be sensitive to these while X-ray diffraction will not. As many phases are non-quenchable, in situ techniques offer great advantages. Classical methods to study phase transitions, such as dilatometry or calorimetry, do not identify the

newly-formed phases and are also sensitive to changes in chemical composition, e. g. rearrangement of the oxygen atoms, without formation of a new, distinct phase. Not surprisingly, neutrons have played a vital role over the past decades in elucidating the properties of nuclear fuels.

Series editors

Ian S. Anderson, Oak Ridge National Laboratory, Oak Ridge, TN, USA Alan J. Hurd, Los Alamos National Laboratory, Los Alamos, NM, USA Robert L. McGreevy, ISIS, Didcot, UK

Neutron scattering has grown from being a physics and chemistry centred technique in which lattice dynamics and crystallography were the mainstays, to a more interdisciplinary field which includes areas as diverse as engineering, archaeology, food, plastics and all manner of nanostructured materials. The study of sustainable — energy materials is highly cross-disciplinary in nature, with the diverse activities frequently having some background in neutron-scattering techniques, which reflects the innate applicability of neutron scattering in this area. It is not surprising to find that many neutron-scattering centres are supporting an “energy project” of some description that brings together the in-house and user activities. It is refreshing to see scientists with widely-varying expertise making a joint approach to under­standing and improving energy materials. Progress is made through a number of avenues:

1. Increasing performance of neutron sources and their associated instrumentation.

2. Improvement of specialised instrumentation, sample environments, and ancillary equipment (such as in situ cells), aided by collaboration, workshops, and conferences.

3. Computational resources and algorithms adapted to modelling structure and dynamics in increasingly complex materials are being both validated and used in the study of sustainable energy with neutrons.

4. Studentships and fellowships in the field are increasing, bringing fresh ideas and new approaches to the way neutron scattering is used and the data analysed.

5. Increasing awareness of the importance of sustainable energy in society, and the role that neutron techniques of analysis plays, helping to increase the resources that are allocated to this area.

This book brings together some of the core aspects of sustainable-energy materials that can be studied with neutrons, but there are obviously many other important neutron-based studies in the area that fall outside this core, for example in the fields of wind, hydro, and biomass. Similarly, a large number of non-neutron techniques are used to study the materials that we discuss in this book, and it is

frequently the combination of information from a number of techniques that leads to the final understanding.

Although the book does not contain dedicated theory or instrumental sections, the volume is aimed at those who have little or no knowledge of neutron-based techniques of analysis, and instead we refer to an earlier volume in this series in which these topics are presented in detail. The contents of this book are aimed at professionals at all levels in the field of sustainable energy, to show the types of question that can be addressed using neutrons. Some chapters in the book take the form of a review, whilst others use case studies to provide a more targeted approach. The book is structured chronologically, beginning with energy genera­tion, moving onto storage, and then to use, although each chapter can be read independently of the others. The loose theme of application of sustainability- materials in transport applications that runs through many of the chapters is rather artificial, because each chapter has at least some bearing on stationary applications.

We are sincerely grateful to all the authors for their willingness to find the time to provide their chapters. They are all active in research and can only make their contribution to this book by taking time away from their research projects. We do understand.

Lucas Heights, NSW, Australia, October 2014 Gordon J. Kearley

Vanessa K. Peterson

Neutron powder diffraction (NPD) has been used extensively to determine the location of guest molecules in porous framework materials, and this work extends to CO2 [31-36]. Two MOFs that have been explored intensively for their selective sorption properties are M2(dobdc) (M = Mg, Mn, Co, Ni, Zn; dobdc = 2, 5- dioxido-1, 4-benzenedicarboxylate), also known as MOF-74 or CPO-27, and M3(btc)2 (M = Cu, Cr, Mo; btc =1,3, 5-benzenetricarboxylate) with Cu3(btc)2 also known as HKUST-1. Both materials contain exposed M2+ sites, with the M2(dobdc) material possessing exceptionally large densities of such sites. The location of CO2 in the two MOF materials Mg2(dobdc) and Cu3(btc)2, along with the host-CO2 structure, was determined using NPD. The nature of the host-CO2 interaction in both materials was identified to be binding at metal sites via an oxygen with the remainder of the molecule remaining relatively free (see Fig. 3.1), where the adsorbed CO2 is clearly located above the open Mg ions in Mg2(dobdc) [32]. Importantly, the presence of coordinatively-unsaturated metal sites in MOFs such as M2(dobdc) and Cu3(btc)2 leads to enhanced interactions between adsorbates such

as CO2 and the host framework, but also guest molecules such as CH4 and H2. Indeed, we will show that the application of NPD to examine competitive binding between CO2 and these other gases represents an area of significant current interest.

Density-functional theory (DFT) calculations performed using the NPD-deter — mined structures allowed evaluation of the representative CO2 motions in Mg2(dobdc) and Cu3(btc)2. These calculations show that the O bound to the open metal can be approximated as the rotational centre. In both materials the open metal and the associated carboxyls from the ligands form a nearly square-planar surface at the CO2 binding-site, such that the metal-CO2 interaction closely represents a surface normal. The CO2 rotations are shown by arrows in Fig. 3.1(b-c), occurring about the surface normal (red arrows) and away from the surface-normal (blue arrows). The mode energies for the motions denoted by the red and blue arrows are

4.3 and 8.5 meV for Mg2(dobdc), respectively, and 0.2 and 3.4 meV for Cu3(btc)2, respectively. To gain more direct information about the CO2-host interaction in Cu3(btc)2 the energy at the open-metal sites (assuming a rigid host framework) was calculated for these two CO2 motions as a function of CO2 rotational angle, and is shown in Fig. 3.1d. As expected, the energy curves are shallow, particularly in the ±10° region, allowing for significant CO2 orientational disorder in the MOF at this site with little effect on the total energy of the MOF-CO2 system. These findings are in excellent agreement with the relatively-large atomic displacement parameters of CO2 adsorbed at open metal sites obtained from the NPD measure­ments, in particular for Cu3(btc)2. These results also point to the presence of dis­order (either static or dynamic) in the orientation of the CO2 molecule, resulting in a relatively large apparent O-C-O bond bend obtained from the NPD data, which is a structural average and strongly biased by the relatively large disorder of the adsorbed CO2. Since CO2 is reversibly physisorbed on these open metal sites, a large degree of CO2 bond activation and bending is unlikely.

Vacancy-containing Prussian blue analogues of the formula M(1)II3[M

(2) III(CN)6]2 (where M(1) and M(2) are transition metals) are excellent candidate gas adsorbents as 1/3 of their octahedral MIII(CN)3 units are vacant for charge neutrality, generating both non-vacancy and vacancy pores. Each vacancy pore will possess some of the six bare-metal sites per formula unit (eight per unit cell). The M (1)3[Co(CN)6]2 system (M(1) = Mn, Co, Ni, Cu, Zn, Fig. 3.2) displays good selectivity for CO2 over CH4 and N2 [38], with a NPD study revealing two sites for CO2 binding in the Fe3[Co(CN)6]2 material, which has a CO2 uptake of

2.20 mmol g-1 at 35 °C and 1 bar [39]. At one of these sites CO2 was found to bridge between two open-metal sites, with the quadrupolar CO2 molecule inter­acting strongly with the positively-charged Fe sites. The saturation of this site by CO2 at relatively-low CO2 concentrations indicated the favourable nature of the interaction, explaining the selectivity of the material.

CO2 hydrates, consisting of an H2O-cage encapsulating a CO2, are another porous material that have great potential for application as CO2 adsorbents, and these too have been studied using NPD to determine the locations of CO2 within the cage [40]. This study used a cage in which D was substituted for H, allowing structural details of the cage atoms and their interaction with the CO2 to be determined. The study also included the temperature-dependence of this CO2-cage interaction. Data indicate that the CO2 molecule in the tetrakaidecahedral cage rotates rapidly even at low temperatures and that the interaction between the CO2 molecule and the D atoms of the cage is strong enough to provide the site dependence of the atomic displacement parameters of the D atoms. Further work on CO2 hydrates [41] using NPD found CO2 to have different motions in the small and large cages of this system. In both cages the CO2 resides at the cage centre, however, in the small cage the O atoms revolved freely around the C atom, in contrast to the large cage where the O atoms revolved around the C atom along the plane parallel to the hexagonal facets of the cage. The analysis of CO2 hydrates using NPD has also been extended to studies of their formation, including kinetics, using in situ NPD [42]. This work also derived the occupancy of CO2 in the small and large cage during the formation of the hydrate.

Solids whose crystal structures are based on tetrahedrally-coordinated ions may show the intriguing property of negative thermal-expansion.

The ternary AIBIIICVI semiconductors (A = Ag, Cu; B = Al, Ga, In; C = S, Se, Te), exhibit such a tetrahedral coordination (see Fig. 5.7). The coordination tetra­hedron around an anion (sulfur or selenium) consists of two monovalent and two trivalent cations. The chemical bonds within such a tetrahedron are of mixed covalent and ionic character, whereby the ionicity of the bonds is different for the AI-CVI and BIII-CVI bonds. These different interactions result in different bond lengths (RAC Ф RBC) as well as bond angles and lead to a displacement of the anions from the ideal tetrahedral site by a quantity u = lx — %l (where x is the anion x coordinate).

The linear thermal-expansion coefficients are closely related to the Gruneisen parameters у of lattice vibrations [25]. The occurrence of a negative thermal — expansion can be understood using the notation of a balance between acoustic shear and compression modes of the observed crystal structure. The Gruneisen parameters of the shear modes show a tendency to negative values, while those of the com­pression modes are positive [25-27]. Hence, the temperature dependence of the thermal expansion is determined by the degree of excitations of the various modes and can change its sign when the relative thermal-population of the modes varies.

In AIBIIIC2VI chalcopyrite-type semiconductors the thermal-expansion behaviour is described by the independent linear thermal-expansion coefficients aa and ac with

The uniaxial chalcopyrite-type structure comes with two independent Gruneisen parameters ya and yc, which are related to aa and ac according to [28]:

Ca = C [(C11 + C13H + 4^] and Ус = pm [2c13aa + 4,«c] . (5.4)

Cp Cp

Here Vm is the molar volume, Cp the molar specific-heat at constant pressure and cy are the adiabatic elastic-stiffnesses. With increasing ionicity the Gruneisen parameter should become more negative [29]. Thus, the covalent character of the chemical bond is expected to strongly affect the Gruneisen parameter.

The determination of linear thermal-expansion coefficients by dilatometry or X-ray diffraction [30-33] has shown that aa and ac vary independently with tem­perature. This is caused by the axial symmetry of the chalcopyrite-type crystal structure and the difference in strength of the Cu-CVI and BIII-CVI cation-anion bonds.

The investigation of the negative thermal-expansion is conveniently achieved by neutron powder diffraction. One aspect for the use of neutrons is again the high intensity in the diffraction pattern at high Q-values, important for an exact deter­mination of the chalcogen position. It is important to monitor the change of this position at low temperatures to describe the bond stretching during cooling. The negative thermal-expansion has been studied for several chalcopyrite-type com­pounds, whereby the focus now lies on Cu(InxGa1-x)Se2 once with high (x = 0.918) and once with low indium content (x = 0.096), to show the effect of different bond ionicities on the negative thermal-expansion. Neutron powder diffraction patterns were collected for temperatures between 1.5 K > T > 300 K and structures refined by the Rietveld method according to the previously-described sequence. The ion — icity can be calculated following Phillip’s definition [34]:

(XA-XB)2

= 1 — e 4

with XA and XB the electronegativity of the elements A and B (XCu = 1.9; XGa = 1.81; XIn = 1.78). According to Phillip’s definition the bond ionicities increase from Cu-Se (fi = 0.1002) to In-Se (fi = 0.115) and Ga-Se (f — = 0.128). Thus the ionicity of the BIII-Se cation-anion bond is increasing with increasing substitution of indium by gallium. From this it follows that with a high amount of gallium the difference in bond ionicity between the Cu-Se and BIII-Se cation-anion bond increases, which lead to an increased anisotropy.

The higher anisotropy affects the change of lattice parameters with decreasing temperature, which is stronger for the gallium-rich Cu(In, Ga)Se2 and pure CuGaSe2 than for indium-rich Cu(In, Ga)Se2 (see Fig. 5.8). Applying a third-order polynomial fit to the lattice parameters the thermal-expansion coefficients aa and ac, can be derived. The temperature at which the linear thermal-expansion becomes negative (T0), is seen to vary with the chemical composition (see Table 5.2).

The variation of T0 with chemical composition should be discussed in context with the bond ionicity of the BnI-Se bonds, which increase with increasing sub­stitution of indium by gallium. Thus, with increasing gallium content the ionicity increases and the temperature, for which the linear thermal-expansion coefficient changes its sign, increases. This is observed for Cu(InxGa1-x)Se2 with different x-values as summarized in Table 5.2.

The In/(In + Ga) ratio strongly influences the character of the covalent-ionic BIII-Se cation-anion bond, and therefore the behaviour of the linear thermal — expansion coefficients of the two lattice constants aa and ac.

Also, the x-parameter of the selenium anion as a function of temperature is strongly affected by the different bond ionicities. In the Ga-rich sample the tetragonal deformation u = 0.25-x(Se) strongly tends to zero with decreasing temperature, whereas it stays almost constant for the In-rich sample (Fig. 5.9). This effect is explained by the higher bond-ionicity for the Ga-Se bond compared to the In-Se bond.

The change of the tetragonal distortion and the anion position parameter x(Se) is reflected by the change in the average cation-anion bond distances and angles, which change markedly for the Ga-rich sample compared to In-rich sample (see

Fig. 5.10).

The term quasielastic neutron-scattering (QENS) can be used to describe any broadening of the elastic peak regardless of its origin, but use of the term in this book is always with reference to a stochastic or diffusive non-periodic motion. The technique is important for energy materials because it provides the relevant time and length scales on which the atomic-scale dynamics of protons and small molecules typically occur, for example in proton-conducting perovskites (Chap. 9). Various molecular processes can be distinguished from the data, which can be quite straightforward, although in systems of any complexity it is now common to use molecular-dynamics simulations from which it is now easy to produce a calculated QENS spectrum.

Michael Law, David G. Carr and Sven C. Vogel

Abstract Current and future nuclear-technologies such as fission and fusion reactor-systems depend on well-characterized structural materials, underpinned by reliable material-models. The response of the material must be understood with science-based models, under operating and accident conditions which include irradiation, high temperature and stress, corrosive environments, and magnetic fields. Neutron beams offer methods of characterizing and understanding the effects of radiation on material behaviour such as yield and tensile strength, toughness, embrittlement, fatigue and corrosion resistance. Neutron-analysis techniques improve our understanding of radiation damage, which is essential in guiding the development of new materials.

4.1 Introduction

Radiation damage changes structural materials; the role of microstructure, stress, and radiation flux on swelling, creep, embrittlement, and phase transformations must all be understood. This knowledge will allow development of materials with superior resistance to fast-neutron fluence and high temperatures.

The ability to model and predict the performance and life of materials in nuclear power plants is essential for the reliability and safety of these technologies. These systems may include new environments such as high-pressure water, molten salt, molten metal, and helium which all increase the potential for material degradation

M. Law (H) • D. G. Carr

Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW, Australia e-mail: michael. law@ansto. gov. au

D. G. Carr

e-mail: david. carr@ansto. gov. au S. C. Vogel

Los Alamos National Laboratories, Los Alamos, MN, United States e-mail: sven@lanl. gov

© Springer International Publishing Switzerland 2015 61

G. J. Kearley and V. K. Peterson (eds.), Neutron Applications in Materials for Energy, Neutron Scattering Applications and Techniques,

DOI 10.1007/978-3-319-06656-1_4

and corrosion. Significant material-development challenges must be met as com­ponents of Generation IV reactor systems will experience higher fluxes, tempera­tures, and sometimes stresses, than conventional light-water reactor systems. The same applies for fusion reactors which in the current developmental phase pose significant challenges to available structural materials. Only by improved charac­terization can we move to science-based models of material behaviour. Under­standing material behaviour from the atomic level up to the full-component scale is essential in developing new materials for these applications.

Creep and creep-fatigue of reactor materials is poorly understood. When the effects of irradiation are added, it is obvious that a better understanding is required. These same effects are intensified in welds due to texture, material inhomogeneity, residual stress, and thermal-expansion mismatch.

Irradiation can cause significant microstructural changes including atomic dis­placement, helium bubble formation, irradiation-induced swelling and irradiation — induced creep, crystalline-to-amorphous phase transitions, and the generation of point defects or solute aggregates in crystalline lattices. Irradiation also creates defects resulting from atomic displacement or from transmutation products. These defects increase the yield and tensile strengths while reducing ductility and causing embrittlement.

The neutron-beam techniques relevant to the nuclear-energy sector are residual stress and texture measurements, crystallographic phase analysis to establish phase diagrams and reaction kinetics, neutron radiography and tomography, prompt — gamma activation analysis, and small-angle neutron scattering. As exposure to neutron beams activates many materials, neutron facilities generally have the infrastructure to accommodate radioactive materials which allows post-irradiation examination of samples.

The ability to characterize materials in situ is essential, at the appropriate tem­perature and environment, rather than bringing the sample back to ambient con­ditions. This also allows the evolution of material behaviour to be studied rather than just the properties at the start and endpoint.

Although pure DLCs generally show a poor absorption in the visible spectral domain, mixtures of the electron-donating discoids with non-discogenic electron acceptors could exhibit absorption bands in the visible due to the formation of a CT complex. In many cases CT complexation even causes a considerable increase in the stability of the columnar mesophase. For good performance of a photovoltaic device the donor and acceptor molecules must form separate columns, i. e., enable charge separation and subsequent charge transport along the columnar wires. The position of the electron acceptors within the columnar mesophases is still contro­versial. Acceptor molecules such as TNF have been reported to be sandwiched between discotic molecules within the same column [31—34] but also inter­columnar, i. e., between the columns within the aliphatic tails of the discotic mol­ecules [29, 35]. Only the inter-columnar juxtaposition could provide a morphology with separate continuous columns for electron and hole transport. Another issue is that the characterization of (photo-induced) electron transfer and relaxation pro­cesses in self-assembled aggregates such as DLCs and DLC-CTs is in its infancy. The addition of electron acceptors such as TNF has been shown to increase the conductivity of DLCs. On the other hand, it has been proposed that recombination processes limit the hole photocurrent in DLC-CT compounds. Charge carriers in CT compounds are supposed to be trapped and readily annihilated through rapid, phonon-assisted relaxation and recombination processes.

In this section the morphology issue is elucidated by considering the prototypical discotic CT compound HAT6-TNF (Fig. 6.8), where HAT6 is used as electron — donating discoid and TNF as electron acceptor. HAT6-TNF forms a CT compound exhibiting a stable columnar phase from below room-temperature and up to 237 °C. The high symmetry and moderate molecular size of discogens such as HAT6 make these systems attractive for exploring the effects of increasing molecular complexity by comparing their photo-physical properties with those of the fundamental building blocks: benzene, and large poly-aromatic hydrocarbons. For discotic liquid crystal­line CT-complexes it is generally accepted that intermolecular charge-transfer occurs in the excited state, but not in the ground state. Mixtures of the electron-donating discoids with non-discogenic electron-acceptors exhibit absorption bands in the visible region due to excited-state charge-transfer. Support for these indications can be found from a combination of NMR and Raman spectroscopy measurements. Furthermore, the electronic transitions involved in the CT-band of HAT6-TNF can be characterized by combining UV-visible absorption and resonant Raman

spectroscopies. Additionally, the UV-visible and Raman measurements are accompanied by DFT calculations, to identify the vibrational modes that assist charge-carrier relaxation in the hot-band of HAT6 and in the CT-band of HAT6-TNF.

The ND pattern of HAT6-TNF (Fig. 6.16) is characteristic for a columnar mesophase, with sharp reflections in the small 20 region ((100), (010), etc.) origi­nating from the two-dimensional columnar lattice, a broad liquid-like band from the distribution in tail-tail distances, and a broad (001) peak from the intra-columnar distances [36]. Two observations are important. First, there is no superstructure peak visible with a double co-facial distance as would result in an intra-columnar juxta­position of TNF where TNF and HAT6 alternate, doubling the cell dimensions in the z-direction. This essentially makes such configurations unlikely. Second, the large decrease in lattice parameters is not accompanied by a similar increase in density that would result when simply shrinking the HAT6 cell to the new dimensions.[10]

These observations indicate that the columnar morphology has drastically changed in the charge-transfer compound. For a hexagonal columnar-structure with the TNF sandwiched between the HAT6 molecules, the column-column distance should be about 17 % smaller compared to pure HAT6. This only appears possible if the HAT6 and TNF are also alternately packed in the hexagonal plane. Other distributions result in energetically unfavourable inter-digitation of the aliphatic tails, such as the often suggested alternating intra-columnar packing with the dis — cotic molecules positioned in the same hexagonal plane. The intra-columnar jux­taposition of TNF, on the other hand, is only consistent with the experiments if the HAT6 columns are tilted on an oblique lattice. Such an arrangement, with discotic molecules slid laterally, has already been observed for highly polar HAT2-NO2 molecules. However, the tilted HAT6 columns leave such small spaces within the tail region that the TNF molecules should mainly adopt a vertical orientation.

The sandwich (with HAT6 and TNF alternating in the horizontal plane) and intra-columnar (with tilted HAT6) models were further analysed with Rietveld refinement. Remarkably, the refinement does not favour either of the juxtapositions of TNF over the other. Both models reproduce the characteristic features of the experimental diffraction patterns with comparable agreement. In addition, the effect of deuteration on the diffraction patterns also fits well with the measurements in both cases, particularly considering that no extra refinement step has been made; i. e., only the deuterium atoms of TNF were replaced with protons. Even the ori­entation dependences of the diffraction patterns on macroscopically aligned samples show little difference between the two models. As expected, the two-dimensional lattice peaks have maximum intensity when the diffraction beam is perpendicular to the column director, and the intra-columnar peak is at a maximum for the parallel orientation.

Clearly, it is difficult to determine the juxtaposition of TNF using only diffrac­tion. Nevertheless, the sandwich and inter-columnar models only fit with the observed density and diffraction patterns under the conditions illustrated in Fig. 6.17. As anticipated, the HAT6 columns in the inter-columnar model are tilted, with a co-facial slide of about 3.5 A between two neighbouring molecules in a column. We defined d column-column as the average separation between the column directors of two neighbouring columns, which is significantly larger for the inter-column juxtaposition of TNF. The minimal distance dHAT6core-TNF between the core of HAT6 and TNF predominantly determines the charge-transfer

#HAT*i>-TNFd

HAT. d TNFp

Fig. 6.18 Left Solid state CP-MAS 13C NMR spectra for HAT6 (a), HAT6-TNFD (b), TNF (c), and HAT6-TNF (d). The temperature and CP mixing time are 358 K, 10 ms for (a, b), 300 K, 2 ms for (c), and 300 K, 5 ms for (d). The HAT6 carbon assignment in (a) follows the labelling of Fig. 6.8. The blue (red) arrows indicate the downfield (up-field) shifts of HAT6 (TNF) peaks in the composite. Right 1H13C two-dimensional hetero-nuclear correlation spectra for HAT6-TNFD (coloured contours) and HAT6D-TNF (grey contours) at 290 K with a CP mixing time of 10 ms. The red numbers indicate cross-polarization between protons on the tail of HAT6 (H2H5 and H6) and specific TNF carbons shown in the inset. The circles in the inset surrounding the numbered carbons illustrate the strength of these interactions and the possible HAT6 hydrogens involved (pink for H2H5, green for H6). The HAT6 Cc and Co carbon and the Hcore hydrogen chemical shifts are indicated by the green, blue, and brown dashed lines, respectively. The signals marked with yellow arrows are due to imperfect deuteration of TNF behaviour of the complex. For the inter-columnar arrangement dHAT6core-TNF is difficult to estimate, since there is considerable freedom left in the refinement of the TNF position. We refined several inter-column models with different initial vertical positions of TNF from which we estimated that dHAT6core-TNF should be within the range of 4-10 A. Typically, the closest distance between TNF and HAT6 for the intra-columnar juxtaposition involved a CH C atom of HAT6 and TNF NO2 group.

Figure 6.18 (left) shows the solid state 13C cross-polarization (CP) magic-angle spinning nuclear magnetic resonance spectra of liquid-crystalline HAT6, TNF, and the CT complex. On the basis of these spectra of the un-complexed samples, all the peaks in the CT-complex spectrum are assigned to specific HAT6 or TNF carbons. In the CT compounds, however, the chemical shifts of the HAT6 and TNF carbons are changed significantly. All the TNF carbon signals are shifted up-field, reflecting a stronger local magnetic-field for the TNF in the mixture compared with the pure compound. In contrast to TNF, the HAT6 lines show both downfield and up-field shifts.

Chemical-shift changes in charge-transfer complexes have been attributed to partial electron-transfer from donor to acceptor molecules in the electronic ground — state. According to Mulliken’s theory, partial transfer of electron density occurs from the HOMO of the donor to the LUMO of the acceptor in the electronic GS. This is observed for the HAT6 donor. The anticipated general shift-sign (lower
electron density, lower field) that would result from partial electron-transfer to the acceptor TNF. For HAT6, the largest down-field shifts are observed for 13C nuclei in the outer part of the aromatic core, which is consistent with the spatial distri­bution of the HOMO [37].

A clear conclusion on the juxtaposition of TNF can be drawn from the two­dimensional 1H13C hetero-correlation nuclear magnetic resonance measurements. The signals indicated with red arrows in Fig. 6.18 (right) result from coherence transfer between the HAT6 tail-proton spins H1H6 and specific TNF carbons labelled in the inset. The TNF should be, at least for the major part, within the tail region of the HAT6 molecules. On the other hand, no interaction between TNF and the core of HAT6 was observed. The proton Hcore attached to the HAT6 core only shows cross-polarization with HAT6 carbons. For the differently deuterated sample, HAT6D-TNF, very little coherence transfer between TNF protons and any of the HAT6 carbons was observed. The absence of a significant reverse CP from TNF to the HAT6 tails is likely due to the different number of protons involved (12 for HAT6 tail protons H2H5, 1 for TNF protons). The combination of a rapid rigid-like CP build-up and the narrowing of the 1H line widths are indicative of selective averaging or quenching of weak longer range homo-nuclear 1H1H dipolar inter­actions by anisotropic motion of the HAT6 and TNF molecules in the liquid crystalline phase. Larger molecular displacements of HAT6, related to dynamic defects in the liquid-crystalline phase, occur on time scales up to milliseconds. These motions can quench the long-range dipolar interactions and contribute to the narrowing of the 1H lines in the data sets without decoupling, while allowing at the same time for the high CP rates for the 13C in the aromatic core by strong short — range hetero-nuclear dipolar interactions.

The nuclear magnetic resonance analyses seem to indicate that charge transfer from the HAT6 core to TNF already takes place in the ground state of the complex. Excited-state or ground-state charge-transfer from the HAT6 core to TNF requires that the electron acceptor should not be too far from the aromatic core. For the inter­columnar CT-complex structure the diffraction analyses resulted in HAT6core-TNF distances of 4-10 A, mostly involving the optically active NO2 groups of TNF. To ensure a sufficient orbital overlap for CT electron delocalization, most of the TNF molecules should be in the lower part of this range. The consistent analysis reveals a CT-complex morphology with dynamically disordered TNF molecules that are vertically oriented between the HAT6 columns, i. e., within the aliphatic-tail region. What does this mean for PV applications?

A promising observation is that there is a hole-conducting column present that is well separated from the electron acceptors. The columnar morphology has changed drastically in the composites, with a time-averaged tilted orientation and smaller average distances between the neighbouring HAT6 molecules within the column than in pure HAT6. In the CT complex the hole transport through the column will thus still be possible, while the CT process enables efficient charge separation. The liquid-crystalline structure and its facile alignment over macroscopic distances is an important asset for device realization.

The future PV application of CT complexes such as HAT6-TNF thus relies on improving the electron transport channel (Fig. 6.19). For instance, by searching for better acceptors that self-assemble into a separate channel, designing molecularly — connected donor and acceptor groups. The persistence of the hole conducting HAT6 column in the CT complex is promising for future application in organic PV systems. The major challenge towards such an application is to achieve a mor­phology that enables a BHJ PV device architecture.

Atomic hydrogen diffusion in metal lattices has been studied for a long time by QENS, e. g. [12]. The diffusion of atomic hydrogen on the surface of catalysts has
been studied scarcely [13, 14], this could be revisited since the neutron flux and the instrumentation has been tremendously improved since. Dihydrogen diffusion in porous media is a more recent topic. H2 diffusion is of interest for a number of industrial processes, including membranes for fuel cells or reactions with membrane reactors, where the zeolite acts as a porous membrane. Over the last years, a large number of studies have been conducted on MOFs, to explore the performance of these solids as energy carrier for H2. In general, for applications involving gas separation, a fast diffusion rate of the gas molecules into the porous system is as important as the adsorption uptake, the selectivity, or the regenerability.

There are few measurements on the diffusion of H2 in zeolites and MOFs, this is due to the difficulty in measuring fast sorption rates by macroscopic methods. There are also few theoretical studies, a classical approach being not sufficient at low temperatures since the de Broglie thermal wavelength is comparable to the diameter of the confining pores, requiring in this case a quantum mechanical treatment.

The self-diffusivity, Ds, of H2 in several zeolites was studied by pulsed-field gradient nuclear magnetic resonance (PFG-NMR) and QENS [15]. The self-diffu­sion coefficients were found to decrease with decreasing pore sizes of the zeolite structures, with one notable exception in silicalite. In this case, the diffusivities were found to be exceptionally small, the explanation being that H2 molecules are trapped within the pentasil chains forming the structure.

While the self-diffusivity can be determined from incoherent neutron-scattering, the transport diffusivity, Dt, can be derived from coherent neutron-scattering [16]. Normally, Dt is measured under the influence of concentration gradients, i. e. under non-equilibrium conditions. It may seem strange to derive a non-equilibrium quantity from experiments performed at equilibrium. This is because the scattering function corresponding to coherent scattering is related to the full correlation function, G(r, t), which is itself connected with the evolution of the particle density around equilibrium. This approach was already formulated by Onsager [17] in his regression hypothesis. It was later proven that the response of a system to an external perturbation can be evaluated from correlation functions of the sample at equilibrium.

For a comparison of the different results, the transport diffusivity is often rep­resented in terms of the so-called corrected diffusivity, D0, which is defined by the relation

(2.1)

where c denotes the adsorbate concentration in equilibrium with the pressurep. The term d lnp/dlnc is the thermodynamic factor. When the adsorption isotherms can be fitted with the Langmuir model, the thermodynamic factor is equal to or larger than 1 [18].

High neutron-flux and improved detectors allow faster data acquisition; if suffi­ciently fast compared to the reaction rate, in situ kinetic studies are possible. The sensitivity of neutron diffraction to the crystallographic phases and the rate of data acquisition can allow not only the equilibrium state, but any intermediate phases and reaction rates to be determined.

To study the kinetics of phase transformations, hyper-stoichiometric uranium oxide was cycled across a phase boundary [57]. Desgranges et al. [58] performed in situ studies on transitions between four different uranium-oxide phases which depended on both temperature and oxygen partial-pressure. Knowledge of inter­mediate phases led to a better understanding of the phase-transition process and growth kinetics.