Analysis of stacking disorder in ice I using pair distribution functions

Manipulating disorder within cathodes of alkali-ion batteries

The fact that ordered materials are rarely perfectly crystalline is widely acknowledged among materials scientists, but its impact is often overlooked or underestimated when studying how structure relates to properties. Various investigations demonstrate that intrinsic and extrinsic defects, and disorder generated by physicochemical reactions, are responsible for unexpectedly detrimental or beneficial functionalities. The task remains to modulate the disorder to produce desired properties in materials. As disorder is often correlated with local interactions, it is controllable. In this Review, we explore the structural disorder in cathode materials as a novel approach for improving their electrochemical performance. We revisit cathode materials for alkali-ion batteries and outline the origins and beneficial consequences of disorder. Focusing on layered, cubic rocksalt and other metal oxides, we discuss how disorder improves electrochemical properties of cathode materials and which interactions generate the disorder. We also present the potential pitfalls of disorder that must be considered. We conclude with perspectives for enhancing the electrochemical performance of cathode materials by using disorder.

Canyon Diablo lonsdaleite is a nanocomposite containing c/h stacking disordered diamond and diaphite

In 1967, a diamond polymorph was reported from hard, diamond-like grains of the Canyon Diablo iron meteorite and named lonsdaleite. This mineral was defined and identified by powder X-ray diffraction (XRD) features that were indexed with a hexagonal unit cell. Since 1967, several natural and synthetic diamond-like materials with XRD data matching lonsdaleite have been reported and the name lonsdaleite was used interchangeably with hexagonal diamond. Its hexagonal structure was speculated to lead to physical properties superior to cubic diamond, and as such has stimulated attempts to synthesize lonsdaleite. Despite numerous reports, several recent studies have provided alternative explanations for the XRD, transmission electron microscopy and Raman data used to identify lonsdaleite. Here, we show that lonsdaleite from the Canyon Diablo diamond-like grains are a nanocomposite material dominated by subnanometre-scale cubic/hexagonal stacking disordered diamond and diaphite domains. These nanostructured elements are intimately intergrown, giving rise to structural features erroneously associated with h diamond. Our data suggest that the diffuse scattering in XRD and the hexagonal features in transmission electron microscopy images reported from various natural and laboratory-prepared samples that were previously used for lonsdaleite identification, in fact arise from cubic/hexagonal stacking disordered diamond and diaphite domains. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 2)'.

Structural Analysis of Molecular Materials Using the Pair Distribution Function

This is a review of atomic pair distribution function (PDF) analysis as applied to the study of molecular materials. The PDF method is a powerful approach to study short- and intermediate-range order in materials on the nanoscale. It may be obtained from total scattering measurements using X-rays, neutrons, or electrons, and it provides structural details when defects, disorder, or structural ambiguities obscure their elucidation directly in reciprocal space. While its uses in the study of inorganic crystals, glasses, and nanomaterials have been recently highlighted, significant progress has also been made in its application to molecular materials such as carbons, pharmaceuticals, polymers, liquids, coordination compounds, composites, and more. Here, an overview of applications toward a wide variety of molecular compounds (organic and inorganic) and systems with molecular components is presented. We then present pedagogical descriptions and tips for further implementation. Successful utilization of the method requires an interdisciplinary consolidation of material preparation, high quality scattering experimentation, data processing, model formulation, and attentive scrutiny of the results. It is hoped that this article will provide a useful reference to practitioners for PDF applications in a wide realm of molecular sciences, and help new practitioners to get started with this technique.

Comparison of the phase transitions of high-pressure phases of ammonium fluoride and ice at ambient pressure

The phase diagrams of water and ammonium fluoride (NH₄F) display some interesting parallels. Several crystalline NH₄F phases have isostructural ice counterparts and one of the famous anomalies of water, the fact that the liquid is denser than ice Ih, is also found for NH₄F. Here, we investigate the phase transitions of the pressure-quenched high-pressure phases of NH₄F upon heating at ambient pressure with x-ray diffraction and calorimetry, and we compare the results with the corresponding ices. NH₄F II transforms to NH₄F Isd, which is a stacking-disordered variant of the stable hexagonal NH₄F Ih polymorph. Heating NH₄F III gives a complex mixture of NH₄F II and NH₄F Isd, while some NH₄F III remains initially. Complete conversion to NH₄F Isd is achieved above ∼220 K. The NH₄F II obtained from NH₄F III persists to much higher temperatures compared to the corresponding pressure-quenched NH₄F II. Quantification of the stacking disorder in NH₄F Isd reveals a more sluggish conversion to NH₄F Ih for NH₄F Isd from NH₄F III. In general, the presence of stress and strain in the samples appears to have pronounced effects on the phase transition temperatures. NH₄F shows a complete lack of amorphous forms at low temperatures either upon low-temperature compression of NH₄F Ih or heating NH₄F III at ambient pressure. The amorphous forms of ice are often used to explain the anomalies of water. It will, therefore, be interesting to explore if liquid NH₄F displays more water-like anomalies despite the apparent lack of amorphous forms at low temperatures.

Long-Range Structures of Amorphous Solid Water

High-energy X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) of amorphous solid water (ASW) were studied during vapor deposition and the heating process. From the diffraction patterns, the oxygen–oxygen pair distribution functions (PDFs) were calculated up to the eighth coordination shell and an r = 23 Å. The PDF of ASW obtained both during vapor deposition at 80 K as well as the subsequent heating are consistent with that of low-density amorphous ice. The formation and temperature-induced collapse of micropores were observed in the XRD data and in the FTIR measurements, more specifically, in the OH stretch and the dangling mode. Above 140 K, ASW crystallizes into a stacking disordered ice, I_sd. It is observed that the fourth, fifth, and sixth peaks in the PDF, corresponding to structural arrangements between 8 and 12 Å, are the most sensitive to the onset of crystallization.

Designing disorder into crystalline materials

Crystals are a state of matter characterized by periodic order. Yet, crystalline materials can harbour disorder in many guises, such as non-repeating variations in composition, atom displacements, bonding arrangements, molecular orientations, conformations, charge states, orbital occupancies or magnetic structure. Disorder can sometimes be random but, more usually, it is correlated. Frontier research into disordered crystals now seeks to control and exploit the unusual patterns that persist within these correlated disordered states in order to access functional responses inaccessible to conventional crystals. In this Review, we survey the core design principles that guide targeted control over correlated disorder. We show how these principles - often informed by long-studied statistical mechanical models - can be applied across an unexpectedly broad range of materials, including organics, supramolecular assemblies, oxide ceramics and metal-organic frameworks. We conclude with a forward-looking discussion of the exciting link between disorder and function in responsive media, thermoelectrics and topological phases.

Cubic ice Ic without stacking defects obtained from ice XVII

Amongst the more than 18 different forms of water ice, only the common hexagonal phase and the cubic phase are present in nature on Earth. Nonetheless, it is now widely recognized that all samples of 'cubic ice' discovered so far do not have a fully cubic crystal structure but instead are stacking-disordered forms of ice I (namely, ice Isd), which contain both hexagonal and cubic stacking sequences of hydrogen-bonded water molecules. Here, we describe a method to obtain large quantities of cubic ice Ic with high structural purity. Cubic ice Ic is formed by heating a powder of D₂O ice XVII obtained from annealing of pristine C₀ hydrate samples under dynamic vacuum. Neutron diffraction experiments performed on two different instruments and Raman spectroscopy measurements confirm the structural purity of the cubic ice, Ic. These findings contribute to a better understanding of ice I polymorphism and the existence of the two natural ice forms.

On the role of intermolecular vibrational motions for ice polymorphs. II. Atomic vibrational amplitudes and localization of phonons in ordered and disordered ices

We investigate the vibrational amplitudes and the degree of the phonon localization in 19 ice forms, both crystalline and amorphous, by a quasi-harmonic approximation with a reliable classical intermolecular interaction model for water. The amplitude in the low pressure ices increases with compression, while the opposite trend is observed in the medium and high pressure ices. The amplitude of the oxygen atom does not differ from that of hydrogen in low pressure ices apart from the contribution from the zero-point vibrations. This is accounted for by the coherent but opposite phase motions in the mixed translational and rotational vibrations. A decoupling of translation-dominant and rotation-dominant motions significantly reduces the vibrational amplitudes in any ice form. The amplitudes in ice III are found to be much larger than any other crystalline ice form. In order to investigate the vibrational mode characteristics, the moment ratio of the atomic displacements for individual phonon modes, called the inverse participation ratio, is calculated and the degree of the phonon localization in crystalline and amorphous ices is discussed. It is found that the phonon modes in the hydrogen-ordered ice forms are remarkably spread over the entire crystal having propagative or diffusive characteristic, while many localized modes appear at the edges of the vibrational bands, called dissipative modes, in the hydrogen-disordered counterparts. The degree of localization is little pronounced in low density amorphous and high density amorphous due to disordering of oxygen atoms.

Quantifying hexagonal stacking in diamond

Diamond is a material of immense technological importance and an ancient signifier for wealth and societal status. In geology, diamond forms as part of the deep carbon cycle and typically displays a highly ordered cubic crystal structure. Impact diamonds, however, often exhibit structural disorder in the form of complex combinations of cubic and hexagonal stacking motifs. The structural characterization of such diamonds remains a challenge. Here, impact diamonds from the Popigai crater were characterized with a range of techniques. Using the MCDIFFaX approach for analysing X-ray diffraction data, hexagonality indices up to 40% were found. The effects of increasing amounts of hexagonal stacking on the Raman spectra of diamond were investigated computationally and found to be in excellent agreement with trends in the experimental spectra. Electron microscopy revealed nanoscale twinning within the cubic diamond structure. Our analyses lead us to propose a systematic protocol for assigning specific hexagonality attributes to the mineral designated as lonsdaleite among natural and synthetic samples.

Advances in the experimental exploration of water's phase diagram

Water's phase diagram displays enormous complexity with currently 17 experimentally confirmed polymorphs of ice and several more predicted computationally. For almost 120 years, it has been a stomping ground for scientific discovery, and ice research has often been a trailblazer for investigations into a wide range of materials-related phenomena. Here, the experimental progress of the last couple of years is reviewed, and open questions as well as future challenges are discussed. The specific topics include (i) the polytypism and stacking disorder of ice I, (ii) the mechanism of the pressure amorphization of ice I, (iii) the emptying of gas-filled clathrate hydrates to give new low-density ice polymorphs, (iv) the effects of acid/base doping on hydrogen-ordering phase transitions as well as (v) the formation of solid solutions between salts and the ice polymorphs, and the effect this has on the appearance of the phase diagram. In addition to continuing efforts to push the boundaries in terms of the extremes of pressure and temperature, the exploration of the "chemical" dimensions of ice research appears to now be a newly emerging trend. It is without question that ice research has entered a very exciting era.

Structural manifestation of partial proton ordering and defect mobility in ice Ih

High precision lattice-parameter measurements provide a potential roadmap to producing partially-ordered states of water ice.