arXiv subjects

Christoph G. Salzmann

Publications and source records attributed to Christoph G. Salzmann.

16 recordsLinked to original sources

arXiv2026

Crystallization of pristine cubic ice from liquid at ambient pressure

The phase diagram of frozen water is famously rich: to date, over twenty crystalline polymorphs have been identified. Of the low-pressure 'ice I' family, hexagonal (Ih) is the principal form on Earth, while cubic (Ic) is much more elusive. Fundamental questions remain open as to whether cubic ice Ic can form directly from the liquid state, its thermodynamic stability and natural occurrence. Here we show that pristine cubic ice Ic can be formed at atmospheric pressure simply by cooling an aqueous solution confined within mesoporous silica. Using primarily neutron scattering, we show unambiguously that under these conditions, cubic ice Ic forms reproducibly and is the only thermodynamically stable crystalline phase of water. The discovery that cubic ice Ic is directly accessible from the liquid state, and stable at atmospheric pressure, strongly suggests that this polymorph plays a much more significant role in natural and synthetic processes than previously thought.

cond-mat.mtrl-sci↗

arXiv2024

How Crystalline is Low-Density Amorphous Ice?

Low-density amorphous ice (LDA) is one of the most common solid materials in the Universe and a key material for understanding the many famous anomalies of liquid water. Yet, despite its significance and its discovery dating nearly 90 years, the structure of LDA is debated. It is unclear if LDA is a glassy state representing a liquid or a heavily disordered crystal; indeed, two forms (LDA-I and LDA-II) have been discussed as amorphous and partially crystalline in the literature, respectively. Here, with two widely used water models, we show that the experimental structure factor of LDA is best reproduced computationally by a partially crystalline structure. Models for both LDA-I and LDA-II are highly similar, with differences only due to subtle differences in crystallinity and/or experimental error. Further support for this structural model of LDA comes from experiment: if LDA is partially crystalline, then its route to formation should result in different nanocrystallite cubicities, and thus give rise to different cubicities upon recrystallisation. This memory effect of LDA's creation route is observed and it is incompatible with a fully amorphous material. The results present a unified computational and experimental view that LDA is not fully amorphous but instead a partially crystalline material. This impacts LDA's many roles in nature and potentially our understanding of liquid water. Furthermore, the "re-identification" of such an intensely studied material highlights that great care will be needed when classifying the nature of glassy materials going forward.

physics.chem-ph↗

arXiv2019

Deep-glassy ice VI confirmed with a combination of neutron spectroscopy and diffraction

The recent discovery of a low-temperature endotherm upon heating hydrochloric-acid doped ice VI has sparked a vivid controversy. The two competing explanations aiming to explain its origin range from a new distinct crystalline phase of ice to deep-glassy states of the well-known ice VI. Problems with the slow kinetics of deuterated phases have been raised, which we circumvent here entirely by simultaneously measuring the inelastic neutron spectra and neutron diffraction data of H2O samples. These measurements clearly confirm the deep-glassy ice VI scenario and rule out alternative explanations. Additionally, we show that the crystallographic model of D2O ice XV, the ordered counterpart of ice VI, also applies to the corresponding H2O phase. The discovery of deep-glassy ice VI now provides a fascinating new example of ultra-stable glasses which are encountered across a wide range of other materials.

cond-mat.mtrl-sci↗

arXiv2019

Ammonium Fluoride as a Hydrogen-disordering Agent for Ice

The removal of residual hydrogen disorder from various phases of ice with acid or base dopants at low temperatures has been a focus of intense research for many decades. As an antipode to these efforts, we now show using neutron diffraction that ammonium fluoride (NH4F) is a hydrogen-disordering agent for the hydrogen-ordered ice VIII. Cooling its hydrogen-disordered counterpart ice VII doped with 2.5 mol% ND4F under pressure leads to a hydrogen-disordered ice VIII with ~31% residual hydrogen disorder illustrating the long-range hydrogen-disordering effect of ND4F. The doped ice VII could be supercooled by ~20 K with respect to the hydrogen-ordering temperature of pure ice VII after which the hydrogen-ordering took place slowly over a ~60 K temperature window. These findings demonstrate that ND4F-doping slows down the hydrogen-ordering kinetics quite substantially. The partial hydrogen order of the doped sample is consistent with the antiferroelectric ordering of pure ice VIII. Yet, we argue that local ferroelectric domains must exist between ionic point defects of opposite charge. In addition to the long-range effect of NH4F-doping on hydrogen-ordered water structures, the design principle of using topological charges should be applicable to a wide range of other 'ice-rule' systems including spin ices and related polar materials.

cond-mat.mtrl-sci↗

arXiv2018

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 the polytypism and stacking disorder of ice I, the mechanism of the pressure amorphization of ice I, the emptying of gas-filled clathrate hydrates to give new low-density ice polymorphs, the effects of acid / base doping on hydrogen-ordering phase transitions as well as the formation of solids 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.

cond-mat.mtrl-sci↗

arXiv2018

Benchmarking acid and base dopants with respect to enabling the ice V to XIII and ice VI to XV hydrogen-ordering phase transitions

Doping the hydrogen-disordered phases of ice V, VI and XII with hydrochloric acid (HCl) has led to the discovery of their hydrogen-ordered counterparts ices XIII, XV and XIV. Yet, the mechanistic details of the hydrogen-ordering phase transitions are still not fully understood. This includes in particular the role of the acid dopant and the defect dynamics that it creates within the ices. Here we investigate the effects of several acid and base dopants on the hydrogen ordering of ices V and VI with calorimetry and X-ray diffraction. HCl is found to be most effective for both phases which is attributed to a favourable combination of high solubility and strong acid properties which create mobile H3O+ defects that enable the hydrogen-ordering processes. Hydrofluoric acid (HF) is the second most effective dopant highlighting that the acid strengths of HCl and HF are much more similar in ice than they are in liquid water. Surprisingly, hydrobromic acid doping facilitates hydrogen ordering in ice VI whereas only a very small effect is observed for ice V. Conversely, lithium hydroxide (LiOH) doping achieves a performance comparable to HF-doping in ice V but it is ineffective in the case of ice VI. Sodium hydroxide, potassium hydroxide (as previously shown) and perchloric acid doping are ineffective for both phases. These findings highlight the need for future computational studies but also raise the question why LiOH-doping achieves hydrogen-ordering of ice V whereas potassium hydroxide doping is most effective for the 'ordinary' ice Ih.

cond-mat.mtrl-sci↗

arXiv2017

Hydrogen mean force and anharmonicity in polycrystalline and amorphous ice

The hydrogen mean force from experimental neutron Compton profiles is derived using deep inelastic neutron scattering on amorphous and polycrystalline ice. The formalism of mean force is extended to probe its sensitivity to anharmonicity in the hydrogen-nucleus effective potential. The shape of the mean force for amorphous and polycrystalline ice is primarily determined by the anisotropy of the underlying quasi-harmonic effective potential. The data from amorphous ice show an additional curvature reflecting the more pronounced anharmonicity of the effective potential with respect to that of ice Ih.

physics.chem-ph↗

arXiv2017

Double-layer ice from first principles

The formation of monolayer and multilayer ice with a square lattice structure has recently been reported on the basis of transmission electron microscopy experiments, renewing interest in confined two dimensional ice. Here we report a systematic density functional theory study of double-layer ice in nano-confinement. A phase diagram as a function of confinement width and lateral pressure is presented. Included in the phase diagram are honeycomb hexagonal, square-tube, hexagonal-close-packed and buckled-rhombic structures. However, contrary to experimental observations, square structures do not feature: our most stable double-layer square structure is predicted to be metastable. This study provides general insight into the phase transitions of double-layer confined ice and a fresh theoretical perspective on the stability of square ice in graphene nanocapillary experiments.

cond-mat.mtrl-sci↗

arXiv2017

Is High-density Amorphous Ice Simply a 'Derailed' State along the Ice I to Ice IV Pathway?

The structural nature of high-density amorphous ice (HDA), which forms through low-temperature pressure-induced amorphization of the 'ordinary' ice I, is heavily debated. Clarifying this question is not only important for understanding the complex condensed states of H$_2$O but also in the wider context of pressure-induced amorphization processes, which are encountered across the entire materials spectrum. We first show that ammonium fluoride (NH$_4$F), which has a similar hydrogen-bonded network to ice I, also undergoes a pressure collapse upon compression at 77 K. However, the product material is not amorphous but NH$_4$F II, a high-pressure phase isostructural with ice IV. This collapse can be rationalized in terms of a highly effective mechanism. In the case of ice I, the orientational disorder of the water molecules leads to a deviation from this mechanism and we therefore classify HDA as a 'derailed' state along the ice I to ice IV pathway.

cond-mat.mtrl-sci↗

arXiv2016

Detailed crystallographic analysis of the ice VI to ice XV hydrogen ordering phase transition

The D2O ice VI to ice XV hydrogen ordering phase transition at ambient pressure is investigated in detail with neutron diffraction. The lattice constants are found to be sensitive indicators for hydrogen ordering. The a and b lattice constants contract whereas a pronounced expansion in c is found upon hydrogen ordering. Overall, the hydrogen ordering transition goes along with a small increase in volume which explains why the phase transition is more difficult to observe upon cooling under pressure. Slow-cooling ice VI at 1.4 GPa gives essentially fully hydrogen-disordered ice VI. Consistent with earlier studies, the ice XV obtained after slow-cooling at ambient pressure is best described with P-1 space group symmetry. Using a new computational approach, we achieve the atomistic reconstruction of a supercell structure that is consistent with the average partially ordered structure derived from Rietveld refinements. This shows that C-type networks are most prevalent in ice XV but other structural motifs outside of the classifications of the fully hydrogen-ordered networks are identified as well. The recently proposed Pmmn structural model for ice XV is found to be incompatible with our diffraction data and we argue that only structural models that are capable of describing full hydrogen order should be used.

cond-mat.mtrl-sci↗

arXiv2016

A new structural relaxation pathway of low-density amorphous ice

Low-density amorphous ice (LDA) is involved in critical cosmological processes and has gained prominence as one of the at least two distinct amorphous forms of ice. Despite these accolades, we still have an incomplete understanding of the structural diversity that is encompassed within the LDA state and the dynamic processes that take place upon heating LDA. Heating the high-pressure ice VIII phase at ambient pressure is a remarkable example of temperature-induced amorphisation yielding LDA. We investigate this process in detail using X-ray diffraction and Raman spectroscopy, and show that the LDA obtained from ice VIII is structurally different from the more 'traditional' states of LDA which are approached upon thermal annealing. This new structural relaxation pathway involves an increase of structural order on the intermediate range length scale. In contrast with other LDA materials the local structure is more ordered initially and becomes slightly more disordered upon annealing. We also show that the cascade of phase transitions upon heating ice VIII at ambient pressure includes the formation of ice IX which may be connected with the structural peculiarities of LDA from ice VIII. Overall, this study shows that LDA is a structurally more diverse material than previously appreciated.

cond-mat.mtrl-sci↗

arXiv2015

2D ice from first principles: structures and phase transitions

Despite relevance to disparate areas such as cloud microphysics and tribology, major gaps in the understanding of the structures and phase transitions of low-dimensional water ice remain. Here we report a first principles study of confined 2D ice as a function of pressure. We find that at ambient pressure hexagonal and pentagonal monolayer structures are the two lowest enthalpy phases identified. Upon mild compression the pentagonal structure becomes the most stable and persists up to \textit{ca.} 2 GPa at which point square and rhombic phases are stable. The square phase agrees with recent experimental observations of square ice confined within graphene sheets. We also find a double layer AA stacked square ice phase, which clarifies the difference between experimental observations and earlier force field simulations. This work provides a fresh perspective on 2D confined ice, highlighting the sensitivity of the structures observed to both the confining pressure and width.

cond-mat.mtrl-sci↗

arXiv2015

The complex kinetics of the ice VI to ice XV hydrogen ordering phase transition

The reversible phase transition from hydrochloric-acid-doped ice VI to its hydrogen-ordered counterpart ice XV is followed using differential scanning calorimetry. Upon cooling at ambient pressure fast hydrogen ordering is observed at first followed by a slower process which manifests as a tail to the initial sharp exotherm. The residual hydrogen disorder in H2O and D2O ice XV is determined as a function of the cooling rate. We conclude that it will be difficult to obtain fully hydrogen-ordered ice XV by cooling at ambient pressure. Our new experimental findings are discussed in the context of recent computational work on ice XV.

cond-mat.mtrl-sci↗

arXiv2015

Extent of stacking disorder in diamond

Hexagonal diamond has been predicted computationally to display extraordinary physical properties including a hardness that exceeds cubic diamond. However, a recent electron microscopy study has shown that so-called hexagonal diamond samples are in fact not discrete materials but faulted and twinned cubic diamond. We now provide a quantitative analysis of cubic and hexagonal stacking in diamond samples by analysing X-ray diffraction data with the DIFFaX software package. The highest fractions of hexagonal stacking we find in materials which were previously referred to as hexagonal diamond are below 60%. The remainder of the stacking sequences are cubic. We show that the cubic and hexagonal sequences are interlaced in a complex way and that naturally occurring Lonsdaleite is not a simple phase mixture of cubic and hexagonal diamond. Instead, it is structurally best described as stacking disordered diamond. The future experimental challenge will be to prepare diamond samples beyond 60% hexagonality and towards the so far elusive 'perfect' hexagonal diamond.

cond-mat.mtrl-sci↗

arXiv2009

Edge-carboxylated Carbon Nanoflakes from Nitric Acid Oxidised Arc-discharge Material

Carbon Nanoflakes (CNFs) with average diameters of about 30 nm have been prepared and isolated in bulk quantities by a single-step oxidation procedure using single-wall carbon nanotube arc-discharge material and nitric acid. The CNFs are predominately single, graphenic sheets containing a small number of internal defects. The edges are decorated with primarily carboxylic acid groups which allow facile chemical functionalisation and cross-linking of the fragments using multivalent cations.

cond-mat.mtrl-sci↗

arXiv2009

Ice XV: a new thermodynamically stable phase of ice

A new phase of ice, named ice XV, has been identified and its structure determined by neutron diffraction. Ice XV is the hydrogen-ordered counterpart of ice VI and is thermodynamically stable at temperatures below ~130 K in the 0.8 to 1.5 GPa pressure range. The regions of stability in the medium pressure range of the phase diagram have thus been finally mapped, with only hydrogen-ordered phases stable at 0 K. The ordered ice XV structure is antiferroelectric, in clear disagreement with recent theoretical calculations predicting ferroelectric ordering.

cond-mat.mtrl-sci↗