Kinetic isotope effects on hydrogen/deuterium disordering and ordering in ice crystals: A Raman and dielectric study of ice VI, XV, and XIX.

Ice XIX and ice XV are both partly hydrogen-ordered counterparts to disordered ice VI. The ice XIX → XV transition represents the only order-to-order transition in ice physics. Using Raman and dielectric spectroscopies, we investigate the ambient-pressure kinetics of the two individual steps in this transition in real time (of hours), that is, ice XIX → transient ice VI (the latter called VI‡) and ice VI‡ → ice XV. Hydrogen-disordered ice VI‡ appears intermittent between 101 and 120 K, as inferred from the appearance and subsequent disappearance of the ice VI Raman marker bands. A comparison of the rate constants for the H2O ices reported here with those from D2O samples [Thoeny et al., J. Chem. Phys. 156, 154507 (2022)] reveals a large kinetic isotope effect for the ice XIX decay, but a much smaller one for the ice XV buildup. An enhancement of the classical overbarrier rate through quantum tunneling for the former can provide a possible explanation for this finding. The activation barriers for both transitions are in the 18-24 kJ/mol range, which corresponds to the energy required to break a single hydrogen bond. These barriers do not show an H/D isotope effect and are the same, no matter whether they are derived from Raman scattering or from dielectric spectroscopy. These findings favor the notion that a dipolar reorientation, involving the breakage of a hydrogen bond, is the rate determining step at the order-to-order transition.

The impact of alcohol and ammonium fluoride on pressure-induced amorphization of cubic structure I clathrate hydrates.

We have investigated pressure-induced amorphization (PIA) of an alcohol clathrate hydrate (CH) of cubic structure type I (sI) in the presence of NH4F utilizing dilatometry and x-ray powder diffraction. PIA occurs at 0.98 GPa at 77 K, which is at a much lower pressure than for other CHs of the same structure type. The amorphized CH also shows remarkable resistance against crystallization upon decompression. While amorphized sI CHs could not be recovered previously at all, this is possible in the present case. By contrast to other CHs, the recovery of the amorphized CHs to ambient pressure does not even require a high-pressure annealing step, where recovery without any loss of amorphicity is possible at 120 K and below. Furthermore, PIA is accessible upon compression at unusually high temperatures of up to 140 K, where it reaches the highest degree of amorphicity. Molecular dynamics simulations confirm that polar alcoholic guests, as opposed to non-polar guests, induce cage deformation at lower pressure. The substitution of NH4F into the host-lattice stabilizes the collapsed state more than the crystalline state, thereby enhancing the collapse kinetics and lowering the pressure of collapse.

Configurational entropy of ice XIX and its isotope effect.

Ice XIX is a partly hydrogen-ordered polymorph related to disordered ice VI, similar to ice XV. We here investigate the order-order-disorder sequence ice XIX→ice XV→ice VI based on calorimetry at ambient pressure both for D2O and H2O-ice XIX. From these data we extract configurational entropy differences between ice XIX, ice XV and ice VI. This task is complex because, unlike for all other ices, the order-disorder transition from ice XIX to ice VI takes place in two steps via ice XV. Even more challenging, these two steps take place in an overlapping manner, so that careful separation of slow kinetics is necessary. This is evidenced best by changing the heating rate in calorimetry experiments: For fast heating experiments the second step, disordering of ice XV, is suppressed because the first step, formation of ice XV from ice XIX, is too slow. The transient state ice VI‡ that is initially produced upon ice XIX decay then does not have enough time to convert to ice XV, but remains disordered all along. In order to tackle the challenge to determine the entropy difference between ice XIX and VI as well as the entropy difference between ice XV and VI we employ two different approaches that allow assessing the impact of kinetics on the entropy change. "Single peak integration" defines a kinetically limited result, but "combined peak integration" allows estimation of the true thermodynamic values. Our best estimate for the true value shows ice XIX to be much more ordered than ice XV (25 ± 3% vs 9 ± 4% of the Pauling entropy). For D2Oice XIX samples we obtain 28% of order, but only when a small number of fast H-isotope defects are used. In the second part we use these results to estimate the location of the ice XIX phase boundary both for protiated and deuterated ice XIX. The initial Clapeyron slope at ambient pressure is determined from the combination of neutron powder diffraction volume differences and calorimetry entropy differences data to be 21 K GPa-1 with an order-disorder transition temperature To-d(0.0 GPa) = 103 ± 1 K. An in situ bracketing experiment at 1.8 GPa yields To-d(1.8 GPa) = 116 ± 3 K, i.e., the phase boundary slope flattens at higher pressures. These data allow us to determine the region of thermodynamic stability of ice XIX in the phase diagram and to explain the surprising isotope shift reversal at 1.6 GPa compared to 0.0 GPa, i.e., why D2O-ice XIX disorders at lower temperatures than H2O-ice XIX at 1.6 GPa, but at higher temperatures at ambient pressures.

Tuning the low-temperature phase behavior of aqueous ionic liquids.

Water's anomalous behavior is often explained using a two-liquid model, where two types of water, high-density liquid (HDL) and low-density liquid (LDL), can be separated via a liquid-liquid phase transition (LLPT) at low temperature. Mixtures of water and the ionic liquid hydrazinium trifluoroacetate were suggested to also show an LLPT but with the advantage that there is no rapid ice crystallization hampering its observation. It remains controversial whether these solutions exhibit an LLPT or are instead associated with complex phase separation phenomena. We here show detailed low-temperature calorimetry and diffraction experiments on aqueous solutions containing hydrazinium trifluoroacetate and other similar ionic liquids, all at a solute mole fraction of x = 0.175. Hydrazinium trifluoroacetate, ammonium trifluoroacetate, ethylammonium trifluoroacetate and hydrazinium pentafluoropropionate all boast exothermic transitions unrelated to crystallization as well as remarkable structural changes upon cooling into the glassy state. We propose a model inspired by micelle formation and decomposition in surfactant solutions, which is complemented by MD simulations and allows rationalizing the rich phase behavior of our mixtures during cooling. The fundamental aspect of the model is the hydrophobic nature of fluorinated anions that enables aggregation, which is reversed upon cooling and culminates in the remarkable exothermic first-order transition observed at low temperature. That is, we assign the first-order transition not to an LLPT but to phase-separations similar to the ones when falling below the Krafft temperature. All other solutions merely show simple vitrification behavior. Still, they exhibit distinct differences in liquid fragility, which is decreased continuously with decreasing hydrophobicity of the anions. This might enable the systematic tuning of ionic liquids with the goal of designing aqueous solutions of specific fragility.

Thermodynamically Stable Intermediate in the Course of Hydrogen Ordering from Ice V to Ice XIII.

Even though many partially ordered ices are known, it remains elusive to understand and categorize them. In this study, we study the ordering from ice V to XIII using calorimetry at ambient pressure and discover that the transition takes place via an intermediate that is thermodynamically stable at 113-120 K. Our isothermal ordering approach allows us to highlight the distinction of this intermediate from ice V and XIII, where there are clear differences both in terms of enthalpy and ordering kinetics. We suggest that the approach developed in the present work can also reveal the nature of partially ordered forms in the hydrogen order-disorder series of other ice phases.

A laboratory-based multifunctional near ambient pressure X-ray photoelectron spectroscopy system for electrochemical, catalytic, and cryogenic studies.

A versatile multifunctional laboratory-based near ambient pressure x-ray photoelectron spectroscopy (XPS) instrument is presented. The entire device is highly customized regarding geometry, exchangeable manipulators and sample stages for liquid- and solid-state electrochemistry, cryochemistry, and heterogeneous catalysis. It therefore delivers novel and unique access to a variety of experimental approaches toward a broad choice of functional materials and their specific surface processes. The high-temperature (electro)catalysis manipulator is designed for probing solid state/gas phase interactions for heterogeneous catalysts including solid electrolyzer/fuel cell electrocatalysts at pressures up to 15 mbar and temperatures from room temperature to 1000 °C. The liquid electrochemistry manipulator is specifically designed for in situ spectroscopic investigations of polarized solid/liquid interfaces using aqueous electrolytes and the third one for experiments for ice and ice-like materials at cryogenic temperatures to approximately -190 °C. The flexible and modular combination of these setups provides the opportunity to address a broad spectrum of in situ and operando XPS experiments on a laboratory-based system, circumventing the limited accessibility of experiments at synchrotron facilities.

Investigating freezing-induced acidity changes in citrate buffers.

Citrate buffers are commonly utilized in the field of biomolecule stabilization. We investigate their applicability in the frozen state within a range of initial pHs (2.5 to 8.0) and concentrations (0.02 to 0.60 M). Citrate buffer solutions subjected to various cooling and heating temperatures are examined in terms of the freezing-induced acidity changes, revealing that citrate buffers acidify upon cooling. The acidity is assessed with sulfonephthalein molecular probes frozen in the samples. Optical cryomicroscopy combined with differential scanning calorimetry was employed to investigate the causes of the observed acidity changes. The buffers partly crystallize and partly vitrify in the ice matrix; these processes influence the resulting pH and allow designing the optimal storage temperatures in the frozen state. The freezing-induced acidification apparently depends on the buffer concentration; at each pH, we suggest pertinent concentration, at which freezing causes minimal acidification.

Neutron scattering study of polyamorphic THF·17(H2O) - toward a generalized picture of amorphous states and structures derived from clathrate hydrates.

From crystalline tetrahydrofuran clathrate hydrate, THF-CH (THF·17H2O, cubic structure II), three distinct polyamorphs can be derived. First, THF-CH undergoes pressure-induced amorphization when pressurized to 1.3 GPa in the temperature range 77-140 K to a form which, in analogy to pure ice, may be called high-density amorphous (HDA). Second, HDA can be converted to a densified form, VHDA, upon heat-cycling at 1.8 GPa to 180 K. Decompression of VHDA to atmospheric pressure below 130 K produces the third form, recovered amorphous (RA). Results from neutron scattering experiments and molecular dynamics simulations provide a generalized picture of the structure of amorphous THF hydrates with respect to crystalline THF-CH and liquid THF·17H2O solution (∼2.5 M). Although fully amorphous, HDA is heterogeneous with two length scales for water-water correlations (less dense local water structure) and guest-water correlations (denser THF hydration structure). The hydration structure of THF is influenced by guest-host hydrogen bonding. THF molecules maintain a quasiregular array, reminiscent of the crystalline state, and their hydration structure (out to 5 Å) constitutes ∼23H2O. The local water structure in HDA is reminiscent of pure HDA-ice featuring 5-coordinated H2O. In VHDA, the hydration structure of HDA is maintained but the local water structure is densified and resembles pure VHDA-ice with 6-coordinated H2O. The hydration structure of THF in RA constitutes ∼18 H2O molecules and the water structure corresponds to a strictly 4-coordinated network, as in the liquid. Both VHDA and RA can be considered as homogeneous.

Enthalpy Change from Pure Cubic Ice Ic to Hexagonal Ice Ih.

The preparation of pure cubic ice without hexagonal stacking faults has been realized only recently by del Rosso et al. ( Nat. Mater. 2020, 19, 663-668) and Komatsu et al. ( Nat. Commun. 2020, 11, 464). With our present calorimetric study on the transition from pure cubic ice to hexagonal ice we are able to clarify the value of the enthalpy change ΔHc→h to be -37.7 ± 2.3 J mol-1. The transition temperature is identified as 226 K, much higher than in previous work on ice Isd. This is due to a catalytic effect of hexagonal faults on the transition, but even more importantly due to a relaxation exotherm that was not properly identified in the past.

Glass Polymorphism in Hyperquenched Aqueous LiCl Solutions.

We investigate the glass polymorphism of dilute LiCl-H2O in the composition range of 0-5.8 mol % LiCl. The solutions are vitrified at ambient pressure (requires hyperquenching with ∼106 K s-1) and transformed to their high-density state using a special high-pressure annealing protocol. Ex situ characterization was performed via isobaric heating experiments using X-ray diffraction and differential scanning calorimetry. We observe signatures from a distinct high-density and a distinct low-density glass for all solutions with a mole fraction xLiCl of ≤ 4.3 mol %, where the most notable are (i) the jumplike polyamorphic transition from high-density to low-density glass and (ii) two well-separated glass-to-liquid transitions Tg,1 and Tg,2, each pertaining to one glass polymorph. These features are absent for solutions with xLiCl ≥ 5.8 mol %, which show only continuous densification and relaxation behavior. That is, a switch from water-dominated to solute-dominated region occurs between 4.3 mol % LiCl and 5.8 mol % LiCl. For the water-dominated region, we find that LiCl has a huge impact only on the low-density form. This is manifested as a shift in halo peak position to denser local structures, a lowering of Tg,1, and a significant change in relaxation dynamics. These effects of LiCl are observed both for hyperquenched samples and low-density samples obtained via heating of the high-density glasses, suggesting path independence. Such behavior further necessitates that LiCl is distributed homogeneously in the low-density glass. This contrasts earlier studies in which structural heterogeneity is claimed: ions were believed to be surrounded by only high-density states, thereby enforcing a phase separation into ion-rich high-density and ion-poor low-density glasses. We speculate the difference arises from the difference in cooling rates, which are higher by at least 1 order of magnitude in our case.

Nucleation and growth of crystalline ices from amorphous ices.

We here review mostly experimental and some computational work devoted to nucleation in amorphous ices. In fact, there are only a handful of studies in which nucleation and growth in amorphous ices are investigated as two separate processes. In most studies, crystallization temperatures Tx or crystallization rates RJG are accessed for the combined process. Our Review deals with different amorphous ices, namely, vapor-deposited amorphous solid water (ASW) encountered in many astrophysical environments; hyperquenched glassy water (HGW) produced from µm-droplets of liquid water; and low density amorphous (LDA), high density amorphous (HDA), and very high density amorphous (VHDA) ices produced via pressure-induced amorphization of ice I or from high-pressure polymorphs. We cover the pressure range of up to about 6 GPa and the temperature range of up to 270 K, where only the presence of salts allows for the observation of amorphous ices at such high temperatures. In the case of ASW, its microporosity and very high internal surface to volume ratio are the key factors determining its crystallization kinetics. For HGW, the role of interfaces between individual glassy droplets is crucial but mostly neglected in nucleation or crystallization studies. In the case of LDA, HDA, and VHDA, parallel crystallization kinetics to different ice phases is observed, where the fraction of crystallized ices is controlled by the heating rate. A key aspect here is that in different experiments, amorphous ices of different "purities" are obtained, where "purity" here means the "absence of crystalline nuclei." For this reason, "preseeded amorphous ice" and "nuclei-free amorphous ice" should be distinguished carefully, which has not been done properly in most studies. This makes a direct comparison of results obtained in different laboratories very hard, and even results obtained in the same laboratory are affected by very small changes in the preparation protocol. In terms of mechanism, the results are consistent with amorphous ices turning into an ultraviscous, deeply supercooled liquid prior to nucleation. However, especially in preseeded amorphous ices, crystallization from the preexisting nuclei takes place simultaneously. To separate the time scales of crystallization from the time scale of structure relaxation cleanly, the goal needs to be to produce amorphous ices free from crystalline ice nuclei. Such ices have only been produced in very few studies.

Raman Marker Bands for Secondary Structure Changes of Frozen Therapeutic Monoclonal Antibody Formulations During Thawing.

In this work we use Raman spectroscopy for protein characterization in the frozen state. We investigate the behavior of frozen therapeutic monoclonal antibody IgG1 formulation upon thawing by Raman spectroscopy. Secondary and tertiary structure of the protein in three different mab formulations in the frozen state are followed through observation of marker bands for α-helix, ß-sheet and random coil. We identify the tyrosine intensity ratio I856/I830 as a marker for mab aggregation. Upon fast cooling (40 °C/min) to -80 °C we observe a significant increase of random coil and α -helical structures, while this is not the case for slower cooling (20 °C/min) to -80 °C. Most changes in the protein's secondary structure are observed in the course of thawing in the range up to -20 °C, when passing through the glass transitions and cold-crystallization of the two types of freeze-concentrated solutions formed through macro- and microcryoconcentration. An increase of protein concentration and the addition of mannitol suppress secondary structural changes but do no impact on aggregation.

Infrared Spectroscopy on Equilibrated High-Density Amorphous Ice.

High-density (HDA) and low-density amorphous ices (LDA) are believed to be counterparts of the high- and low-density liquid phases of water, respectively. In order to better understand how the vibrational modes change during the transition between the two solid states, we present infrared spectroscopy measurements, following the change of the decoupled OD-stretch (vOD) (∼2460 cm-1) and OH-combinational mode (vOH + v2, vOH + 2vR) (∼5000 cm-1). We observe a redshift from HDA to LDA, accompanied with a drastic decrease of the bandwidth. The hydrogen bonds are stronger in LDA, which is caused by a change in the coordination number and number of water molecules interstitial between the first and second hydration shell. The unusually broad uncoupled OD band also clearly distinguishes HDA from other crystalline high-pressure phases, while the shape and position of the in situ prepared LDA are comparable to those of vapor-deposited amorphous ice.

Pressure-annealed high-density amorphous ice made from vitrified water droplets: A systematic calorimetry study on water's second glass transition.

In previous work, water's second glass transition was investigated based on an amorphous sample made from crystalline ice [Amann-Winkel et al., Proc. Natl. Acad. Sci. U. S. A. 110, 17720 (2013)]. In the present work, we investigate water's second glass transition based on the genuine glassy state of high-density water as prepared from micron-sized liquid water droplets, avoiding crystallinity at all stages. All the calorimetric features of water's second glass transition observed in the previous work are also observed here on the genuine glassy samples. This suggests that the glass transition indeed thermodynamically links amorphous ices continuously with deeply supercooled water. We proceed to extend the earlier study by investigating the effect of preparation history on the calorimetric glass transition temperature. The best samples prepared here feature both a lower glass transition temperature Tg,2 and a higher polyamorphic transition temperature Tons, thereby extending the range of thermal stability in which the deeply supercooled liquid can be observed by about 4 K. Just before the polyamorphic transition, we observe a spike-like increase of heat capacity that we interpret in terms of nucleation of low-density water. Without this spike, the width of water's second glass transition is 15 K, and the Δcp amounts to 3 ± 1 J K-1 mol-1, making the case for the high-density liquid being a strong liquid. We suggest that samples annealed at 1.9 GPa to 175 K and decompressed at 140 K to ≥0.10 GPa are free from such nuclei and represent the most ideal high-density amorphous glasses.

Increase of Radiative Forcing through Midinfrared Absorption by Stable CO2 Dimers?

We performed matrix-isolation infrared (MI-IR) spectroscopy of carbon dioxide monomers, CO2, and dimers, (CO2)2, trapped in neon and in air. On the basis of vibration configuration interaction (VCI) calculations accounting for mode coupling and anharmonicity, we identify additional infrared-active bands in the MI-IR spectra due to the (CO2)2 dimer. These bands are satellite bands next to the established CO2 monomer bands, which appear in the infrared window of Earth's atmosphere at around 4 and 15 µm. In a systematic carbon dioxide mixing ratio study using neon matrixes, we observe a significant fraction of the dimer at mixing ratios above 300 ppm, with a steep increase up to 1000 ppm. In neon matrix, the dimer increases the IR absorbance by about 15% at 400 ppm compared to the monomer absorbance alone. This suggests a high fraction of the (CO2)2 dimer in our matrix experiments. In atmospheric conditions, such increased absorbance would significantly amplify radiative forcings and, thus, the greenhouse warming. To enable a comparison of our laboratory experiment with various atmospheric conditions (Earth, Mars, Venus), we compute the thermodynamics of the dimerization accordingly. The dimerization is favored at low temperatures and/or high carbon dioxide partial pressures. Thus, we argue that matrix isolation does not trap the gas composition "as is". Instead, the gas is precooled to 40 K, where CO2 dimerizes before being trapped in the matrix, already at very low carbon dioxide partial pressures. In the context of planetary atmospheres, our results improve understanding of the greenhouse effect for planets of rather thick CO2 atmospheres such as Venus, where a significant fraction of the (CO2)2 dimer can be expected. There, the necessity of including the mid-IR absorption by stable (CO2)2 dimers in databases used for modeling radiative forcing, such as HITRAN, arises.

Raman spectroscopy study of the slow order-order transformation of deuterium atoms: Ice XIX decay and ice XV formation.

The nature of the hydrogen substructure of a deuterated and deuterium chloride (DCI)-doped ice VI sample after cooling at 1.8 GPa has been a topic of recent interest-especially because the novel ice polymorph ice XIX was discovered in the course of such studies. We here investigate deuterated samples containing 5% H2O using Raman spectroscopy to probe for transitions associated with rearrangement of D-atoms in ice XIX. The protocol involving heating at subambient pressure (10 mbar) in this study follows closely the one used in our earlier neutron diffraction study. Heating of ice XIX induces a complex cascade of processes involving both ordering and disordering of D atoms. Our Raman spectra demonstrate that the transition sequence is ice XIX → ice VI‡ → ice XV, in accordance with our earlier neutron diffraction result. First signs for ice XIX decay are evident at 100 K, while ice XV build-up is seen only at 108 K and above. Between 100 and 108 K, a transiently disordered D-substructure appears, where at 108 K, ice VI‡ forms from ice XIX and simultaneously decays to produce ice XV-thereby establishing a dynamic equilibrium. Using isothermal, time-resolved Raman spectroscopy in real time, we here determine rate constants, Avrami exponents, and activation energies for both slow processes, ice XIX decay and ice XV build-up. The first transition in this sequence, ice XIX decay, is faster than the second transition, ice XV build-up, so that ice VI‡ accumulates. On the basis of the Johnson-Mehl-Avrami-Kolmogorov data obtained from the isothermal Raman experiment, we additionally report kinetic models for the development of fractions of ices XIX, XV, and VI‡ in non-isothermal heating experiments at different heating rates. These models consider the two coupled first-order transitions as separated processes, where the phase fractions are calculated for incrementally small temperature (or time) steps. These models compare favorably with our previous observations for slowly or rapidly heated ice XIX samples, such as in calorimetry or neutron diffraction experiments.

Oxygen NMR of high-density and low-density amorphous ice.

Using oxygen-17 as a nuclear probe, spin relaxometry was applied to study the high-density and low-density states of amorphous ice, covering temperatures below and somewhat above their glass transitions. These findings are put in perspective with results from deuteron nuclear magnetic resonance and with calculations based on dielectrically detected correlation times. This comparison reveals the presence of a wide distribution of correlation times. Furthermore, oxygen-17 central-transition echo spectra were recorded for wide ranges of temperature and pulse spacing. The spectra cannot be described by a single set of quadrupolar parameters, suggesting a distribution of H-O-H opening angles that is broader for high-density than for low-density amorphous ice. Simulations of the pulse separation dependent spin-echo spectra for various scenarios demonstrate that a small-step frequency diffusion process, assigned to the presence of homonuclear oxygen-oxygen interactions, determines the shape evolution of the pulse-separation-dependent spectra.

The impact of temperature and unwanted impurities on slow compression of ice.

For slowly compressed hexagonal ice pressure-induced amorphisation to high-density amorphous ice (HDA) takes place below and at 130 K, but polymorphic transformation to ice IX takes place at 140-170 K. Stable ice II only forms above 170 K. Ice IX impurities trigger ice IX growth even at 120 K. HDA and ice IX are equally long-lived, where both can be regarded as metastable phases.

Advances in the study of supercooled water.

In this review, we report recent progress in the field of supercooled water. Due to its uniqueness, water presents numerous anomalies with respect to most simple liquids, showing polyamorphism both in the liquid and in the glassy state. We first describe the thermodynamic scenarios hypothesized for the supercooled region and in particular among them the liquid-liquid critical point scenario that has so far received more experimental evidence. We then review the most recent structural indicators, the two-state model picture of water, and the importance of cooperative effects related to the fact that water is a hydrogen-bonded network liquid. We show throughout the review that water's peculiar properties come into play also when water is in solution, confined, and close to biological molecules. Concerning dynamics, upon mild supercooling water behaves as a fragile glass former following the mode coupling theory, and it turns into a strong glass former upon further cooling. Connections between the slow dynamics and the thermodynamics are discussed. The translational relaxation times of density fluctuations show in fact the fragile-to-strong crossover connected to the thermodynamics arising from the existence of two liquids. When considering also rotations, additional crossovers come to play. Mobility-viscosity decoupling is also discussed in supercooled water and aqueous solutions. Finally, the polyamorphism of glassy water is considered through experimental and simulation results both in bulk and in salty aqueous solutions. Grains and grain boundaries are also discussed.

Calorimetric Investigation of Hydrogen-Atom Sublattice Transitions in the Ice VI/XV/XIX Trio.

Ice XIX represents the latest discovery of ice polymorphs and exists in the medium pressure range near 1-2 GPa. Ice XIX is a partially hydrogen-ordered phase, by contrast to its disordered mother phase ice VI, which shares the same oxygen-atom network with ice XIX. Ice XIX differs in terms of the ordering of the hydrogen-atom sublattice, and hence the space group, from its hydrogen-ordered sibling ice XV, which also features the same type of oxygen network. Together, ice VI, XV, and XIX form the only known trio of ice polymorphs, where polymorphic transformations from order to order, order to disorder, and disorder to order are possible, which also compete with each other depending on the thermodynamic path taken and the cooling/heating rates employed. These transitions in the H-sublattice have barely been investigated, so we study here the unique triangular relation in the ice VI/XV/XIX trio based on calorimetry experiments. We reveal the following key features for H-sublattice transitions: (i) upon cooling ice VI, domains of ice XV and XIX develop simultaneously, where pure ice XV forms at ≤0.85 GPa and pure ice XIX forms at ≥1.60 GPa, (ii) ice XIX transforms into ice XV via a transient disordered state, (iii) ice XV recooled at ambient pressure features a complex domain structure, possibly containing an unknown H-ordered polymorph, (iv) recooled ice XV partly transforms back into ice XIX at 1.80 GPa, and (v) partial deuteration slows down domain reordering strongly. These findings not only are of interest in understanding possible hydrogen-ordering and -disordering processes in the interior of icy moons and planets but, more importantly, also provide a challenging benchmark for our understanding and parameterizing many-body interactions in H-bonded networks.