Cryoflotation: densities of amorphous and crystalline ices.

We present an experimental method aimed at measuring mass densities of solids at ambient pressure. The principle of the method is flotation in a mixture of liquid nitrogen and liquid argon, where the mixing ratio is varied until the solid hovers in the liquid mixture. The temperature of such mixtures is in the range of 77-87 K, and therefore, the main advantage of the method is the possibility of determining densities of solid samples, which are instable above 90 K. The accessible density range (~0.81-1.40 g cm(-3)) is perfectly suitable for the study of crystalline ice polymorphs and amorphous ices. As a benchmark, we here determine densities of crystalline polymorphs (ices I(h), I(c), II, IV, V, VI, IX, and XII) by flotation and compare them with crystallographic densities. The reproducibility of the method is about ±0.005 g cm(-3), and in general, the agreement with crystallographic densities is very good. Furthermore, we show measurements on a range of amorphous ice samples and correlate the density with the d spacing of the first broad halo peak in diffraction experiments. Finally, we discuss the influence of microstructure, in particular voids, on the density for the case of hyperquenched glassy water and cubic ice samples prepared by deposition of micrometer-sized liquid droplets.

Equilibrated high-density amorphous ice and its first-order transition to the low-density form.

We investigate the downstroke transition from high- (HDA) to low-density amorphous ice (LDA) at 140 (H(2)O) and 143 K (D(2)O). The visual observation of sudden phase separation at 0.07 GPa is evidence of the first-order character of the transition. Powder X-ray diffractograms recorded on chips recovered from the propagating front show a double halo peak indicative of the simultaneous presence of LDA and HDA. By contrast, chips picked from different parts of the sample cylinder show either HDA or LDA. Growth of the low-density form takes place randomly somewhere inside of the high-density matrix. The thermal stability of HDA against transformation to LDA at ambient pressure significantly increases with decreasing recovery pressure and reaches its maximum at 0.07 GPa. A sample decompressed to 0.07 GPa is by ~17 K more stable than an unannealed HDA sample. An increasingly relaxed nature of the sample is also evident from the progressive disappearance of the broad calorimetric relaxation exotherm, preceding the sharp transition to LDA. Finally, we show that two independent thermodynamic paths lead to a very similar state of (relaxed) HDA at 140 K and 0.2 GPa. We argue that these observations imply an equilibrated nature of the amorphous sample in the pressure range of p ≲ 0.2 GPa and speculate that the observation of macroscopic phase separation involves two ultraviscous liquid phases at 140 K. This supports the scenario of a first-order liquid-liquid transition in bulk water.

How many amorphous ices are there?

Many acronyms are used in the literature for describing different kinds of amorphous ice, mainly because many different preparation routes and many different sample histories need to be distinguished. We here introduce these amorphous ices and discuss the question of how many of these forms are of relevance in the context of polyamorphism. We employ the criterion of reversible transitions between amorphous "states" in finite intervals of pressure and temperature to discriminate between independent metastable amorphous "states" and between "substates" of the same amorphous "state". We argue that the experimental evidence suggests we should consider there to be three polyamorphic "states" of ice, namely low-(LDA), high-(HDA) and very high-density amorphous ice (VHDA). In addition to the realization of reversible transitions between them, they differ in terms of their properties, e.g., compressibility, or number of "interstitial" water molecules. Thus they cannot be regarded as structurally relaxed variants of each other and so we suggest considering them as three distinct megabasins in an energy landscape visualization.

Pressure-amorphized cubic structure II clathrate hydrate: crystallization in slow motion.

A range of techniques has so far been employed for producing amorphous aqueous solutions. In case of aqueous tetrahydrofuran (THF) this comprises hyperquenching of liquid droplets, vapour co-deposition and pressure-induced amorphization of the crystalline cubic structure II clathrate. All of these samples are thermally labile and crystallize at temperatures above 110 K. We here outline a variant of the pressure-amorphization protocol developed by Suzuki [Phys. Rev. B, 2004, 70, 172108], which results in a highly crystallization resistant amorphous THF hydrate. The hydrate produced according to our protocol (annealing to 180 K at 1.8 GPa rather than to 150 K at 1.5 GPa) does not transform to the cubic structure II THF clathrate even at 150 K. We track the reason for this higher stability to the presence of crystalline remnants when following the Suzuki protocol, which are removed when using our protocol involving higher pressures and an annealing step. These crystalline remnants later serve as crystallization seeds lowering the thermal stability of the amorphous sample. Our protocol thus makes a purely amorphous THF hydrate available to the research community. We use powder X-ray diffraction to study the process of nucleation and slow crystal growth in the temperature range 160-200 K and find that the local cage structure and periodicity of the fully crystalline hydrate develops even at the earliest stages of crystallization, when the "clathrate crystal" has a size of about two unit cells.

Formation of mixed-phase particles during the freezing of polar stratospheric ice clouds.

Polar stratospheric clouds (PSCs) are extremely efficient at catalysing the transformation of photostable chlorine reservoirs into photolabile species, which are actively involved in springtime ozone-depletion events. Why PSCs are such efficient catalysts, however, is not well understood. Here, we investigate the freezing behaviour of ternary HNO3-H2SO4-H2O droplets of micrometric size, which form type II PSC ice particles. We show that on freezing, a phase separation into pure ice and a residual solution coating occurs; this coating does not freeze but transforms into glass below ∼150 K. We find that the coating, which is thicker around young ice crystals, can still be approximately 30 nm around older ice crystals of diameter about 10 µm. These results affect our understanding of PSC microphysics and chemistry and suggest that chlorine-activation reactions are better studied on supercooled HNO3-H2SO4-H2O solutions rather than on a pure ice surface.

Reversibility and isotope effect of the calorimetric glass --> liquid transition of low-density amorphous ice.

We here report differential scanning calorimetry (DSC) scans recorded by repeatedly heating the H(2)O (D(2)O) low density amorph (LDA) which was made by isothermal decompression of very high-density amorphous ice (VHDA) at 140 K from 1.1 to 0.006 GPa. These DSC scans show a glass --> liquid transition endotherm with an onset temperature (T(g)) of approximately 137 (140) K at a heating rate of 30 K min(-1) accompanied by an increase in heat capacity of approximately 1.7 (1.5) J K(-1) mol(-1). We establish the reversibility of this effect by thermally cycling between its glassy state below 137 K and its highly viscous liquid state at 149 K. All calorimetric signatures, including H/D isotope effect, are highly similar to the signatures in hyperquenched glassy water (HGW). We argue that the observation of almost identical calorimetric traces for HGW and LDA implies that there is no need to reassign HGWs T(g) to higher temperatures provided that the viscous liquid state connected to both LDA and HGW behaves as an ideally "strong" liquid in the Angell classification. We furthermore show that LDA prepared by isothermal decompression of VHDA is more crystallization-resistant than LDA made from high-density amorphous ice (HDA) by isobaric warming. We suggest that the former route via VHDA removes "nanocrystalline remnants" in LDA which are still present in the latter after pressure-amorphization of hexagonal ice to HDA at 77 K.

Hexagonal ice transforms at high pressures and compression rates directly into "doubly metastable" ice phases.

We report compression and decompression experiments of hexagonal ice in a piston cylinder setup in the temperature range of 170-220 K up to pressures of 1.6 GPa. The main focus is on establishing the effect that an increase in compression rate up to 4000 MPa/min has on the phase changes incurred at high pressures. While at low compression rates, a phase change to stable ice II takes place (in agreement with earlier comprehensive studies), we find that at higher compression rates, increasing fractions and even pure ice III forms from hexagonal ice. We show that the critical compression rate, above which mainly the metastable ice III polymorph is produced, decreases by a factor of 30 when decreasing the temperature from 220 to 170 K. At the highest rate capable with our equipment, we even find formation of an ice V fraction in the mixture, which is metastable with respect to ice II and also metastable with respect to ice III. This indicates that at increasing compression rates, progressively more metastable phases of ice grow from hexagonal ice. Since ices II, III, and V differ very much in, e.g., strength and rheological properties, we have prepared solids of very different mechanical properties just by variation in compression rate. In addition, these metastable phases have stability regions in the phase diagrams only at much higher pressures and temperatures. Therefore, we anticipate that the method of isothermal compression at low temperatures and high compression rates is a tool for the academic and industrial polymorph search with great potential.

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 approximately 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 (P1), in clear disagreement with recent theoretical calculations predicting ferroelectric ordering (Cc).

Raman spectroscopic study of the phase transition of amorphous to crystalline beta-carbonic acid.

What's the matter? The laboratory Raman spectra for carbonic acid (H(2)CO(3)), both for the beta-polymorph and its amorphous state, are required to detect carbonic acid on the surface of the pole caps of Mars in 2009, when the Mars Microbeam Raman Spectrometer lands on the planet. The picture shows a martian crater with ice of unknown composition, possibly containing carbonic acid (image adapted from DLR, with permission from ESA, DLR, and FU Berlin--G. Neukum).

A calorimetric study on the low temperature dynamics of doped ice V and its reversible phase transition to hydrogen ordered ice XIII.

Doped ice V samples made from solutions containing 0.01 M HCl (DCl), HF (DF), or KOH (KOD) in H(2)O (D(2)O) were slow-cooled from 250 to 77 K at 0.5 GPa. The effect of the dopant on the hydrogen disorder --> order transition and formation of hydrogen ordered ice XIII was studied by differential scanning calorimetry (DSC) with samples recovered at 77 K. DSC scans of acid-doped samples are consistent with a reversible ice XIII <--> ice V phase transition at ambient pressure, showing an endothermic peak on heating due to the hydrogen ordered ice XIII --> disordered ice V phase transition, and an exothermic peak on subsequent cooling due to the ice V --> ice XIII phase transition. The equilibrium temperature (T(o)) for the ice V <--> ice XIII phase transition is 112 K for both HCl doped H(2)O and DCl doped D(2)O. From the maximal enthalpy change of 250 J mol(-1) on the ice XIII --> ice V phase transition and T(o) of 112 K, the change in configurational entropy for the ice XIII --> ice V transition is calculated as 2.23 J mol(-1) K(-1) which is 66% of the Pauling entropy. For HCl, the most effective dopant, the influence of HCl concentration on the formation of ice XIII was determined: on decreasing the concentration of HCl from 0.01 to 0.001 M, its effectiveness is only slightly lowered. However, further HCl decrease to 0.0001 M drastically lowered its effectiveness. HF (DF) doping is less effective in inducing formation of ice XIII than HCl (DCl) doping. On heating at a rate of 5 K min(-1), kinetic unfreezing starts in pure ice V at approximately 132 K, whereas in acid doped ice XIII it starts at about 105 K due to acceleration of reorientation of water molecules. KOH doping does not lead to formation of hydrogen ordered ice XIII, a result which is consistent with our powder neutron diffraction study (C. G. Salzmann, P. G. Radaelli, A. Hallbrucker, E. Mayer, J. L. Finney, Science, 2006, 311, 1758). We further conjecture whether or not ice XIII has a stable region in the water/ice phase diagram, and on a metastable triple point where ice XIII, ice V and ice II are in equilibrium.

Radicals produced by gamma-irradiation of hyperquenched glassy water containing 2'-deoxyguanosine-5'-monophosphate.

Hyperquenched glassy water (HGW) has been suggested as the best model for liquid water, to be used in low-temperature studies of indirect radiation effects on dissolved biomolecules (Bednarek et al. J. Am. Chem. Soc. 1996, 118, 9387). In the present work, these effects are examined by X-band electron spin resonance spectroscopy (ESR) in gamma-irradiated HGW matrix containing 2'-deoxyguanosine-5'-monophosphate. Analysis of the complex ESR spectra indicates that, in addition to OH(*) and HO2(*) radicals generated by water radiolysis, three species are trapped at 77 K:(i) G(C8)H(*) radical, the H-adduct to the double bond at C8; (ii) G(- *) radical anion, the product of electron scavenging by the aromatic ring of the base; and (iii) dR(-H)(*) radicals formed by H abstraction from the sugar moiety, predominantly at the C'5 position. We discuss the yields of the radicals, their thermal stability and transformations, as well as the effect of photobleaching. This study confirms our earlier suggestion that in HGW the H atom addition/abstraction products are created at 77 K in competition with HO2(*) radicals, in a concerted process following ionization of water molecule at L-type defect sites of the H-bonded matrix. The lack of OH(*) reactivity toward the solute suggests that the H-bonded structure in HGW is much more effective in recombining OH(*) radicals than that of aqueous glasses obtained from highly concentrated electrolyte solutions. Furthermore, complementary experiments for the neat matrix have provided evidence that HO2(*) radicals are not the product of H atom reaction with molecular oxygen, possibly generated by ultrasounds used in the process of sample preparation.

Water polyamorphism: reversibility and (dis)continuity.

An understanding of water's anomalies is closely linked to an understanding of the phase diagram of water's metastable noncrystalline states. Despite the considerable effort, such an understanding has remained elusive and many puzzles regarding phase transitions in supercooled liquid water and their possible amorphous proxies at low temperatures remain. Here, decompression of very high density amorphous ice (VHDA) from 1.1 to 0.02 GPa at 140 K is studied by means of dilatometry and powder x-ray diffraction of quench-recovered states. It is shown that the three amorphous states of ice are reversibly connected to each other, i.e., LDA<-->e-HDA<-->VHDA. However, while the downstroke VHDA-->e-HDA transition takes place in the pressure range of 0.06 GPaLDA transition takes place quasi-discontinuously at p approximately 0.06 GPa. That is, two amorphous-amorphous transitions of a distinct nature are observed for the first time in a one-component system-a first-order-like transition (e-HDA-->LDA) and a transition which is not first-order like but possibly of higher order (VHDA-->e-HDA). VHDA and e-HDA are established as the most stable and limiting states in the course of the transition. We interpret this as evidence disfavoring the hypothesis of multiple first-order liquid-liquid transitions (and the option of a third critical point), but favoring a single first-order liquid-liquid transition (and the option of a second critical point).

Carbonic acid: from polyamorphism to polymorphism.

Layers of glassy methanolic (aqueous) solutions of KHCO3 and HCl were deposited sequentially at 78 K on a CsI window, and their reaction on heating in vacuo in steps from 78 to 230 K was followed by Fourier transform infrared (FTIR) spectroscopy. After removal of solvent and excess HCl, IR spectra revealed formation of two distinct states of amorphous carbonic acid (H2CO3), depending on whether KHCO3 and HCl had been dissolved in methanol or in water, and of their phase transition to either crystalline alpha- or beta-H2CO3. The main spectral features in the IR spectra of alpha- and beta-H2CO3 are observable already in those of the two amorphous H2CO3 forms. This indicates that H-bond connectivity or conformational state in the two crystalline phases is on the whole already developed in the two amorphous forms. The amorphous nature of the precursors to the two crystalline polymorphs is confirmed using powder X-ray diffraction. These diffractograms also show that alpha- and beta-amorphous H2CO3 are two distinct structural states. The variety of structural motifs found within a few kJ/mol in a computational search for possible crystal structures provides a plausible rationalization for (a) the observation of more than one amorphous form and (b) the retention of the motif observed in the amorphous form in the corresponding crystalline form. The polyamorphism inferred for carbonic acid from our FTIR spectroscopic and powder X-ray diffraction studies is special since two different crystalline states are linked to two distinct amorphous states. We surmise that the two amorphous states of H2CO3 are connected by a first-order-like phase transition.

Novel method to detect the volumetric glass --> liquid transition at high pressures: glycerol as a test case.

We report a novel method of detecting the glass --> liquid transition at high pressures, which comprises measuring the relative volume change incurred upon heating glassy samples into the liquid state. We show data on glycerol in the pressure range 0.050-1.00 GPa to demonstrate the viability of the method. The reversible glass --> liquid transition is observed by means of a kink in the relative volume change on heating the sample isobarically, which is attributed to the glass --> liquid transition temperature Tg. This kink can only be observed in the second and subsequent heating cycles since it is superposed by a compaction in the first heating cycle. The isobaric thermal expansivity beta, which is closely related to the first derivative of this curve, shows the features expected for a glass --> liquid transition, including a sharp rise of beta(glass) in a narrow temperature interval to beta(viscous liquid) and an accompanying overshoot effect. Both Tg and the size of the overshoot effect vary in accordance with theory upon changing the ratio of cooling to heating rates. From the shape of this curve the onset, inflection, overshoot peak, and endpoint of the glass --> liquid transition can be extracted, which can be employed to calculate the reduced glass transition width as a measure for the fragility of the liquid. Comparison with literature data allows quantifying the accuracy of the liquid's thermal expansivity beta to be at least +/-10%, while the error in beta is significantly larger for the expansivity of the glassy state. The reproducibility of the glass --> liquid transition temperature Tg is better than +/-2 K. Our glycerol data confirms literature studies showing a nonlinear increase of Tg with increasing pressure (approximately 35 K/GPa on average), which is accompanied by an increase in fragility.

Raman spectroscopic study of hydrogen ordered ice XIII and of its reversible phase transition to disordered ice V.

Raman spectra of recovered ordered H(2)O (D(2)O) ice XIII doped with 0.01 M HCl (DCl) recorded in vacuo at 80 K are reported in the range 3600-200 cm(-1). The bands are assigned to the various types of modes on the basis of isotope ratios. On thermal cycling between 80 and 120 K, the reversible phase transition to disordered ice V is observed. The remarkable effect of HCl (DCl) on orientational ordering in ice V and its phase transition to ordered ice XIII, first reported in a powder neutron diffraction study of DCl doped D(2)O ice V (C. G. Salzmann, P. G. Radaelli, A. Hallbrucker, E. Mayer, J. L. Finney, Science, 2006, 311, 1758), is demonstrated by Raman spectroscopy and discussed. The dopants KOH and HF have only a minor effect on hydrogen ordering in ice V, as shown by the Raman spectra.

The relation between high-density and very-high-density amorphous ice.

The exact nature of the relationship between high-density (HDA) and very-high-density (VHDA) amorphous ice is unknown at present. Here we review the relation between HDA and VHDA, concentrating on experimental aspects and discuss these with respect to the relation between low-density amorphous ice (LDA) and HDA. On compressing LDA at 125 K up to 1.5 GPa, two distinct density steps are observable in the pressure-density curves which correspond to the LDA --> HDA and HDA --> VHDA conversion. This stepwise formation process LDA --> HDA --> VHDA at 125 K is the first unambiguous observation of a stepwise amorphous-amorphous-amorphous transformation sequence. Density values of amorphous ice obtained in situ between 0.3 and 1.9 GPa on isobaric heating up to the temperatures of crystallization show a pronounced change of slope at ca. 0.8 GPa which could indicate formation of a distinct phase. We infer that the relation between HDA and VHDA is very similar to that between LDA and HDA except for a higher activation barrier between the former. We further discuss the two options of thermodynamic phase transition versus kinetic densification for the HDA --> VHDA conversion.

The preparation and structures of hydrogen ordered phases of ice.

Two hydrogen ordered phases of ice were prepared by cooling the hydrogen disordered ices V and XII under pressure. Previous attempts to unlock the geometrical frustration in hydrogen-bonded structures have focused on doping with potassium hydroxide and have had success in partially increasing the hydrogen ordering in hexagonal ice I (ice Ih). By doping ices V and XII with hydrochloric acid, we have prepared ice XIII and ice XIV, and we analyzed their structures by powder neutron diffraction. The use of hydrogen chloride to release geometrical frustration opens up the possibility of completing the phase diagram of ice.