Hda Ice Research Articles

The metastable T−P phase diagram and anomalous thermodynamic properties of supercooled water

The metastable T−P phase diagram and the anomalies of the thermodynamic properties of supercooled water are calculated on the basis of a two-level thermodynamic model. Water is considered as a mixture of two components which differ in atomic configurations and correspond to low-density amorphous (lda) and high-density amorphous (hda) ice. The expression for the Gibbs potential of water is written in the form which is analogous to that of usual regular binary solutions. But this model considers the concentration, c, of the components, as a pressure and temperature-dependent internal parameter. There are only four constants in the expression for the Gibbs potential: the differences in the specific volumes, entropies, and energies of the two components and the mixing energy of the components whose values are ΔV0=−3.8 cm3/mol, ΔS0=4.225 J/mol, ΔE0=1037 J/mol, and U=3824 J/mol, respectively. The lda−hda phase equilibrium line terminates at the critical point, Tcr=230 K and Pcr=0.173 kbar, the second critical point in the phase diagram of water. The anomalous thermal dependence of the specific volume, the heat expansion coefficient, and the specific heat of water calculated for the atmospheric pressure is in a good quantitative agreement with the available experimental data. Thus anomalous properties of supercooled water are well explained by the occurrence of the second critical point close to the atmospheric pressure. The absolute value of parameter c is not crucial for the thermal behavior of properties, instead, the anomalies in water are due to the dependence on pressure and temperature. The parameter c behavior is analyzed in various pressure and temperature ranges around the second critical point. The thermal dependence of parameter c is very weak in the temperature range of 290–350 K at atmospheric pressure. As a consequence, the thermodynamic properties of water behave in this range like those of a normal liquid though water stays a mixture of two components, lda-like and hda-like, in an approximate proportion 2:3.

Neutron-Scattering Studies of Ice Prepared by Different Thermobaric Treatments

On heating the recovered high-pressure phase ice VIII at ambient pressure, the transitions to high-density amorphous (hda) and low-density amorphous ices, to cubic ice Ic, and finally to hexagonal ice Ih were observed using real-time neutron diffraction. A new form of ice, “defective ice VI”, was prepared for the fist time by pressurizing ice Ih to 30 kbar at liquid nitrogen temperature. Vibrational spectra of ices Ih, hda, VI, VI‘, and VIII prepared by different thermobaric treatments were studied by inelastic neutron scattering. The translational band of the intermolecular part of ice VI‘ vibrational spectrum was shown to be similar to that of hda ice, whereas its librational band was similar to that of ice VI. We conclude that ices VI, VI‘, and hda have essentially the same atomic correlations and force constants, though with the different degree of disorder in the case of ices VI‘ and hda.

Anomalies of the baric and temperature dependences of the elastic characteristics of ice during solid-phase amorphization and the phase transition in the amorphous state

The elastic characteristics of ice up to pressures of 1.7 GPa are determined for the first time at a temperature of 77 K, along with features of their variation associated with the phase transformation of hexagonal ice Ih into high-density amorphous ice hda. The elastic instability of the ice lattice before solid-phase amorphization is experimentally confirmed. Elastic instability during a transition from one amorphous state to another amorphous state was also observed for the first time; this took place when hda ice was warmed at p=0.05 GPa from T=77 K.

Phase behaviors of supercooled water: Reconciling a critical point of amorphous ices with spinodal instability

The anomalies of supercooled water in thermodynamic response functions at atmospheric pressure, the phase transition between low and high density amorphous ices (LDA and HDA), and a predicted fragile–strong transition are accounted for in a unified manner by reconciling an idea due to Stanley and co-workers introducing a second critical point separating LDA and HDA ices with a conjecture proposed by Speedy that LDA is a different phase from a normal water, called water II. The reconciliation is made on the basis of results from extensive molecular dynamics simulations at constant pressure and temperature. It is found that there exist large gaps around temperature 213 K in thermodynamic, structural, and dynamic properties at atmospheric pressure, suggesting liquid–liquid phase transition. This transition is identified with an extension of the experimentally observed LDA–HDA transition in high pressure to atmospheric pressure. Thus, we propose a new phase diagram where the locus of the second critical point is moved into negative pressure region. With this simple modification, it becomes possible to account for the divergence of the thermodynamic response functions at atmospheric pressure in terms of the critical point and the spinodal-like instability of HDA. The unstable HDA undergoes a transition to LDA phase in lower temperature. The transition is also observed in high pressure region such as 200 MPa while it disappears at negative pressure, −200 MPa. This reinforces our proposed phase diagram in which there is no continuous path from a supercooled state to LDA at atmospheric pressure. It is argued that the HDA–LDA transition is accompanied by a fragile–strong transition. A possible mechanism of avoiding crystallization of aqueous solutions is also discussed in terms of a difference in hydrogen bond number distribution between LDA and HDA.

Similarity of vibrational spectra of high-density amorphous ice and high-pressure phase ice VI

The vibrational spectra of high-density amorphous (hda) ice, hexagonal ice Ih and high-pressure ice VI have been studied by inelastic neutron scattering. The observed similarity of the spectra of hda ice and ice VI means that the dynamics of these ice phases should be determined by the same atomic correlations and force constants.

Low-frequency modes in amorphous ice

The vibrational spectra of high density amorphous (hda) ice and hexagonal crystalline ice (ice Ih) have been determined for frequencies -0.10 ≤ ν ≤ 3.66THz by means of high resolution incoherent inelastic neutron scattering measurements on H 2O samples at 11 and 80 K. The measurements at 11 K clearly reveal an excess of low-frequency modes for the amorphous phase.

A 1H and 2H nuclear magnetic resonance study of amorphous ices at 77 K

The 2H quadrupole coupling constants (QCC) of deuteriated high-density amorphous (hda) ice and low-density amorphous (lda) ice were measured to be 226.6 and 218.6 kHz with asymmetry parameters (η) 0.106 and 0.104, respectively, at 77 K. Correlations of the QCC’s with near-neighbor O–O distances in the crystalline phases of ice imply near-neighbor O–O distances of 2.84 and 2.79 Å in high- and low-density amorphous ice, respectively, compared with values of 2.79 and 2.77 Å for cubic ice (Ic) and hexagonal ice (Ih). The 1H second moment for hda ice of 37.5 G2 can be understood in terms of a short water interproton distance of 1.565 Å and the greater density of hda ice with respect to ice Ih. The 1H spin-lattice relaxation times at 77 K for hda ice and lda ice at 124 and 773 s are much shorter than the value found for the crystalline ices. It is suggested that the T1 parameter may be useful in characterizing different amorphous ice phases.

Pressure-induced phase transformations in ice.

Using molecular-dynamics calculations, we show that at 80 K, ice Ih undergoes a transition to a high-density amorphous (hda) form at about 13 kbar. At higher pressures, the calculated oxygen atom radial distribution function shows strong resemblance to that of ice VIII. The most characteristic feature of hda ice, namely the peak observed around 3.5 A\r{} in the oxygen radial distribution function, is ascribed to molecules occupying interstitial-like sites. On warming of a sample of hda ice recovered at zero pressure, the structure evolves to a lower-density form. These observations are in good accord with available experimental data.