Anomalous pressure-induced transition(s) in ice XI

Nature of the Volume Isotope Effect in Ice

The substitution of hydrogen (H) by deuterium (D) in ice Ih and in its H-ordered version, ice XI, produces an anomalous form of volume isotope effect (VIE), i.e., volume expansion. This VIE contrasts with the normal VIE (volume contraction) predicted in ice-VIII and in its H-disordered form, ice VII. Here we investigate the VIE in ice XI and in ice VIII using first principles quasiharmonic calculations. We conclude that normal and anomalous VIEs can be produced in ice VIII and ice XI in sequence by application of pressure (ice XI starting at negative pressures) followed by a third type-anomalous VIE with zero-point volume contraction. The latter should also contribute to the isotope effect in the ice VII → ice X transition. The predicted change between normal and anomalous VIE in ice VIII at 14.3 GPa and 300 K is well reproduced experimentally in ice VII using x-ray diffraction measurements. The present discussion of the VIE is general, and conclusions should be applicable to other solid phases of H(2)O, possibly to liquid water under pressure, and to other H-bonded materials.

Universal elastic-hardening-driven mechanical instability in α-quartz and quartz homeotypes under pressure

As a fundamental property of pressure-induced amorphization (PIA) in ice and ice-like materials (notably α-quartz), the occurrence of mechanical instability can be related to violation of Born criteria for elasticity. The most outstanding elastic feature of α-quartz before PIA has been experimentally reported to be the linear softening of shear modulus C₄₄, which was proposed to trigger the transition through Born criteria B₃. However, by using density-functional theory, we surprisingly found that both C₄₄ and C₆₆ in α-quartz exhibit strong nonlinearity under compression and the Born criteria B₃ vanishes dominated by stiffening of C₁₄, instead of by decreasing of C₄₄. Further studies of archetypal quartz homeotypes (GeO₂ and AlPO₄) repeatedly reproduced the same elastic-hardening-driven mechanical instability, suggesting a universal feature of this family of crystals and challenging the long-standing idea that negative pressure derivatives of individual elastic moduli can be interpreted as the precursor effect to an intrinsic structural instability preceding PIA. The implications of this elastic anomaly in relation to the dispersive softening of the lowest acoustic branch and the possible transformation mechanism were also discussed.

First-principles codes for computational crystallography in the Quantum-ESPRESSO package

The Quantum-ESPRESSO package is a multi-purpose and multi-platform software for ab-initio calculations of condensed matter (periodic and disordered) systems. Codes in the package are based on density functional theory and on a plane wave/pseudopotential description of the electronic ground state and are ideally suited for structural optimizations (both at zero and at finite temperature), linear response calculations (phonons, elastic constants, dielectric and Raman tensors, etc.) and high-temperature molecular dynamics. Examples of applications of the codes included in the package are briefly discussed.

Assessment of the pairwise additive approximation and evaluation of many-body terms for water clusters

We have tested the ability of four commonly used density functionals (three of which are semilocal and one of which is nonlocal) to outperform accurate pairwise additive approximations in the prediction of binding energies for a series of water clusters ranging in size from dimer to pentamer. Comparison to results obtained with the Weizmann-1 (W1) level of wave function theory shows that while all density functionals are capable of outperforming the accurate pairwise data, the choice of basis set used is crucial to the performance of the method, and if a poor choice of basis set is made the errors obtained with density functional theory (DFT) can be larger than those obtained with the simple pairwise approximation. We have also compared the binding energies and many-body terms determined with DFT to those obtained with W1, and have found that all semilocal functionals have significant errors in the many-body components of the full interactions energy. Despite this limitation, however, we have found that, of the four functionals tested, PBE1W/MG3S is the most accurate for predicting the binding energies of the clusters.

Unusual Grüneisen and Bridgman parameters of low-density amorphous ice and their implications on pressure induced amorphization

The low-temperature limiting value of the Grüneisen parameter for low-frequency phonons and the density dependence of the thermal conductivity (Bridgman parameter) of low-density amorphous (LDA) ice, high-density amorphous (HDA) ice, hexagonal ice Ih, and cubic ice Ic were calculated from high-pressure sound velocity and thermal conductivity measurements, yielding negative values for all states except HDA ice. LDA ice is the first amorphous state to exhibit a negative Bridgman parameter, and negative Grüneisen parameters are relatively unusual. Since Ih, Ic, and LDA ice all transform to HDA upon pressurization at low temperatures and share the unusual feature of negative Grüneisen parameters, this seems to be a prerequisite for pressure induced amorphization. We estimate that the Grüneisen parameter increases at the ice Ih to XI transition, and may become positive in ice XI, which indicates that proton-ordered ice XI does not amorphize like ice Ih on pressurization.

Phonon dispersion of ice under pressure

We report measurements of the phonon dispersion of ice Ih under hydrostatic pressure up to 0.5 GPa, at 140 K, using inelastic neutron scattering. They reveal a pronounced softening of various low-energy modes, in particular, those of the transverse acoustic phonon branch in the [100] direction and polarization in the hexagonal plane. We demonstrate with the aid of a lattice dynamical model that these anomalous features in the phonon dispersion are at the origin of the negative thermal expansion (NTE) coefficient in ice below 60 K. Moreover, extrapolation to higher pressures shows that the mode frequencies responsible for the NTE approach zero at approximately 2.5 GPa, which explains the known pressure-induced amorphization (PIA) in ice. These results give the first clear experimental evidence that PIA in ice is due to a lattice instability, i.e., mechanical melting.