Regensburg 2016 – wissenschaftliches Programm

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MM: Fachverband Metall- und Materialphysik

MM 21: Microstructure and Phase Transformations I

MM 21.6: Vortrag

Dienstag, 8. März 2016, 13:00–13:15, H38

Anharmonic quantum nuclear vibrations and the stability of hexagonal ice — Edgar Engel, •Bartomeu Monserrat, and Richard Needs — TCM Group, Cavendish Laboratory, 19 JJ Thomson Avenue, Cambridge, CB3 0HE, UK

We use a recently developed vibrational self-consistent field approach in conjunction with first-principles density functional theory (DFT) calculations to calculate anharmonic quantum nuclear vibrations in the hexagonal and cubic forms of water ice. The very similar free energies of hexagonal and cubic have so far prevented even state-of-the-art first-principles quantum mechanical calculations using non-local DFT and quantum Monte Carlo methods from reproducing, let alone explaining, the stability of hexagonal ice. We show that, although the static lattice and harmonic vibrational energies are almost identical, anharmonic vibrations crucially stabilise hexagonal ice compared with cubic ice by at least 1.4 meV/H2O, in agreement with experimental estimates. The difference in anharmonicity arises predominantly from the difference in vibrational frequencies of the O-H bond stretching modes, which is in agreement with recent IR absorption measurements for stacking disordered ice. We account for proton-disorder and relate our results to recent IR absorption measurements for partially proton-ordered cubic ice. Moreover, we trace the difference in anharmonicity back to the difference in structure.