Seminar

TYC Seminar: The molecular description of amorphous ices and the mechanism governing their pressure induced interconversion

Dr Fausto Martelli - IBM Research

Friday 26th October 2018
Time: 3-4pm
Venue: Ramsay Lecture Theatre, Christopher Ingold Building, UCL
Contact: Karen Stoneham
Tel: 0207 6797306

Water exhibits polyamorphism, i.e., it exists in more than one amorphous state.  The most common forms of glassy water are the low-density amorphous (LDA) and the high-density amorphous (HDA) ices.  LDA, the most avundant form of ice in the Universe, transorms into HDA upon isothermal compression.  We model the amorphisation of liquid water in LDA and the transormation of LDA in HDA under isothermal compression with classical molecular dynamics simulations.  We analyse the molecular structures with a recently introduced scalar order metric [1] to measure short and intermediate range order, and we investigate the large-scale structure by the extent to which long range density fluctuations are suppressed.

Our results indicate that the structural properties of LDA differ from those of liquid water, and that LDA and HDA are indeed amorphous, i.e., they lack polydispersed ice domains.  Interestingly, HDA contains a small number of domans that are reminiscent of the unit cell of ice IV, although the hydrogen-bond network (HBN) of these domains differs from the HBN of ice IV [2].  Both HDA and LDA are characterised by an anomalous suppression of large-scale density fluctuations [3].  By contrast, in correspondence with the non-equilibrium phase transitions to HDA, an extensive rearrangement of the hydrogen bond network introduces strong long range density fluctuations.  Our investigations challenge the largely accepted "frozen-liquid" picture, which views glasses as structurally arrested liquids.  Beyond implications for water, our findings also enrich our understanding of pressure-induced structural transformations in glasses.

[1] F. Mertelli, H.-Y. Ko, E. C. Oguz and R. Car, Phys. Rev. B, 97, 064105 (2018)

[2] F. Martelli, N. Giovambattista, S. Torquato and R. Car, Phys. Rev. Materials, 2, 075601 (2018)

[3] F. Martelli, S. Torquato, N. Giovambattista and R. Car, Phys. Rev. Lett., 119, 136002 (2017)

Biography

Dr Fausto Martelli received his MSc from the University of Milan, and a PhD from the same institution for a collaborative project with the Ecole Mormale Superieure in Paris.  After his PhD, Dr Martelli joined the CNRS in Paris as a postdoctoral scholar developing classical potentials to describe the interaction of water with heavy ions of interest in the nuclear industry.  He then moved to Princeton University (USA) as a postdoctoral assiciate first, in the group of Prof Roberto Car, and as a Faculty Associate Research next, working in the undertanding of the source of water anomalies.  Dr Martelli moved to IBM Research 2018, where ie continues his research on the statistical mechanics of liquids and glassy materials.

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