dc.contributor.author Whaley-Baldwin, Jack en dc.contributor.author Needs, Richard en dc.date.accessioned 2019-11-15T00:30:50Z dc.date.available 2019-11-15T00:30:50Z dc.date.issued 2020-01-01 en dc.identifier.issn 1367-2630 dc.identifier.uri https://www.repository.cam.ac.uk/handle/1810/298906 dc.description.abstract We use first-principles Density Functional Theory (DFT) to conduct an extensive structure search using the AIRSS package for elemental sulfur in the range $50$-$550$ GPa. We then obtain the low-temperature phase diagram of sulfur in the same pressure range, including vibrational effects through the harmonic approximation. We do not find any structures lower in energy than those already reported in experiment, although the phase diagram below $100$ GPa is found to be crowded with structures separated by only a few meV. We report the transition sequence $I4_1/acd \rightarrow P\bar{1} \rightarrow$ ICM (Incommensurately Modulated) $\rightarrow C2/m \rightarrow R\bar{3}m \rightarrow Im\bar{3}m$ and obtain accurate pressures for each transition, although we find the second-order $C2/m \rightarrow R\bar{3}m$ transition particularly difficult to define. Contrary to previous first-principles works, we do not reproduce a trigonal $\rightarrow$ simple cubic (SC) transition at either the static lattice or harmonic level. We also undertake a detailed analysis of the incommensurately modulated phase of sulfur (ICM) phase using a commensurate approximant found in the structure search. We find that the modulation is zero above $96$ GPa, some $50$ GPa below the experimentally reported transition to the unmodulated phase. We find that the body-centred atoms in the relaxed ICM approximant are, in addition to the dominant transverse modulation, slightly displaced longitudinally in the $b$-direction. We subsequently discover that this (small) longitudinal modulation is coupled to the transverse mode, and hence report previously unnoticed weak-mode coupling between transverse and longitudinal optical phonons in the ICM phase. dc.publisher Institute of Physics (IoP) and Deutsche Physikalische Gesellschaft dc.rights All rights reserved dc.rights.uri dc.title First-principles high pressure structure searching, longitudinal-transverse mode coupling and absence of simple cubic phase in sulfur en dc.type Article prism.issueIdentifier 2 en prism.publicationDate 2020 en prism.publicationName New Journal of Physics en prism.volume 22 en dc.identifier.doi 10.17863/CAM.45963 dcterms.dateAccepted 2019-11-11 en rioxxterms.versionofrecord 10.1088/1367-2630/ab6068 en rioxxterms.version AM rioxxterms.licenseref.uri http://www.rioxx.net/licenses/all-rights-reserved en rioxxterms.licenseref.startdate 2020-01-01 en dc.contributor.orcid Whaley-Baldwin, Jack [0000-0001-9350-7115] dc.contributor.orcid Needs, Richard [0000-0002-5497-9440] dc.identifier.eissn 1367-2630 rioxxterms.type Journal Article/Review en cam.orpheus.counter 2 * rioxxterms.freetoread.startdate 2022-11-14
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