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@PHDTHESIS{Herrmann:864791,
      author       = {Herrmann, Markus Guido},
      title        = {{C}rystal structures and vibrational properties of
                      chalcogenides: the role of temperature and pressure},
      volume       = {201},
      school       = {RWTH Aachen},
      type         = {Dr.},
      address      = {Jülich},
      publisher    = {Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag},
      reportid     = {FZJ-2019-04448},
      isbn         = {978-3-95806-421-8},
      series       = {Schriften des Forschungszentrums Jülich. Reihe
                      Schlüsseltechnologien / Key Technologies},
      pages        = {XI, 156 S.},
      year         = {2019},
      note         = {RWTH Aachen, Diss., 2019},
      abstract     = {Phase-change materials (e. g. GeTe, GeSe, Sb$_{2}$Te$_{3}$
                      and Sb$_{2}$Se$_{3}$) are characterized by an ultra-fast
                      switching between an amorphous and a crystalline phase and
                      thus, these materials are considered to be hopeful
                      candidates for applications in future computer-based
                      memories which allow a more reliable, more efficient and
                      more capable data storage. In the crystalline phase a strong
                      properties contrast between the tellurides and selenides is
                      observed and and it was concluded that this is due
                      todifferent bonding schemes. While the tellurides are
                      believed to exhibit a metavalent bonding scheme, the
                      selenides are covalently bonded. However, up to now, the
                      metavalent bonding scheme is still under discussion and it
                      is not completely understood. This study is focused on the
                      GeSe$_{x}$Te$_{1-x}$ and Sb$_{2}$Te$_{3-x}$Se$_{x}$ solid
                      solutions which are both relevant systems of phase-change
                      materials. According to the conception of the metavalent
                      bonding scheme it is expected that composition-induced
                      changes and the application of low-temperature and/or
                      high-pressure lead to either a weakening or a collapse of
                      this bonding, however, an experimental conformation is
                      missing. The influence of composition-induced changes,
                      low-temperature and high-pressure on the structural
                      parameter of compounds from the GeSe$_{x}$Te$_{1-x}$ (x=0,
                      0.2, 0.5, 0.75, 1) solid solution was investigated by a
                      combination of powder and single crystal diffraction. At
                      ambient conditions, three phases, a rhombohedral
                      (0$\le$x(Se)$\le$0.52), a hexagonal
                      (0.58$\le$x(Se)$\le$0.86) and an orthorhombic
                      (0.91$\le$x(Se)$\le$1) one, exist in this system. All phases
                      are stable down to at least 25K and no evidences for
                      structural phase transition have been observed. The thermal
                      behavior of the rhombohedral compounds differs significantly
                      from the one of hexagonal GeSe$_{0.75}$Te$_{0.25}$ which is
                      explained by the additional Ge$\cdot$ $\cdot$Ge and
                      Se/Te$\cdot$ $\cdot$Se/Te interactions which are only
                      present in the crystal structure of the hexagonal phase.
                      Compounds from the stability field of the rhombohedral phase
                      (x=0, 0.2, 0.5) follow the pressure transition pathway:
                      GeTe-I (R3$\textit{m}$)$\rightarrow$GeTe-II
                      (FCC)$\rightarrow$GeTe-III (Pnma). A first-order nature for
                      all observed phase transitions is suggested. It was found
                      that all phase transitions are reversible. For the GeTe-III
                      polymorph, a new crystal structure was determined and it was
                      demonstrated that earlier structural models are erroneously.
                      GeTe-III is isostructural to $\beta$-GeSe, a high-pressure
                      and high-temperature polymorph of GeSe, and crystallizes in
                      the orthorhombic space group $\textit{Pnma}$ with lattice
                      parameter of $\textit{a}$=7.3690(18) Å,
                      $\textit{b}$=3.9249(10)Å and $\textit{c}$=5.698(9) Å. In
                      the structure, Ge$\cdot$ $\cdot$Ge and long-ranged Te$\cdot$
                      $\cdot$Te interactions are present. Hexagonal
                      GeSe$_{0.75}$Te$_{0.25}$ is [...]},
      cin          = {JCNS-2 / PGI-4 / JARA-FIT},
      cid          = {I:(DE-Juel1)JCNS-2-20110106 / I:(DE-Juel1)PGI-4-20110106 /
                      $I:(DE-82)080009_20140620$},
      pnm          = {144 - Controlling Collective States (POF3-144) / 524 -
                      Controlling Collective States (POF3-524) / 6212 - Quantum
                      Condensed Matter: Magnetism, Superconductivity (POF3-621) /
                      6213 - Materials and Processes for Energy and Transport
                      Technologies (POF3-621) / 6G4 - Jülich Centre for Neutron
                      Research (JCNS) (POF3-623)},
      pid          = {G:(DE-HGF)POF3-144 / G:(DE-HGF)POF3-524 /
                      G:(DE-HGF)POF3-6212 / G:(DE-HGF)POF3-6213 /
                      G:(DE-HGF)POF3-6G4},
      experiment   = {EXP:(DE-MLZ)POLI-HEIDI-20140101 /
                      EXP:(DE-MLZ)TOF-TOF-20140101 / EXP:(DE-H253)P-P01-20150101},
      typ          = {PUB:(DE-HGF)3 / PUB:(DE-HGF)11},
      url          = {https://juser.fz-juelich.de/record/864791},
}