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Title: High-pressure X-ray diffraction, Raman, and computational studies of MgCl2 up to 1 Mbar: Extensive pressure stability of the β-MgCl2 layered structure

We studied magnesium chloride (MgCl2) with the rhombohedral layered CdCl2-type structure (α-MgCl2), experimentally, using synchrotron angle-dispersive powder x-ray diffraction and Raman spectroscopy using a diamond-anvil cell up to 100 GPa at room temperature and theoretically using first-principles density functional calculations. Our results reveal a pressure-induced second-order structural phase transition to a hexagonal layered CdI2-type structure (β-MgCl2) at 0.7 GPa: the stacking sequence of the Cl anions are altered resulting in a reduction of the c-axis length. Theoretical calculations confirm this phase transition sequence and the calculated transition pressure is in excellent agreement with the experiment. Lattice dynamics calculations also reproduce the experimental Raman spectra measured for the ambient and high-pressure phase. According to our experimental results MgCl2 remains in a 2D layered phase up to 100 GPa and further, the 6-fold coordination of Mg cations is retained. Theoretical calculations of relative enthalpy suggest that this extensive pressure stability is due to a low enthalpy of the layered structure ruling out kinetic barrier effects. Our observation is unusual, as it contradicts with the general structural behavior of highly compressed AB2 compounds.
Authors:
 [1] ;  [2] ;  [1] ;  [1] ;  [3] ;  [4] ;  [5]
  1. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
  2. Univ. of Saskatchewan, Saskatoon, SK (Canada). Dept. of Physics and Engineering Physics; Canadian Light Sources, Inc., Saskatoon, SK (Canada)
  3. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States); Hacettepe Univ., Ankara (Turkey)
  4. DESY Photon Science, Hamburg (Germany)
  5. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
Publication Date:
OSTI Identifier:
1321438
Report Number(s):
LLNL-JRNL--688100
Journal ID: ISSN 2045-2322
Grant/Contract Number:
AC52-07NA27344; HDTRA1-11-1-4538I; AC02-05CH11231
Type:
Accepted Manuscript
Journal Name:
Scientific Reports
Additional Journal Information:
Journal Volume: 6; Journal ID: ISSN 2045-2322
Publisher:
Nature Publishing Group
Research Org:
Lawrence Livermore National Laboratory (LLNL), Livermore, CA
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; 75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY