High pressure elasticity and phase transformation in brucite, Mg(OH)2 |
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| Authors: | K P Jayachandran Lin-gun Liu |
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| Institution: | (1) Institute of Earth Sciences, Academia Sinica, Nankang, Taipei, 11529, Taiwan, ROC;(2) Present address: Centro de Projecto Mecanico, IDMEC–Instituto de Engenharia Mecanica, Pólo IST, Av. RoviscoPais, 1, 1049-001 Lisbon, Portugal |
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| Abstract: | The first pressure derivatives of the second-order elastic constants
 have been calculated for brucite, Mg(OH)2 from the second- and third-order elastic constants. The deformation theory and finite strain elasticity theory have been used to obtain the second- and third-order elastic constants of Mg(OH)2 from the strain energy of the lattice. The strain energy ϕ is calculated by taking into account the interactions up to third nearest neighbors in the Mg(OH)2 lattice. ϕ is then compared with the strain dependent lattice energy from continuum model approximation to obtain the expressions of elastic constants. The complete set of six second-order elastic constants C
IJ
of brucite exhibits large anisotropy. Since C
33 (= 21.6 GPa), which corresponds to the strength of the material along the c-axis direction, is less than the longitudinal mode C
11 (= 156.7 GPa), the interlayer binding forces are weaker than the binding forces along the basal plane of Mg(OH)2. The 14 nonvanishing components of the third-order elastic constants, C
IJK
, of brucite have been obtained. All the C
IJK
of brucite are negative except the values of C
114 (= 230.36 GPa), C
124 (= 75.45 GPa) and C
134 (= 36.98 GPa). The absolute values of the C
IJK
are, in general, one order of magnitude greater than the C
IJ
’s in the Mg(OH)2 system as usually expected for a crystalline material. To our knowledge, no previous data are available to compare the pressure derivatives of brucite. The pressure derivatives of the two components viz., C
14 and C
33 become negative
 indicating an elastic instability in brucite while under pressure. This may be related to the phase transition of brucite largely involving rearrangements of H atoms revealed in the Raman spectroscopic, powder neutron diffraction and synchrotron X-ray diffraction studies. |
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| Keywords: | Brucite Elastic properties Phase transition High pressure |
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