The nonlinear anomalous lattice elasticity associated with the high-pressure phase transition in spodumene: a high-precision static compression study |
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| Authors: | Angela Ullrich Wilfried Schranz Ronald Miletich |
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| Institution: | 1.Institut für Geowissenschaften, Forschungsgruppe Mineralphysik,Universit?t Heidelberg,Heidelberg,Germany;2.Fakult?t für Physik,Nichtlineare Physik,Vienna,Austria |
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| Abstract: | The high-pressure behavior of the lattice elasticity of spodumene, LiAlSi2O6, was studied by static compression in a diamond-anvil cell up to 9.3 GPa. Investigations by means of single-crystal XRD and
Raman spectroscopy within the hydrostatic limits of the pressure medium focus on the pressure ranges around ~3.2 and ~7.7 GPa,
which have been reported previously to comprise two independent structural phase transitions. While our measurements confirm
the well-established first-order C2/c–P21/c transformation at 3.19 GPa (with 1.2% volume discontinuity and a hysteresis between 0.02 and 0.06 GPa), both unit-cell dimensions
and the spectral changes observed in high-pressure Raman spectra give no evidence for structural changes related to a second
phase transition. Monoclinic lattice parameters and unit-cell volumes at in total 59 different pressure points have been used
to re-calculate the lattice-related properties of spontaneous strain, volume strain, and the bulk moduli as a function of
pressure across the transition. A modified Landau free energy expansion in terms of a one component order parameter has been
developed and tested against these experimentally determined data. The Landau solution provides a much better reproduction
of the observed anomalies than any equation-of-state fit to data sets truncated below and above P
tr, thus giving Landau parameters of K
0 = 138.3(2) GPa, K′ = 7.46(5), λ
V
= 33.6(2) GPa, a = 0.486(3), b = −29.4(6) GPa and c = 551(11) GPa. |
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