Cordierite under hydrostatic compression : anomalous elastic behavior as a precursor for a pressure-induced phase transition

The high-pressure behavior of cordierite was investigated by means of in situ experiments using piston-cylinder press and diamond-anvil cell. Static compression in diamond-anvil cells was conducted with various penetrating and non-penetrating pressure media (H2O up to 2 GPa, argon and 4:1-methanolethanol up to 7 GPa). The measurement of lattice parameters revealed neither a significant influence on the elasticity nor any indication for effects in analogy to over-hydration within the experimental pressure ranges. Volumetric compression experiments at constant rates up to 1.2 GPa in a piston-cylinder apparatus insinuate subtle irregularities in the low-pressure range at around 0.35 and 0.85 GPa. The \u394V/V contribution related to the anomalous compression behavior in that pressure range is of the order of 5 7 10 -4. The results obtained from single-crystal X-ray diffraction between 10-4 and 7 GPa revealed an unexpected and anomalous linear volume decrease, corresponding to KT,298 = 131\ub11 GPa for the bulk modulus and K\u2032 = -0.4\ub10.3 for its pressure derivative for a third-order Birch-Murnaghan equation of state. The compressional behavior of the main axis directions is anisotropic with \u3b2a-1 48 \u3b2b-1 > \u3b2c-1 for an initial pressure regime up to 3 GPa. At pressures above ~4 GPa, the compression of the a- and b-axis starts to differ significantly, with the b-axis showing elastic softening as indicated by negative values for 02(\u3b2 b-1)/ 02P. The diversification between the a- and b-axis is also expressed by the pressure-depending increase of the distortion parameter \u394. The pronounced elastic softening in both the b-axis and c-axis directions 02(\u3b2b-1)/ 02P = -4.3\ub10.9, 02(\u3b2c-1)/ 02P = -1.2\ub10.8) are responsible for the apparent linear bulk compression, which indicates the structural instability and precedes a so far not reported ferroelastic phase transition to a triclinic polymorph, following a primitive lattice above the critical transition at 6.9 GPa

Die Wohnungsmaerkte Dortmund und Saarbruecken: ein Strukturvergleich ; Text-Teil

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