High-pressure elasticity, shear-mode softening and polymorphism in MnO

Details High-pressure elasticity, shear-mode softening and polymorphism in MnO High-pressure elasticity, shear-mode softening and polymorphism in MnO

Oct 1988

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988pepi...52..117w&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 52, Issue 1-2, p. 117-131.

Physics

21

Scientific paper

Ultrasonic phase comparison measurements incorporating explicit corrections for transducer-bond phase shifts have been conducted on a single crystal of MnO, to 3 GPa hydrostatic pressure. The single-crystal elastic moduli, the calculated bulk and shear moduli for isotropic polycrystals, and their pressure derivatives at zero pressure are as follows:
Cij (GPa)dCij/dP
C11226.4 +/- 0.29.26 +/- 0.09
C12114.9 +/- 0.13.30 +/- 0.09
C4479.0 +/- 0.10.17 +/- 0.02
K152.1 +/- 0.25.28 +/- 0.09
G68.8 +/- 0.11.55 +/- 0.03
The new elasticity data for MnO, and in particular the measured value of dK/dP, provide tighter constraints on the non-crystal-field contribution to the bulk elastic properties of transition-metal monoxides. These data, along with the simple theory developed by Ohnishi and Mizutani for the incorporation of crystal-field effects, provide more reliable estimates of K (179 GPa) and dK/dP (~ 5.3) for stoichiometric wüstite FeO than have previously been available. Although the pressure dependence of the elastic moduli C11 and C12 and hence the bulk modulus K, is found to be normal, the shear modulus C44 is observed to have an initially small but positive pressure derivative which decreases to zero at ~ 1 GPa, thereafter becoming significantly negative. The observed softening of the C44 shear mode is suggestive of ultimate high-pressure polymorphism. Softening of the same mode has previously been observed in association with the paramagnetic --> antiferromagnetic transition at the room pressure Néel point TN (118 K) in MnO, and also as a precursor to pressure-induced B1 --> B2 phase transformation in a variety of halides and oxides. Both of these are possible modes of high pressure transformation in MnO. A very tentative phase diagram has been constructed on the basis of crude extrapolations of relatively low-pressure data describing the pressure dependence of TN and C44/K.

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