Single-crystal elastic constants of fluorite (CaF2) to 9.3 GPa |
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Authors: | S Speziale T S Duffy |
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Institution: | (1) Department of Geosciences, Princeton University, Princeton New Jersey, 08544 USA e-mail: speziale@Princeton.EDU Tel.: 001-609-258-3261; Fax: 001-609-258-1274, US |
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Abstract: | The second-order elastic constants of CaF2 (fluorite) have been determined by Brillouin scattering to 9.3 GPa at 300 K. Acoustic velocities have been measured in the
(111) plane and inverted to simultaneously obtain the elastic constants and the orientation of the crystal. A notable feature
of the present inversion is that only the density at ambient condition was used in the inversion. We obtain high-pressure
densities directly from Brillouin data by conversion to isothermal conditions and iterative integration of the compression
curve. The pressure derivative of the isentropic bulk modulus and of the shear modulus determined in this study are 4.78 ± 0.13
and 1.08 ± 0.07, which differ from previous low-pressure ultrasonic elasticity measurements. The pressure derivative of the
isothermal bulk modulus is 4.83 ± 0.13, 8% lower than the value from static compression, and its uncertainty is lower by a
factor of 3. The elastic constants of fluorite increase almost linearly with pressure over the whole investigated pressure
range. However, at P ≥ 9 GPa, C
11 and C
12 show a subtle structure in their pressure dependence while C
44 does not. The behavior of the elastic constants of fluorite in the 9–9.3 GPa pressure range is probably affected by the onset
of a high-pressure structural transition to a lower symmetry phase (α-PbCl2 type). A single-crystal Raman scattering experiment performed in parallel to the Brillouin measurements shows the appearance
of new features at 8.7 GPa. The new features are continuously observed to 49.2 GPa, confirming that the orthorhombic high-pressure
phase is stable along the whole investigated pressure range, in agreement with a previous X-ray diffraction study of CaF2 to 45 GPa. The high-pressure elasticity data in combination with room-pressure values from previous studies allowed us to
determine an independent room-temperature compression curve of fluorite. The new compression curve yields a maximum discrepancy
of 0.05 GPa at 9.5 GPa with respect to that derived from static compression by Angel (1993). This comparison suggests that
the accuracy of the fluorite pressure scale is better than 1% over the 0–9 GPa pressure range.
Received: 10 July 2001 / Accepted: 7 March 2002 |
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Keywords: | Brillouin scattering Single-crystal elastic constants Raman scattering |
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