Anomalous compression and equation of state of coesite

The compressional behaviour of coesite has been determined to a maximum pre ssure of 9.6 GPa by room-temperature single-crystal X-ray diffraction measu rements on three crystals selected from two different synthetic samples. Th ere is no evidence for any phase transitions in this pressure range. The ne w direct measurements show that coesite is approximately 1% denser at 9-10 GPa (at room temperature) than predicted by extrapolation of the previously reported EoS. The volume-pressure data of all three samples cannot be adeq uately described by any available third-order equation of state. Refined pa rameters for a fourth-order Birch-Murnaghan EoS are K-T0 = 100.8 +/- 0.5 GP a, K-T0'= 1.8 +/- 0.3 and K-T0"= 0.57 +/- 0.06 GPa(-1). The anomalous posit ive value of K-T0" appears to be related to the anomalous compression of th e c-axis of the structure that exhibits an initial softening with increasin g pressure. At pressures in excess of 8 GPa the bulk modulus of coesite is some 20 GPa less than that previously predicted, and the impedence contrast between coesite and stishovite is therefore some 10% greater than would ha ve been estimated on the basis of the previous EoS parameters. (C) 2001 Els evier Science B.V. All rights reserved.