Anomalous compression and equation of state of coesite

The compressional behaviour of coesite has been determined to a maximum pressure of 9.6 GPa by room-temperature single-crystal X-ray diffraction measurements on three crystals selected from two different synthetic samples. There is no evidence for any phase transitions in this pressure range. The new 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 adequately described by any available third-order equation of state. Refined parameters for a fourth-order Birch-Murnaghan EoS are K_TO = 100.8 ± 0.5 GPa, K_TO = 1.8 ± 0.3 and K′_TO = 0.57 ± 0.06 GPa^-1. The anomalous positive value of K″_TO appears to be related to the anomalous compression of the c-axis of the structure that exhibits an initial softening with increasing 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 have been estimated on the basis of the previous EoS parameters.