Compression of ice VII to 500 kbar

The isothermal compression of ice VII up to 500 kbar has been measured at 23±2°C by high-pressure X-ray diffraction technique using a diamond-anvil cell. Water transforms to ice VI at 9.3±0.3 kbar, ice VI transforms to ice VII at 20.8±0.5 kbar, and no other phase has been observed up to 500 kbar. The compression data fitted to the Birch equation of state yield K 0 =240±9 kbar and K ′ 0 =4.5, and the volume for ice VII extrapolated to zero pressure is 0.704±0.003 cm 3 /g. No theoretical equations of state for H 2 O which satisfy the data are available at present. The index of refraction for ice VII is observed to be the same as ruby at the pressure interval 350–400 kbar, and it becomes greater than ruby above these pressures.

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