Abstract
The ice VII-VIII phase transition was observed by the X-ray precession method using diamond-anvil pressure cells. Single crystals of ice VII–cubic, a=3.350±0.005 Å (sample A) and a=3.360±0.005 Å (sample B)–transformed into twin-mosaic crystals of ice VIII–tetragonal, a=4.68±0.01 Å and c=6.81±0.01 Å (sample A) and a=4.70±0.01 Å and c=6.84±0.01 Å (sample B)–. The formation of the twin mosaics caused some misorientation and distortion in the crystal. These disappeared during the VIII→VII transition. Thermal hysteresis of the transition temperature was observed.