The high-pressure phase diagram of ammonia dihydrate

We have investigated the P-T phase diagram of ammonia dihydrate (ADH), ND3 center dot 2D(2)O, using powder neutron diffraction methods over the range 0-9 GPa, 170-300 K. In addition to the ambient pressure phase, ADH I, we have identified three high-pressure phases, ADH II, III, and IV, each of which has been reproduced in at least three separate experiments. Another, apparently body-centred-cubic, phase of ADH has been observed on a single occasion above 6 GPa at 170 K. The existence of a dehydration boundary has been confirmed where, upon compression or warming, ADH IV decomposes to a high-pressure ice phase (ice VII or VIII) and a high-pressure phase of ammonia monohydrate (AMH V or VI).