The high-pressure phase diagram of ammonia dihydrate

We have investigated the P–T phase diagram of ammonia dihydrate (ADH), ND3·2D2O, 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).