• ammonia dihydrate;
  • ammonia monohydrate;
  • ice;
  • neutron diffraction;
  • polymorphism;
  • crystal growth kinetics

The results are described of a series of neutron powder diffraction experiments over the pressure and temperature ranges 0 < P < 2 GPa, 150 < T < 240 K, which were carried out with the objective of determining the phase behaviour and thermoelastic properties of perdeuterated ammonia dihydrate (ND3·2D2O). In addition to the low-pressure cubic crystalline phase, ADH I, two closely related monoclinic polymorphs of ammonia dihydrate have been identified, which commonly occur as a composite in the range 450–550 MPa at 175 K; these are labelled ADH IIa and IIb, and each has unit-cell volume V≃ 310 Å3 and number of formula units per unit cell Z = 4. It has been determined that this composite dissociates to a mixture of ammonia monohydrate (ND3·D2O) phase II (AMH II) and ice II when warmed to ∼190 K at 550 MPa, which in turn partially melts to ice II + liquid at T = 196 K; AMH II has a large orthorhombic unit cell (V≃ 890 Å3, Z = 16). Above 600 MPa, an orthorhombic polymorph of ammonia dihydrate (with V≃ 530 Å3, Z = 8), which has been referred to previously as ADH IV, persists to pressures greater than 2 GPa and appears to be the liquidus phase over this whole pressure range. This phase has been observed co-existing with ice II, ice VI and AMH II. The most plausible synthesis of the high-pressure phase behaviour is described here. This model explains the reported observations, and provides measurements of the densities, thermal expansion, bulk moduli and crystal growth kinetics of the high-pressure ammonia dihydrate, ammonia monohydrate and ice polymorphs.