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We have measured the thermal expansivity of ammonia dihydrate (ND[sub 3]·2D[sub 2]O) phase I from 4.2 to 174 K at ambient pressure, and the incompressibility at 174 K from 0 to 0.45 GPa, using time-of-flight neutron powder diffraction. The unit cell volume as a function of temperature, V(T), was fitted with a Grüneisen approximation to the zero-pressure equation of state (with the lattice vibrational energy calculated from a double-Debye model fitted to heat capacity data) having the following parameters at zero pressure and temperature: V[sub 0,0]=356.464±0.005 Å[sup 3], (K[sub 0,0]/γ)=7.163±0.024 GPa, and K[sub 0,0][sup ′]=5.41±0.33 (where V[sub P,T] is the unit cell volume at pressure P and temperature T, K[sub P,T] is the isothermal bulk modulus, K[sub P,T][sup ′] is its first pressure derivative, and γ is the Grüneisen ratio). The two Debye temperatures are θ[sub D][sup A]=165±3 K and θ[sub D][sup B]=729±4 K. The unit cell volume at 174 K as a function of pressure, V(P), was fitted with a third-order Birch–Murnaghan equation of state having the following parameters: V[sub 0,174]=365.69±0.16 Å[sup 3], K[sub 0,174]=7.02±0.25 GPa, and K[sub 0,174][sup ′]=9.56±1.28. The volume thermal expansion coefficient, α[sub V], at 174 K and atmospheric pressure is 281.3×10[sup -6] K[sup -1]. The proton disorder manifested at high homologous temperatures is seen to be frozen in, on the time scale of these experiments, down to 4.2 K. A high-pressure polymorph of ammonia dihydrate was observed following melting of the sample at 179 K and 0.46 GPa. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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). [ABSTRACT FROM AUTHOR]