We have measured the thermal expansivity of ammonia dihydrate (ND3.O2D(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 Gruneisen 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-0,V-0=356.464+/-0.005 Angstrom(3), (K-0,K-0/gamma)=7.163+/-0.024 GPa, and K'(0,0)=5.41+/-0.33 (where V-P,V-T is the unit cell volume at pressure P and temperature T, K-P,K-T is the isothermal bulk modulus, K'(P,T) is its first pressure derivative, and gamma is the Gruneisen ratio). The two Debye temperatures are theta(A)(D)=165+/-3 K and theta(D)(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-0,V-174=365.69+/-0.16 Angstrom(3), K-0,K-174=7.02+/-0.25 GPa, and K'(0,174)=9.56+/-1.28. The volume thermal expansion coefficient, alpha(V), at 174 K and atmospheric pressure is 281.3x10(-6) K-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. (C) 2003 American Institute of Physics.