The structure, ordering and equation of state of ammonia dihydrate (N[H.sub.3] * 2[H.sub.2]O)

We present the first ab initio simulations of the low-pressure phase of ammonia dihydrate (N[H.sub.3] * [2H.sub.2]O), ADH I, a likely constituent of many volatile-rich solid bodies in the outer Solar System (e.g., Saturn's moons). Ordered monoclinic (space group P[2.sub.1]) and orthorhombic (space group P[2.sub.1][2.sub.1][2.sub.1]) variants of the experimentally observed cubic cell (space group P[2.sub.1] 3) may be constructed, with fully ordered water molecule orientations that obey the ice roles. Our calculations show that the most stable structure at 0 K is orthorhombic (P[2.sub.1] [2.sub.1][2.sub.1]), the monoclinic variants (P[2.sub.1]) being energetically disfavored. We provisionally call this ordered orthorhombic phase ADH III. The, as-yet-unmeasured, bulk modulus, [K.sub.0], is predicted to be [10.67.sup.+0.56.sub.-0.44] GPa at 0 K. Our results are also combined with literature data to arrive at a revised coefficient of volume thermal expansion, [[alpha].sub.v] = 2.81 x [10.sup.-7] [T.sup.1.39] (from 0-176 K), with the density at 0 K, [[rho].sub.0] = 991.7(39) kg [m.sup.-3]. We also present a case, based on literature data, that argues for a gradual transformation from a paraelectrically disordered cubic structure ([P2.sub.1]3) to the proposed antiferroelectrically ordered orthorhombic structure ([P2.sub.1][2.sub.1][2.sub.1] around 130-150 K (cf. ice III [left and right arrow] IX), a temperature regime that applies to the surfaces and interiors of many medium-sized (radii ~500-700 km) icy bodies. Keywords: Ices; Interiors; Mineralogy; Satellites of Saturn; Satellites of Uranus