The structure, ordering and equation of state of ammonia dihydrate (nh3 · 2h2o)

We present the first ab initio simulations of the low-pressure phase of ammonia dihydrate (NH3 · 2H2O), 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 P21) and orthorhombic (space group P212121) variants of the experimentally observed cubic cell (space group P213) may be constructed, with fully ordered water molecule orientations that obey the ice rules. Our calculations show that the most stable structure at 0 K is orthorhombic (P212121), the monoclinic variants (P21) being energetically disfavored. We provisionally call this ordered orthorhombic phase ADH III. The, as-yet-unmeasured, bulk modulus, K0, is predicted to be 10.67−0.44+0.56 GPa at 0 K. Our results are also combined with literature data to arrive at a revised coefficient of volume thermal expansion, αv = 2.81 × 10−7 T1.39 (from 0–176 K), with the density at 0 K, ρ0 = 991.7(39) kg m−3. We also present a case, based on literature data, that argues for a gradual transformation from a paraelectrically disordered cubic structure (P213) to the proposed antiferroelectrically ordered orthorhombic structure (P212121) around 130–150 K (cf. ice III → IX), a temperature regime that applies to the surfaces and interiors of many medium-sized (radii ∼500–700 km) icy bodies.