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Phase behaviour and thermoelastic properties of perdeuterated ammonia hydrate and ice polymorphs from 0 to 2 GPa

Fortes, AD; Wood, IG; Vocadlo, L; Knight, KS; Marshall, WG; Tucker, MG; Fernandez-Alonso, F; (2009) Phase behaviour and thermoelastic properties of perdeuterated ammonia hydrate and ice polymorphs from 0 to 2 GPa. Journal of Applied Crystallography , 42 (5) pp. 846-866. 10.1107/S0021889809027897. Green open access


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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 center dot 2D(2)O). In addition to the low-pressure cubic crystalline phase, ADHI, 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 similar or equal to 310 angstrom(3) 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 center dot D2O) phase II ( AMH II) and ice II when warmed to similar 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 similar or equal to 890 angstrom(3), Z = 16). Above 600 MPa, an orthorhombic polymorph of ammonia dihydrate ( with V similar or equal to 530 angstrom(3), 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.

Type: Article
Title: Phase behaviour and thermoelastic properties of perdeuterated ammonia hydrate and ice polymorphs from 0 to 2 GPa
Open access status: An open access version is available from UCL Discovery
DOI: 10.1107/S0021889809027897
Publisher version: http://dx.doi.org/10.1107/S0021889809027897
Language: English
Additional information: Copyright © International Union of Crystallography. Author(s) of this paper may load this reprint on their own web site or institutional repository provided that the cover page is retained. Republication of this article or its storage in electronic databases other than as specified above is not permitted without prior permission in writing from the IUCr. For further information see http://journals.iucr.org/services/authorrights.html
UCL classification: UCL > School of BEAMS
UCL > School of BEAMS > Faculty of Maths and Physical Sciences
URI: http://discovery.ucl.ac.uk/id/eprint/114386
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