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A high-resolution neutron powder diffraction study of ammonia dihydrate (ND3 center dot 2D(2)O) phase I

Fortes, AD; Wood, IG; Brodholt, JP; Alfredsson, M; Vocadlo, L; McGrady, GS; Knight, KS; (2003) A high-resolution neutron powder diffraction study of ammonia dihydrate (ND3 center dot 2D(2)O) phase I. J CHEM PHYS , 119 (20) 10806 - 10813. 10.1063/1.1619371.

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Abstract

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.

Type:Article
Title:A high-resolution neutron powder diffraction study of ammonia dihydrate (ND3 center dot 2D(2)O) phase I
DOI:10.1063/1.1619371
Keywords:THERMAL-EXPANSION, ICY SATELLITES, WATER SYSTEM, D2O ICE, DIFFERENTIATION, CONSTANTS, PRESSURE, EQUATION, HYDRATE, STATE
UCL classification:UCL > School of BEAMS > Faculty of Maths and Physical Sciences > Earth Sciences

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