Equation of state and phase transition of deuterated ammonia monohydrate (ND_{3}.D_{2}O) measured by highresolution neutron powder diffraction up to 500 MPa
Abstract
We describe the results of a neutron powder diffraction study of perdeuterated ammonia monohydrate (AMH, ND3⋅D2O) carried out in the range 102<P_{<502 MPa at T =180 K using the D2B highresolution diffractometer at the Institut LaueLangevin. This paper reports observations of the phase transformation from the lowpressure P21}2121 phase (AMH I) to the highpressure Pbca phase (AMH II) at 351 MPa, and measurements which have allowed us to determine the volumetric and axial incompressibilities of both polymorphs. At 180 K, the fitted third order BirchMurnaghan equation of state of AMH I has parameters, V_{0}=248.00(2) Å^{3}, K_{0}=7.33(3) GPa with the first pressure derivative of K_{0} fixed at the value obtained in ab initio calculations, (∂K_{0}/∂P)T=K_{0}^{'}=5.3; the implied value of the second derivative is therefore (∂^{2}K_{0}/∂P^{2})T=K_{0}^{″}=0.94(1) GPa^{1}. At 351 MPa, we observed that the transition from AMH I to AMH II occurred over a period of 90 min, with an associated reduction in molar volume of 4.6% and an increase in the incompressibility of 19.6%.
 Publication:

Journal of Chemical Physics
 Pub Date:
 October 2009
 DOI:
 10.1063/1.3245858
 Bibcode:
 2009JChPh.131o4503F
 Keywords:

 ab initio calculations;
 ammonium compounds;
 compressibility;
 equations of state;
 highpressure solidstate phase transformations;
 neutron diffraction;
 polymorphism;
 64.30.t;
 61.66.Fn;
 62.20.F;
 81.40.Lm;
 64.70.kt;
 62.50.p;
 Equations of state of specific substances;
 Inorganic compounds;
 Deformation and plasticity;
 Deformation plasticity and creep;
 Molecular crystals;
 Highpressure effects in solids and liquids