Equation of state and phase transition of deuterated ammonia monohydrate (ND3⋅D2O) measured by high-resolution neutron powder diffraction up to 500 MPa

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 high-resolution diffractometer at the Institut Laue-Langevin. This paper reports observations of the phase transformation from the low-pressure P212121 phase (AMH I) to the high-pressure 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 Birch–Murnaghan equation of state of AMH I has parameters, V0=248.00(2) Å3, K0=7.33(3) GPa with the first pressure derivative of K0 fixed at the value obtained in ab initio calculations, (∂K0/∂P)T=K0′=5.3; the implied value of the second derivative is therefore (∂2K0/∂P2)T=K0″=−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%.