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Full Record Details
Persistent URL
http://purl.org/net/epubs/work/52249532
Record Status
Checked
Record Id
52249532
Title
Ab initio simulations of α- and β-ammonium carbamate (NH4·NH2CO2), and the thermal expansivity of deuterated α-ammonium carbamate from 4.2 to 180 K by neutron powder diffraction
Contributors
CM Howard
,
IG Wood (STFC Rutherford Appleton Lab., and University College London)
,
KS Knight (STFC Rutherford Appleton Lab., and University College London)
,
AD Fortes (STFC Rutherford Appleton Lab., and University College London)
Abstract
Experimental and computational studies of ammonium carbamate have been carried out, with the objective of studying the elastic anisotropy of the framework manifested in (i) the thermal expansion and (ii) the compressibility; furthermore, the relative thermodynamic stability of the two known polymorphs has been evaluated computationally. Using high-resolution neutron powder diffraction data, the crystal structure of α-ammonium carbamate (ND4·ND2CO2) has been refined [space group Pbca, Z = 8, with a = 17.05189 (15), b = 6.43531 (7), c = 6.68093 (7) Å and V = 733.126 (9) Å3 at 4.2 K] and the thermal expansivity of α-ammonium carbamate has been measured over the temperature range 4.2–180 K. The expansivity shows a high degree of anisotropy, with the b axis most expandable. The ab initio computational studies were carried out on the α- and β-polymorphs of ammonium carbamate using density functional theory. Fitting equations of state to the P(V) points of the simulations (run athermally) gave the following values: V0 = 744 (2) Å3 and bulk modulus K0 = 16.5 (4) GPa for the α-polymorph, and V0 = 713.6 (5) Å3 and K0 = 24.4 (4) GPa for the β-polymorph. The simulations show good agreement with the thermoelastic behaviour of α-ammonium carbamate. Both phases show a high-degree of anisotropy; in particular, α-ammonium carbamate shows unusual compressive behaviour, being determined to have negative linear compressibility (NLC) along its a axis above 5 GPa. The thermodynamically stable phase at ambient pressure is the α-polymorph, with a calculated enthalpy difference with respect to the β-polymorph of 0.399 kJ mol−1; a transition to the β-polymorph could occur at ∼0.4 GPa.
Organisation
ISIS
,
ISIS-HRPD
,
STFC
Keywords
Funding Information
STFC
, Consolidated Grant (Astronomy) (ST/K000934/1);
Science and Technology Facilities Council
, Beam-time access (RB1410134)
Related Research Object(s):
10.5286/ISIS.E.55377240
Licence Information:
Language
English (EN)
Type
Details
URI(s)
Local file(s)
Year
Journal Article
Acta Crystallogr B
78, no. 3 (2022): 459-475.
doi:10.1107/S2052520622002645
2022
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