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

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)   Å 3 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)   Å 3 and bulk modulus K0 = 16.5   (4)   GPa for the α -polymorph, and V0 = 713.6   (5)   Å 3 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 negativ...
Source: Acta Crystallographica Section B - Category: Chemistry Authors: Tags: powder diffraction thermal expansion DFT neutron diffraction negative linear compressibility ammonium carbamate research papers Source Type: research
More News: Chemistry | Study