Thermoelastic properties of deuterated melamine, C3N6D6, between 4.2-320 K at 5 kPa and between 0.1-5.0 GPa at 295 K from neutron powder diffraction and DFT calculations

We have determined the lattice parameters of perdeuterated melamine, C3N6D6, as a function of temperature (4.2-320 K) close to atmospheric pressure and as a function of pressure (0-5 GPa) at room temperature, using time-of-flight neutron powder diffraction methods. Our observations confirm that the ambient-pressure P2(1)/a structure persists throughout this range of P and T. Both the thermal expansion and the compressibility are highly anisotropic; the principal directions of greatest thermal expansion and greatest compressibility are roughly perpendicular to the corrugated sheets of H-bonded melamine molecules that form the main structural motif. Complementary ab initio calculations (van der Waals corrected Density Functional Theory) provide an estimate of how the inter- and intra-molecular geometry may alter as a function pressure in the athermal limit.