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

ABSTRACT

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 P2₁/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.

KEYWORDS:

• Melamine
• high-pressure
• thermal expansion
• compressibility
• neutron diffraction

Acknowledgements

The authors thank the STFC ISIS facility for beam-time (RB 1810003 and XB 1890073) as well as access to the SCARF scientific computing cluster. We are grateful to Hannah Shelton (University of Hawaii at Mānoa) for sharing CIFs from their recent single-crystal X-ray study.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

C. L. Bull http://orcid.org/0000-0002-5170-6674

Notes

1. The exact relationship between load and pressure is given in Supplementary Table S2.

Additional information

Funding

The authors are grateful to the STFC ISIS Facility for the award of beamtime.