Abstract
<title>Abstract</title> <p>A simple incompressible model for the thermodynamic properties of graphite is presented based on hybrid 1D/2D Debye harmonic oscillators and a Fermi electron gas. The model represents vibrations of carbon atoms perpendicular to the sp² slip planes using a 1D-Debye oscillator with a constant (temperature-independent) Debye temperature of 735 K, while vibrations parallel to the slip planes are represented by a 2D-Debye oscillator with a constant Debye temperature of 2450 K. The corresponding Debye wavenumbers, 511 and 1700 cm⁻¹, are quantitatively consistent with fundamental lattice-vibration frequencies. The model reproduces NIST–JANAF specific heat and entropy data to within 1% over the temperature range 300–6000 K. These results demonstrate that inclusion of lattice-vibration anharmonicity is less important than previously suggested, even at high temperatures, provided that the anisotropic structure of graphite is incorporated into the Debye model through a hybrid 1D/2D treatment rather than the conventional isotropic 3D treatment. The model is also well suited for calculating thermochemical properties of multi-atom carbon-containing species. When combined with an appropriate sublimation energy and a gas-phase model of comparable rigor (rigid-rotator/harmonic-oscillator), it predicts standard-pressure enthalpies-of-formation and equilibrium constants for species such as CO and CO₂ with accuracy better than 0.2% from 300 K to 6000 K. Finally, some inferences arising from this model regarding dispersion of coherent, electromagnetic energy into less coherent thermal energy are discussed that could have implications in infrared radiation emission, absorption, and scattering by interstellar dust and atmospheric aerosol.</p>