Isotope effects on the vibrational, Invar, and Elinvar properties of pristine and hydrogenated graphene

• Isotopic substitution changes both the phonon frequency and the hybridization.
• Isotopic substitution has an effect on the anharmonicity of phonon modes.
• Isotopic substitution has a negligible effect on the thermodynamic properties.

Materials with Invar (Elinvar) property have a negligible temperature dependence in lattice constant (elasticity), and both pristine and hydrogenated graphene possess these two superior properties. The isotope effects on the vibrational, Invar, and Elinvar properties of them are studied using density-functional simulation. The increase in mass not only lowers the vibrational frequencies of (hydrogenated) graphene, but also influences the inter-atom and inter-branch hybridizations in hydrogenated graphene. However, the Invar and Elinvar properties have a negligible isotopic dependence, and the underlying mechanism is revealed by analyzing the isotope effects on phonon dispersions, anharmonicity, and thermodynamics. When carbon/hydrogen isotope is used to selectively engineer some properties (e.g., thermal transport and kinetic stability), such isotopic independence of the Invar and Elinvar properties can guarantee the minimized side effects on structure and stiffness.