Thermodynamic analysis of the formation of carbon nitrides under pressure

A thermodynamic analysis of the formation of carbon nitrides under pressure is presented. Integration of the pressure-volume equations of state of carbon, nitrogen, and candidate carbon nitride structures show that under pressures achievable with current technology the free energies of reactions to form carbon nitrides can change more than 1000 kJ/mol of C3N4. These changes are greater than the positive free energies estimated for the formation reactions at ambient pressure, indicating that formation of carbon nitrides under pressure should be feasible. The large change in free energy of reaction under pressure is due to the large volume of molecular nitrogen. Because greater changes in free energy of reaction occur with increasing nitrogen content, pressure favors the formation of nitrogen-rich compounds. However, nitrogen-rich compounds tend to have more nitrogen lone-pair repulsions and contain weaker bonds, destabilizing them with respect to compounds that contain less nitrogen. The structure and composition favored under high pressure will be determined by the balance between these opposing factors.