Mode-specific solid state phase transition under high pressure and temperature conditions

The combination of high pressure and high temperature (HPHT) conditions enables the observation of phase-transition behaviour in materials otherwise inaccessible under ambient conditions. Individually, increasing pressure and temperature often have opposite effects on materials properties, and the exact nature and details of the thermal-mechanical coupling necessary for chemical transformations to take place remain poorly understood and difficult to decipher experimentally. We developed an innovative quantum-chemical simulation approach to address this problem, which we applied to investigate phase transition in 2D materials. In general, pressure increases the gradient of the lattice vibration potential of phonon modes. However, for a few specific phonon modes, which we refer to as reaction modes, the lattice vibration potential gradient is inversely proportional to pressure. Under mechanical loadings, the reaction mode can exhibit anharmonic behaviour at much smaller vibration amplitudes than regular phonon modes, introducing instability leading to a lower phase-transition temperature. The electron-vibration interaction in the reaction mode proves crucial for the efficient transfer of thermal energy to chemical bonds and drives bond rearrangement. This work unveils the role of thermal-mechanical coupling in solid-state phase transitions, and the proposed mode-specific reaction mechanism bridges an important knowledge gap between HPHT and mode-selective chemical synthesis.