Diffusionless rotator-crystal transitions in colloidal truncated cubes: lattice distortion and kinetic pathways

Rotationally symmetric, hard faceted particles can form rotator mesophases with particles arranged on a lattice with significant orientational disorder. We study the mechanism of entropy-driven rotator-crystal transitions in such systems focusing on the role of lattice distortion. Monte Carlo simulations are conducted for two selected truncations of cubes: s=0.527 (TC52) & s=0.572 (TC57) that are known to form rotator phases. These systems are chosen because the rotator & crystal lattices are identical for TC57 but dissimilar for TC52.  While TC57 rotator phase successfully transitions to crystal upon compression, the TC52 rotator phase transitions to an orientational salt intermediate with particles forming alternating layers of opposite orientations. The salt lattice is much closer to the rotator than the crystal. Thus, the penalty to accommodate distortion steers the TC52 rotator phase along a pathway where the salt forms before the crystal, at least for moderate supersaturations. We develop order parameters to track rotator-crystal & rotator-salt transitions to calculate the associated free energy barriers via umbrella sampling. This work paves the way for further study of diffusionless transformations in nanoparticles & highlights the role of lattice-distortion on kinetics.