Shape-driven solid–solid transitions in colloids

Significance Despite the fundamental importance of solid–solid transitions for metallurgy, ceramics, earth science, reconfigurable materials, and colloidal matter, the details of how materials transform between two solid structures are poorly understood. We introduce a class of simple model systems in which the direct control of local order, via colloid shape change, induces solid–solid phase transitions and characterize how the transitions happen thermodynamically. We find that, within a single shape family, there are solid–solid transitions that can occur with or without a thermal activation barrier. Our results provide means for the study of solid–solid phase transitions and have implications for designing reconfigurable materials. Solid–solid phase transitions are the most ubiquitous in nature, and many technologies rely on them. However, studying them in detail is difficult because of the extreme conditions (high pressure/temperature) under which many such transitions occur and the high-resolution equipment needed to capture the intermediate states of the transformations. These difficulties mean that basic questions remain unanswered, such as whether so-called diffusionless solid–solid transitions, which have only local particle rearrangement, require thermal activation. Here, we introduce a family of minimal model systems that exhibits solid–solid phase transitions that are driven by changes in the shape of colloidal particles. By using particle shape as the control variable, we entropically reshape the coordination polyhedra of the particles in the system, a change that occurs indirectly in atomic solid–solid phase transitions via changes in temperature, pressure, or density. We carry out a detailed investigation of the thermodynamics of a series of isochoric, diffusionless solid–solid phase transitions within a single shape family and find both transitions that require thermal activation or are “discontinuous” and transitions that occur without thermal activation or are “continuous.” In the discontinuous case, we find that sufficiently large shape changes can drive reconfiguration on timescales comparable with those for self-assembly and without an intermediate fluid phase, and in the continuous case, solid–solid reconfiguration happens on shorter timescales than self-assembly, providing guidance for developing means of generating reconfigurable colloidal materials.