Micrometre-sized colloidal particles can be viewed as large atoms with tailorable size, shape and interactions. These building blocks can assemble into extremely rich structures and phases, in which the thermal motions of particles can be directly imaged and tracked using optical microscopy. Hence, colloidal particles are excellent model systems for studying phase transitions, especially for poorly understood kinetic and non-equilibrium microscale processes. Advances in colloid fabrication, assembly and computer simulations have opened up numerous possibilities for such research. In this Review, we describe recent progress in the study of colloidal crystals composed of tunable isotropic spheres, anisotropic particles and active particles. We focus on advances in crystallization, melting and solidsolid transitions, and highlight challenges and future perspectives in phase-transition studies within colloidal crystals.Micrometre-sized colloidal particles can assemble into numerous structures and phases (FIG.1), and are excellent model systems for the study of phase transitions [1-6] because their thermal motions can be directly visualized by optical video microscopy [7] and tracked by image processing [8]. By contrast, the small spatial and timescales of atomic motions make the microscopic kinetics during phase transitions difficult to resolve.Phase transitions are ubiquitous in nature and industry, and have an important role in materials science, statistical physics, cosmology, biophysics, chemistry and earth science. They depend strongly on dimensionality, surface properties, defects, heating and cooling rates, and external fields. Phase transitions can be classified into first-, secondand infinite-order (in which the free energy is an infinitely differentiable exponential function such as the Koster-litzThouless (KT) transition). The equilibrium behaviour of continuous phase transitions, including second-and infinite-order transitions, can be well described theoretically; however, the fundamental theory to describe first-order phase transitions is lacking. The kinetics of first-order and continuous phase transitions are difficult to predict [9].Colloidal particles can be viewed as large atoms with tailorable size, shape and interactions [6]. Although colloids and atoms differ in some aspects (BOX 1), their phase transitions share many similarities; for example, they both follow classical nucleation theory (CNT) at weak supersaturations [10] and deviate from CNT at strong supersaturations [11]. Hence, colloids can provide general information about phase transitions [4][5][6]. Besides being important as micrometre-scale analogues of atoms, colloids are interesting in their own right. In the past two decades, colloidal model systems have provided vital insights into the microscopic kinetics of melting, crystallization, glass transitions and solid solid transitions. Recent breakthroughs in colloidal fabrication, assembly and computer simulations open new opportunities for phase-transition studies. Comparison between ...

A highZTof up to 0.6 at 773 K with improved mid-temperature thermoelectric performance is achieved for textured SnSe polycrystals without deliberate doping.