Substitutional impurity-induced vitrification in microgel crystals

We study the effect of larger substitutional impurities on the structure of soft microgel crystals. At the size ratio we employ, rsmall/rlarge = 0.67, we observe the unexpected co-crystallisation of the large impurities with the base crystal, at low fractions of impurity particles. A single impurity takes the place of 4 tetrahedrally coordinated small particles within the lattice. However, as this is accompanied by local deformations of the particles, this distortion-minimizing structure transforms into a random surrounding of the impurity particle at higher fractions of large substitutional impurities. The distortions in the lattice become longer ranged through this transformation, and ultimately result in vitrification of the sample.