Beyond Familiar Phases: Discovering New Crystal Chemistry at the Nanoscale

Abstract Colloidal nanocrystal chemistry is highly developed within a narrow range of binary and simple ternary compositions, with reliable control over size, shape, surface chemistry, and the optical, electronic, and magnetic properties they govern. That control covers a small set of structure types, and the mechanisms by which precursors convert to colloidal products remain incompletely understood even within it. Compositionally complex multinary phases have proven a more difficult target. The cooperative phenomena that motivate sustained interest in multinary compounds in bulk, including geometric magnetic frustration, superconductivity, and charge density wave behavior, remain largely inaccessible to size-effect studies because the relevant compositions do not exist as well-defined nanocrystals. Bulk multinary chemistry achieves structural complexity through site differentiation, in which chemically distinct cations or anions occupy crystallographically distinct positions. Colloidal multinary nanocrystals, by contrast, are typically composed of chemically similar elements with comparable coordination and bonding preferences. The structures they form are accordingly limited to solid solutions in simple parent structures, such as wurtzite, zincblende, and rocksalt. This Perspective examines the synthetic, methodological, and analytical infrastructure that will be needed to translate site-differentiated structures from bulk to the nanoscale. Three synthetic strategies that route the reaction through a structurally informative intermediate are surveyed, alongside complementary mechanistic, statistical, and combinatorial approaches to synthetic discovery and the characterization techniques that will be required to identify new phases at sub-100-nm length scales.