Revisiting particle formation beyond classical nucleation: A unified classification framework for nonclassical pathways.

Particle formation is pervasive in both nature and technology, shaping environmental and biological phenomena while underpinning a broad range of industrial products. A mechanistic understanding of particle formation is therefore important both for fundamental science and for the design of particle properties. It is now well established that particle formation can proceed through diverse pathways beyond classical one-step nucleation. At the same time, the proliferation of reported nonclassical pathways has blurred the distinctions among mechanistic categories and obscured their underlying relationships. In this review, we revisit the development of concepts in particle formation pathways and present a unified perspective on this increasingly complex mechanistic landscape. To this end, we classify the nonclassical features of particle formation along three conceptual axes: stepwise transitions between phases or phase-like fluctuations, chemically driven formation of structural units, and aggregation-dominant nucleation and growth processes. This framework places diverse pathways within a common three-dimensional space referenced to classical nucleation, providing a basis for comparing apparently distinct mechanisms within a single picture. We further discuss representative theoretical descriptions in relation to these axes and outline future challenges.