  Traditionally, phase transitions are explored using a combination of
macroscopic functional characterization and scattering techniques, providing
insight into average properties and symmetries of the lattice but local atomic
level mechanisms during phase transitions generally remain unknown. Here we
explore the mechanisms of a phase transition between the trigonal prismatic and
distorted octahedral phases of layered chalogenides in the MoS2 ReS2 system
from the observations of local degrees of freedom, namely atomic positions by
Scanning Transmission Electron Microscopy (STEM). We employ local
crystallographic analysis based on machine learning of atomic environments to
build a picture of the transition from the atomic level up and determine local
and global variables controlling the local symmetry breaking. In particular, we
argue that the dependence of the average symmetry breaking distortion amplitude
on global and local concentration can be used to separate local chemical and
global electronic effects on transition. This approach allows exploring atomic
mechanisms beyond the traditional macroscopic descriptions, utilizing the
imaging of compositional fluctuations in solids to explore phase transitions
over a range of realized and observed local stoichiometries and atomic
configurations.
