  Controlling a state of material between its crystalline and glassy phase has
fostered many real-world applications. Nevertheless, design rules for
crystallization and vitrification kinetics still lack predictive power. Here,
we identify stoichiometry trends for these processes in phase change materials,
i.e. along the GeTe-GeSe, GeTe-SnTe, and GeTe-Sb2Te3 pseudo-binary lines
employing a pump-probe laser setup and calorimetry. We discover a clear
stoichiometry dependence of crystallization speed along a line connecting
regions characterized by two fundamental bonding types, metallic and covalent
bonding. Increasing covalency slows down crystallization by six orders of
magnitude and promotes vitrification. The stoichiometry dependence is
correlated with material properties, such as the optical properties of the
crystalline phase and a bond indicator, the number of electrons shared between
adjacent atoms. A quantum-chemical map explains these trends and provides a
blueprint to design crystallization kinetics.
