Search for a possible flexible-to-rigid transition in models of phase change materials

Structural models of the prototypal phase change material Ge-Sb-Te are generated from first-principles molecular dynamics simulations with a particular attention to the ${\mathrm{Ge}}_{x}{\mathrm{Sb}}_{x}{\mathrm{Te}}_{100\ensuremath{-}2x}$ join. Constraint counting algorithms permit us to analyze the obtained amorphous networks within the framework of topological constraint (rigidity) theory. A flexible-to-rigid transition is found at $x\ensuremath{\simeq}8.5$% which satisfies the Maxwell isostatic criterion and is characterized by an important topological disorder with large amounts of mixed geometries and miscoordinations. The angular constraint count furthermore leads to the identification of tetrahedral Ge sites whose population decreases with growing Ge/Sb content and is about 55% for ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$. Sb sites change from a dominant pyramidal geometry typical of Group V chalcogenides to a defect octahedral one which is reminiscent of the crystalline polymorph of ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$.