Colossal positive and negative thermal expansion and thermosalient effect in a pentamorphic organometallic martensite

Abstract

The thermosalient effect is an extremely rare propensity of certain crystalline solids for self-actuation by elastic deformation or by a ballistic event. Here we present direct evidence for the driving force behind this impressive crystal motility. Crystals of a prototypical thermosalient material, (phenylazophenyl)palladium hexafluoroacetylacetonate, can switch between five crystal structures (α - ε) that are related by four phase transitions including one thermosalient transition (α虠γ). The mechanical effect is driven by a uniaxial negative expansion that is compensated by unusually large positive axial expansion (260 × 10-6 K-1) with volumetric expansion coefficients (≈250 × 10-6 K-1) that are among the highest values reported in molecular solids thus far. The habit plane advances at ∼104 times the rate observed with non-thermosalient transitions. This rapid expansion of the crystal following the phase switching is the driving force for occurrence of the thermosalient effect.