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

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↔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 ~10 4 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. The thermosalient effect is the unusual tendency of some crystals to visibly jump during phase changes. Here, the authors study the multiple phase changes in a prototypic thermosalient material and provide evidence for the factors that drive this self-actuation.