# Thermal Strain and Microstrain in a Polymorphic Schiff Base: Routes to Thermosalience

## TL;DR

This paper studies how a specific chemical compound changes structure with temperature, revealing two different mechanisms for its thermosalient behavior.

## Contribution

The study identifies two distinct thermosalient mechanisms in different polymorphic forms of a Schiff base compound.

## Key findings

- Form I undergoes an explosive phase transition due to anisotropic thermal expansion and negative strain.
- Form III transforms gradually through microstrain accumulation.
- Hirshfeld surface analysis supports a dual-mechanism model for thermosalience.

## Abstract

We present a comprehensive structural and thermomechanical investigation of N-salicylideneaniline, a Schiff base derivative that exhibits remarkable thermosalient phase transition behavior. By combining variable-temperature X-ray powder diffraction (VT-XRPD), differential scanning calorimetry (DSC), hot-stage microscopy, and Hirshfeld surface analysis, we reveal two distinct thermosalient mechanisms operating in different polymorphic forms. Form I displays pronounced anisotropic thermal expansion with negative strain along a principal axis, culminating in a sudden and explosive phase transition into Form IV. In contrast, Form III transforms more gradually through a microstrain accumulation mechanism. Fingerprint plots and contact evolution from Hirshfeld surface analysis further support this dual-mechanism model. These insights highlight the importance of integrating macro- and microscale structural descriptors to fully capture the mechanical behavior of responsive molecular solids. The findings not only enhance the fundamental understanding of thermosalience but also inform the rational design of functional materials for actuating and sensing applications.