Elettra | XRD1 While most people think of crystals as static and rigid, some crystals can behave quite differently. Under specific conditions, they can jump, bend, or even shatter spontaneously. This physical behaviour is generally produced during/ after a transition which involves a rapid change of the crystal structure that results in macroscopic motion. This kind of transition is called thermosalient. Such phenomenon opens the door to new generation devices, from smart actuators to temperature-sensitive switches. Most known examples of these so-called “jumping crystals” occur at relatively high temperatures, but we have discovered a rare case of a crystal that springs into action well below room temperature, around –40 °C. The molecule behind this behavior is a mesogenic hydrocarbon crystal (H2), Fig. 1 (a), which belongs to a family of compounds designed for optical applications in telecommunications. H2 displays an exceptional transformation between two different crystalline forms (phases I and II), triggered simply by cooling. When this temperature-driven transition occurs, single crystals of phase I suddenly move or shatter into the new phase II, often fragmenting into sharp slices with staircase-like profiles. This behavior is a textbook example of a thermosalient effect: a direct conversion of thermal energy into mechanical motion. To understand what drives this dramatic change, we have studied the transition mechanism of the crystal transition. The crystal structure of phase I was solved using x-ray diffraction data from a conventional source while synchrotron radiation was needed to solve the crystal structure of phase II and scan the crystal lattice parameters variation from 200 K up to room temperature. The comparison between the crystal structures showed that the molecules in the two phases remain largely aligned and parallel, but they slide past each other along their long axes during the transition, as shown in Fig. 1 (b). This molecular slipping happens so quickly that internal stress builds up and is released explosively, causing the crystal to jump or break apart. Further insights came from solid state Raman spectroscopy, a technique that reveals how molecules vibrate inside a crystal. There is a basic difference between molecular vibration modes in liquids and gases as compared to crystals. Due to cohesion forces and lattice ordering, molecular motions in a crystal are correlated in space and phase. In a certain sense, the whole crystal should be considered as a unique giant vibrating supermolecule. In our case, a key discovery was made: a very low-frequency lattice vibration of phase I of H2, actually the lowest frequency one, measured at 22.4 cm⁻¹, appears to trigger the transition. This vibrational mode involves neighboring molecules oscillating in opposite directions along their length, effectively reproducing the sliding motion seen in the crystal structures, Fig. 1 (c). Based on this, we have proposed a new perspective: rather than being driven by random thermal fluctuations, the transition is initiated by a coherent vibrational wave propagating through the crystal. In other words, the transformation is not chaotic, it is precisely orchestrated. One of the features that seems to predict such explosive behavior is the presence of colossal and highly anisotropic thermal expansion. Measurements showed that the crystal expands along one direction nearly 200 times more rapidly than along the least responsive axis. This extreme anisotropy is a distinctive feature of materials prone to sudden shape changes. Another diagnostic indicator is the presence of a supercell, a larger repeating unit in the crystal that provides a geometric connection between the two phases. The ability to describe both crystal forms using related supercells reinforces the idea that the transition is coherent and collective. What makes this case particularly exciting is not just the dramatic physical response, but the temperature at which it happens. Most previously reported thermosalient crystals operate near or above room temperature. A transition well below room temperature expands the potential for real-world applications, especially in environments where thermal control is difficult. The study also outlines an overview for predicting such transitions in other materials by focusing on vibrational modes and structural features. By combining crystallography, spectroscopy, and computational modeling, we have added a new piece to the puzzle of how and why crystals can move from one crystal structure to another. Our findings point toward a future where materials can sense and respond to their environment with elegance and precision, even when chilled to the core. Crystals in resonance: how a low-frequency vibration triggers structural change Elettra Highlights 2024–2025 48
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