Researchers Uncover Rare Phenomenon of Symmetry Reduction in Crystals Upon Heating

The study traces this unusual behavior to stereochemically active lone pairs of lead atoms within the crystal, leading to a complex interplay between static and dynamic distortions.

A groundbreaking study has observed a rare phenomenon where the local symmetry of a crystal structure decreases upon heating, contrary to the typical behavior where symmetry increases with rising temperatures. The research, led by Prof. Kanishka Biswas and his team at the Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Bangalore, highlights a novel phenomenon termed "emphanisis", a discovery with significant implications for applications in phononics, thermoelectrics, and solar thermal conversion.

Symmetry breaking is a core concept in chemistry and physics, commonly seen in the transition of gases to liquids and eventually solids, with each phase change involving a reduction in symmetry. Traditionally, heating a material increases its entropy, leading to higher crystal symmetry. However, the JNCASR team discovered that in a specific two-dimensional halide perovskite, Cs₂PbI₂Cl₂, the local crystal structure's symmetry decreases as temperature rises, while the overall global crystal symmetry remains constant.

The researchers identified that this local symmetry breaking occurs due to configurational averaging, where localized distortions, particularly around lead atoms, average out on a larger scale, preserving the global structure. This phenomenon challenges conventional thermodynamic understanding and was investigated using advanced synchrotron X-ray diffraction techniques at DESY in Hamburg, Germany, as part of an India-DESY collaboration.

The study traces this unusual behavior to stereochemically active lone pairs of lead atoms within the crystal, leading to a complex interplay between static and dynamic distortions. The researchers explain that these distortions arise from interactions between chlorine (Cl) and iodine (I) atoms with the lead atom's lone electron pair, resulting in a disordered state at higher temperatures, termed the "emphanitic" phase.

The discovery of emphanisis offers a new approach to achieving low lattice thermal conductivity in crystalline materials, a property essential for various technologies like thermoelectrics and heat management systems. The findings underscore the importance of chemical design in creating unconventional and functional phenomena in crystalline structures, potentially paving the way for innovative materials with broad industrial applications.

The research, published in Advanced Materials, opens up new avenues for understanding and manipulating thermodynamic behavior in crystalline materials for diverse technological applications.