Contrary to the usual trend where heating increases symmetry, this compound exhibits a decrease in local symmetry with rising temperature, while the global crystal symmetry remains unchanged. This occurs due to configurational averaging, where the distorted local symmetries average out at longer length scales, leaving the global structure intact.<\/p>\n
This phenomenon of local symmetry breaking upon heating is termed “emphanisis,” meaning “appearing out of nothing.” The team employed an advanced synchrotron X-ray technique which simultaneously reveals both the local and global structures of solids from their X-ray diffraction patterns, to investigate emphanisis.<\/p>\n
The synchrotron X-ray experiments were done in DESY, Hamburg, Germany under the India-DESY collaboration supported by Department of Science & Technology (DST), India.<\/p>\n
The researchers traced this unusual local symmetry breaking to the stereochemically active lone pair of lead in the compound.<\/p>\n
Interestingly, Cs2<\/sub>PbI2<\/sub>Cl2<\/sub>\u00a0accommodates two types of structural distortion — static distortions in chlorine atoms and dynamic distortions in lead atoms. These distortions result from the complex interplay between different structure-distorting effects driven by the interactions between the mixed halide (Cl and I) motif and the active lone electron pair of lead in Cs2<\/sub>PbI2<\/sub>Cl2<\/sub>. The distortions happen because of a competition between a mix of structure-distorting forces that arise because of interaction of different parts of the material (the mixed anions- Cl and I) with the lone electron pair on the lead atoms in Cs\u2082PbI\u2082Cl\u2082.<\/p>\n The high temperature “emphanitic” phase is characterized as a disordered distorted state, existing at the intersection of an ordered undistorted state and an ordered distorted state.<\/p>\n “Emphanisis” is a promising strategy for achieving intrinsically low lattice thermal conductivity in crystalline materials. Such materials are highly sought after for their fundamental importance and diverse applications, including phononics, thermoelectrics, solar thermal conversion, and various heat management systems.<\/p>\n The study now published in Advanced Materials, underscores the fundamental and functional significance of chemical design in creating unconventional phenomena in crystalline materials. The findings suggest that understanding these thermodynamic subtleties can lead to intriguing structural transformations with broad applications.<\/p>\n Figure 1.\u00a0<\/em><\/strong>Schematic representations showing the evolution of local structure of a system exhibiting emphanisis.<\/em><\/p>\n Prof. Kanishka Biswas (left) and Ivy Maria (right) at Solid State Chemistry Lab, JNCASR, Bangalore.<\/em><\/p>\n","protected":false},"excerpt":{"rendered":" Researchers have made a rare observation where the local crystal structure symmetry or the arrangement of atoms in the immediate vicinity of a given atom, in a crystal, reduces upon warming, contrary to the usual trend of symmetry of crystal structures increasing with rising temperatures. The study underlines the significance of chemical design in triggering …<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[136],"tags":[18139,18136,18142,7788,18137,16776,18138,18141,18143,18140],"class_list":["post-21826","post","type-post","status-publish","format-standard","","category-national","tag-advanced-energy-solutions","tag-crystal-structures","tag-energy-applications","tag-energy-efficiency","tag-hidden-structures","tag-innovative-materials","tag-material-design","tag-materials-science","tag-paradigm-shift","tag-rare-observation"],"yoast_head":"\n![](\"https:\/\/static.pib.gov.in\/WriteReadData\/userfiles\/image\/image0010MFD.jpg\")
<\/p>\n![](\"https:\/\/static.pib.gov.in\/WriteReadData\/userfiles\/image\/image0028CLP.jpg\")
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