Researchers have thoroughly analyzed the mechanisms underlying the flexibility of metal-organic frameworks (MOF) crystals and attributed this flexibility to extensive structural rearrangements associated with soft and hard vibrations within the crystal that strongly couple strain fields, opening the door to novel materials with diverse applications in various analytical industries.
Metal-organic frameworks (MOFs) are a large class of crystalline materials that have remarkable abilities to absorb and store gases such as carbon dioxide as well as act as filters for the purification of crude oil. MOFs derive this ability from the presence of nanopores, which increase their surface areas, and which in turn make them efficient in absorbing and storing gases. However, limited stability and mechanical fragility have hindered their widespread applications.
Addressing this problem, Professor Umesh V. Waghmare and his team from the Theoretical Sciences Unit at Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Bengaluru, an autonomous institute under the Department of Science and Technology (DST), Government of India, have recently proposed a new quantitative measure of mechanical flexibility for a crystal that can be used to screen materials databases to identify next-generation flexible materials.
His paper, “Quantifying the Intrinsic Mechanical Flexibility of Crystalline Materials ”, presents unprecedented insights into the origin of mechanical flexibility and was published in the journal Physical Review B. Professor Waghmare’s research focuses particularly on MOFs, which are known for their beautiful crystalline structure and great flexibility.
Historically, elasticity in any crystal has been evaluated in terms of a parameter called the elastic modulus, which is a measure of the material's resistance to strain-induced deformation. In contrast, this study proposes a unique theoretical measure based on the partial emission of elastic strain or strain energy through internal structural rearrangements under symmetry constraints. This new metric can be easily calculated using standard techniques of simulation.
And it can calibrate the flexibility of crystals on a scale from zero to one, with zero indicating the least flexibility while one indicating the maximum flexibility. Furthermore, this discovery provides a unique and quantitative insight into a dimension of crystal flexibility that was hitherto unknown.
Using theoretical calculations, the team investigated the flexibility of four different systems with different elastic stiffnesses and chemistries. They found that flexibility arises from large structural rearrangements involving soft and hard vibrations within a crystal that connect strongly stressed regions.
The team's research thus goes beyond the traditional view by providing a deeper understanding of the underlying mechanisms of crystal flexibility, in contrast to previous studies that primarily focused on elastic properties. The work also establishes flexibility as an intrinsic property of crystals that is independent of their specific shape or form.