What started as chats over tea between Physics PhD student Connor Wilson (BSc ’21, MSc ’23) and Assistant Professor of Physics Ganesh Ramachandran became a mission to solve something of a cold case in their field.<\/p>\n
The mystery is why solid lithium \u2014 a mineral used to produce rechargeable batteries and some mental health medications \u2014 behaves differently than other solid materials at cold temperatures.<\/p>\n
\u201cIt has been known since 1947 that lithium has strange properties,\u201d says Ramachandran, Wilson\u2019s PhD supervisor. \u201cOver these decades, people have done experiments with contradictory results and lots of hypotheses put forward, but there has been no explanation so far as to why lithium\u2019s structure is disordered in extreme cold.\u201d<\/p>\n
Normally, the arrangement of atoms is most \u201cordered\u201d \u2014 forming a regular pattern \u2014 when a solid is cold. As the solid heats up, defects appear and atoms can become less ordered \u2014 by slipping out of alignment, for example \u2014 until the material eventually melts.<\/p>\n
Solid lithium is highly ordered at room temperature. When cooled below -200 C, however, it suddenly loses its regular arrangement.<\/p>\n
\u201cIt appears to form a random structure with many defects,\u201d says Ramachandran. \u201cUnlike other solids, lithium loses order when cooled, an abnormality that intrigues us.\u201d<\/p>\n
The quest to solve the puzzle began with Eric He, an undergraduate student attending another university who Ramachandran hired during the summers of 2023 and 2024 with support from the National Science and Engineering Research Council of Canada, which is funded by the federal government.<\/p>\n
While working with Ramachandran, He produced a series of numerical calculations related to the arrangement of atoms within lithium.<\/p>\n
Ramachandran and Wilson verified those calculations and Wilson continued to advance He\u2019s work.\u00a0After much discussion, the duo landed on a possible explanation for what happens when lithium is cold.<\/p>\n
Lithium is closely packed with layers of atoms. At warmer temperatures, the layers self-stack on top of each other efficiently so that the atoms take up as little space as possible.<\/p>\n
When lithium cools down, however, the stacks start to shift and \u201cfight\u201d with one another, says Ramachandran, with some layers stacking one way and some layers stacking another way.<\/p>\n
\u201cPast researchers observed that the structure of lithium seems random at low temperatures and doesn\u2019t look like any other solid,\u201d he says. \u201cThey thought it looked distorted but it must have some consistent structure. Our theory is that it\u2019s not a complicated structure; rather lithium is frustrated.\u201d<\/p>\n
In physics, the term \u201cfrustrated\u201d refers to a situation where competing interactions \u2014 such as shifting layers of atoms \u2014 prevent a system from reaching a stable state where all interactions are fulfilled.<\/p>\n
Ramachandran says that, in the short term, insights gained from the team\u2019s research will deepen other physicists\u2019 understanding of lithium so that the theory can be advanced.<\/p>\n