How a ’vegetable ion’ helped scientists unlock theory behind transitions of materials

A puddle freezing on the sidewalk, your humidifier pumping out water vapor, salt trucks melting icy streets-wintertime in Chicago is full of examples of a physics phenomenon called a "phase transition," in which a material changes state. Physicists are fascinated by this phenomenon, which is useful in technology from the basic steam turbine all the way to MRIs.  

In a paper published Dec. 18 in the journal Nature, renowned University of Chicago physicist Peter Littlewood and colleagues propose the most complete picture to date of the transition from conductor to insulator in a type of material called transition metal oxides. Notably, they propose an explanation for a process that had previously baffled theorists, regarding an unexpected role by a part of the molecule called a "vegetable ion."

Such oxides have fascinated scientists because of their many attractive electronic and magnetic properties; the discovery may boost the search for new technologies, such as better batteries and electronics. 

"Tuning and control of this metal-insulator transition has been the source of much exciting new physics and promising materials applications, such as low-power and ultrafast microelectronics," said Littlewood, who has a joint appointment in Argonne National Laboratory ’s Materials Science division. 

A type of material called a correlated oxide is widely useful; scientists have reaped many insights by studying what happens when these oxides change from conducting electricity with little or no resistance (that is, metal-like) to not conductive (i.e., an insulator). This can happen as a result of changes in temperature, pressure, or other external fields.

"The way scientists in the past have typically tuned this metal-insulator transition is by adding electrons," Littlewood said. Research over several decades by others suggests that adjusting the size of an electronically inactive but structurally important "vegetable ion" within the oxide’s crystal structure also has a strong impact on the transition temperature; however, the reason for this effect has not been well understood.

Even though it’s electronically inactive, the size of the "vegetable ion" can alter the temperature at which the metal-insulator transition occurs-from absolute zero to well above room temperature. (The higher the transition temperature, the more attractive the material is for practical applications).

The team’s research focused on an important kind of of transition metal oxide, called  perovskites. Along with oxygen, these oxides combine an electronically active ion and the electronically inactive vegetable ion. The latter ion can be any one of the many rare earth elements or alkaline earth metals. As a consequence, scientists can choose its atomic size to be relatively small or large without changing the related chemistry. 

The pivotal discovery was determining the effect of the size of the rare earth or alkaline earth metal. Varying the size of this element changes the tilt angle introduced into the eight-sided units that make up a perovskite crystal. In turn, increasing the tilt angle results in various distortions and movements in the eight-sided units, which can stretch, shrink and rotate as a result of internal stresses. 

"It is the dynamical fluctuations of these elastic degrees of freedom that are responsible for the observed thermal effects, which occur at temperatures much lower than accounted for in earlier models based purely on the electronically active ion," Littlewood said. 

On the basis of the above mechanism, the team was able to construct a theory that captures the relationship among the tilt angle induced by the vegetable ion size, the temperature of the metal-insulator transition and the degree of disorder in the perovskite crystal structure. Relatively simple calculations with the theory agreed well with experimental results from absolute zero to above 600 degrees Fahrenheit.

"Importantly, our theoretical study applies to not just a single material, but a whole class of materials, and has many possible applications, including some relevant to ongoing and planned research programs at Argonne," said Littlewood. 

In the emerging research area of next-generation microelectronics, for example, improved tuning and control of the metal-insulator transition holds the promise of a great leap forward in low-power and ultrafast microelectronics for computers that simulate brain processes. 

In addition, scientists in Argonne’s world-class battery program might be able to use the theory as inspiration for designing better cathode materials for next-generation lithium-ion batteries. Some of the inspiration for the research by Littlewood’s team was UChicago alum John Goodenough’s trailblazing research on the metal-insulator transition many decades ago. Goodenough translated that understanding into the inspiration to invent the lithium-ion battery, and this year  won the Nobel Prize in chemistry  for his work.

Joining Littlewood in this research project were Gian Guzmán-Verri, then a postdoctoral researcher at Argonne and now at the University of Costa Rica; and Richard Brierley, then a researcher at University of Cambridge and Yale University and now at Nature Communications.