Polar materials such as ferroelectrics are insulators that do not conduct electricity, but polar metals are an exception to this rule. An international joint research group, including Doctoral Student Kantaro Murayama, Associate Professor Hiroshi Takatsu, and Professor Hiroshi Kageyama of the Graduate School of Engineering, Kyoto University, along with Professor Ryotaro Arita of the Graduate School of Science, the University of Tokyo, has demonstrated that a phase transition occurs between polar and non-polar structures in lithium rhenate (LiReO3), a material with metallic conductivity. Furthermore, they revealed that even in the low-temperature region below the transition temperature (Ts), the structure is not statically fixed but rather maintains structural fluctuations. Takatsu said, "While the synthesis of LiReO3 was reported about 40 years ago, its structure and electronic state were poorly understood. By combining experiments and theory, we have clarified that a shallow potential formed by conduction electrons is the origin of structural fluctuations in this polar metal. We hope this discovery leads to new material designs that utilize dynamic fluctuations, born from the competition between conduction electrons and the crystal lattice, as a design principle." The results were published in Science Advances.
Provided by Kyoto University
Polar materials like ferroelectrics are essential functional materials for modern society, used in sensors and optical elements due to their piezoelectricity and nonlinear optical effects. Oxides with a lithium niobate (LiNbO3)-type structure are typical polar materials. On the other hand, it has been believed that a metal in which electricity flows cannot be a polar material because conduction electrons do not cause charge bias (polarity).
The discovery of the polar metal lithium osmate (LiOsO3) in 2013 overturned this common wisdom. However, many mysteries remained regarding the physical origins, such as how a polar structure is stabilized within a metal and what role conduction electrons play in the stability and phase transition dynamics.
The international research group focused on lithium rhenate (LiReO3), which has the same crystal structure as LiNbO3 and LiOsO3. Multiple measurements at SPring-8 and elsewhere have experimentally demonstrated that LiReO3 undergoes a clear polar-nonpolar structural phase transition at 170 K (-103℃), confirming its status as a polar metal.
Next, to verify in detail the effect of conduction electrons on the polar structure, they synthesized a solid solution with LiNbO3, a typical ferroelectric (insulator) with the same type of structure. They found that as the LiReO3 component increased, the transition temperature (Ts) dropped dramatically from 1,480 K (1,207℃) in LiNbO3 to just 170 K in LiReO3.
This is the first direct demonstration that the introduction of conduction electrons significantly destabilizes the polar structure.
"Since the two synthesized substances have electrically opposite properties, metal and insulator, we didn't know what would happen until we tried it," says Murayama.
A notable finding is the existence of structural fluctuations unique to polar metals. In LiReO3, fluctuations between the polar and non-polar structures persist over a wide temperature range across Ts. This fact was confirmed not only through the elastic modulus and thermoelectric response hysteresis measured by ultrasonic waves, but also through Raman scattering experiments that investigate the state of atomic vibrations.
In a normal structural phase transition, fluctuations decay at low temperatures, but in LiReO3, a novel phenomenon was confirmed where structural fluctuations persist down to low temperatures.
To elucidate this mechanism, they performed first-principles calculations based on the self-consistent phonon theory. As a result, it was found that in LiReO3, a "shallow potential" is formed due to the influence of conduction electrons, where the energy difference between the polar and non-polar states is extremely small.
It is thought that this shallow energy landscape realizes a state where both structures continue to fluctuate dynamically even at low temperatures. In fact, these structural fluctuations were observed as a reverberating echo signal at low temperatures in ultrasonic measurements. Normally, at low temperatures after a structural phase transition, atomic vibrations settle down and sound wave absorption decreases. However, in LiReO3, resonance and absorption with sound waves were instead observed on the lower temperature side.
This is decisive evidence showing that the crystal lattice is dynamically fluctuating even at low temperatures, and it is an achievement that clarifies new dynamics inherent in polar metals.
The structural fluctuation found in this study, which persists even below the transition temperature, is an important finding that extends our understanding of conventional phase transition phenomena.
Unlike random disorder derived from impurities or defects, these fluctuations are a phenomenon caused by the dynamic degrees of freedom inherently possessed by the substance.
In such systems, nonlinear and selective responses to external stimuli such as heat, light, and sound waves are expected.
In other words, by reframing fluctuations not as "noise to be eliminated" but as a "source that creates functionality," there is a possibility of bringing a new perspective to material design.
Journal Information
Publication: Science Advances
Title: Lattice softening and diffusive dynamics in the polar metal LiReO3
DOI: 10.1126/sciadv.adt3886
This article has been translated by JST with permission from The Science News Ltd. (https://sci-news.co.jp/). Unauthorized reproduction of the article and photographs is prohibited.