A research team led by Prof. YIN Lihua from the Institute of Solid State Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, and their collaborators have found that TlGaS₂, a two-dimensional material, shows both in-plane and out-of-plane polarization. The team also identified several weak structural changes that provide clues to its unusual behavior.
The results were published in Physical Review B.
Two-dimensional ferroelectric materials are being studied for applications such as ferroelectric transistors and non-volatile memory devices. However, materials that combine in-plane and out-of-plane polarization remain rare, limiting their potential for device design.
In this study, the team prepared high-quality TlGaS₂ single crystals and characterized their properties using several experimental techniques. The results confirmed incipient ferroelectric behavior in the material, with a stronger response along the in-plane direction. Infrared measurements also revealed a soft phonon mode below 75 K, a feature commonly linked to quantum ferroelectricity.
The team further identified several weak structural changes at different temperatures through infrared spectroscopy. Unlike typical phase transitions, these changes were not accompanied by clear heat-capacity signals, suggesting that they may involve local structural rearrangements or weak ordering. This finding helps explain differences among previous studies of TlGaS₂.
This work expands the range of two-dimensional ferroelectric materials and offers new insights into their multifunctional properties, according to the team.
Schematic 2D layered crystal structure of TlGaS2 (left panel). Temperature-dependent in-plane dielectric constant and image of the TlGaS2 single crystal (right panel). (Image by YIN Lihua)