A new study has found that TlGaS₂, a two-dimensional material, shows both in-plane and out-of-plane polarization, while revealing several subtle structural changes that may help explain the material's unusual behavior.

The study, led by Prof. YIN Lihua from the Institute of Solid State Physics, Hefei Institutes of Physical Science of the Chinese Academy of Sciences, and conducted in collaboration with other researchers, was published recently in Physical Review B.

Two-dimensional ferroelectric materials have attracted interest for potential applications in 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 researchers 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.

Using infrared spectroscopy, the researchers further identified several subtle structural changes at different temperatures. 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.

According to the researchers, this work expands the range of two-dimensional ferroelectric materials and offers new insights into their multifunctional properties.

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)