In-plane and out-of-plane electric dipoles and phase transitions in 2D-layered TlGaS2

TL;DR

This study reveals the coexistence of in-plane and out-of-plane electric dipoles and multiple phase transitions in 2D TlGaS2, highlighting its complex ferroelectric behavior and potential for device applications.

Contribution

It provides the first detailed experimental evidence of coexisting electric dipoles and associated phase transitions in 2D layered TlGaS2.

Findings

Observation of quantum paraelectricity in both directions

Correlation between dipoles and Tl+ off-center displacement

Detection of local structural transitions at specific temperatures

Abstract

Out-of-plane and in-plane electric polarization, which rarely coexist in a two-dimensional (2D) ferroelectric material, offer different advantages in ferroelectricity-based devices. Here, we report the coexistence of in-plane and out-of-plane electric dipoles, along with various phase transitions, in 2D van der Waals layered TlGaS2 single crystal. Quantum paraelectricity was observed along both in-plane and out-of-plane directions of the TlGaS2 crystal. Detailed investigation of the quantum paraelectric soft-mode behavior reveals a close correlation between the electric dipoles and the off-center displacement of Tl1+ ions with 6s2 lone pairs in TlGaS2. Anomalies near temperatures of about 120 K and 60-75 K in dielectric and/or infrared spectra indicate the existence of local or weak long-range structural transitions in TlGaS2. Our results provide important experimental evidence for elucidating the phase transitions and coexistence of in-plane and out-of-plane electric dipoles in 2D layered TlGaS2.