The coexistence of out-of-plane and in-plane electric polarization is rare in two-dimensional (2D) ferroelectric materials, yet the two configurations offer distinct advantages for ferroelectric-based devices. Here, we report the coexistence of in-plane and out-of-plane incipient ferroelectricity, along with various phase transitions, in 2D van der Waals layered ${\mathrm{TlGaS}}_2$ single crystal. Detailed investigation of the incipient ferroelectric soft-mode behavior reveals a close correlation between the incipient ferroelectricity and the off-center displacement of $\mathrm{T}{\mathrm{l}}^{1+}$ ions with $6s^2$ lone pairs in ${\mathrm{TlGaS}}_2$. Anomalies near temperatures of about 75, 120, 180, and 220 K in infrared spectra, together with the absence of anomalies in the temperature-dependent specific heat, indicate the existence of local or weak long-range structural transitions in ${\mathrm{TlGaS}}_2$. Our results provide important experimental evidence for elucidating the phase transitions and coexistence of in-plane and out-of-plane incipient ferroelectricity in 2D layered ${\mathrm{TlGaS}}_2$.

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• Electric polarization
• Ferroelectricity
• Phonons
• Quantum phase transitions
• Fourier transform infrared spectroscopy