Ultralow lattice thermal conductivity and electronic properties of monolayer 1T phase semimetal SiTe₂ and SnTe₂

2H phase (trigonal prismatic D_3h) of layered two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted a lot of interests due to the superior electronic and optoelectronic properties. However, flexible electronic devices and thermoelectric performances based on 2H phase have been potentially limited by the strain sensitive electronic band gap and high lattice thermal conductivity (κ_L). Here, we predict and calculate two 1T (octahedral O_h) phase monolayer telluride materials SnTe₂ and SiTe₂ with soft mechanics, ultralow κ_L and electronic properties. The calculated in-plane Young's modulus of monolayer SnTe₂ is softer than most of 1T-MX₂ compounds. Furthermore, monolayer SiTe₂ and SnTe₂ also have relatively flexible electronic properties under large biaxial strain, indicating potential flexible electrode materials. Meanwhile, monolayer SiTe₂ and SnTe₂ both exhibit ultralow κ_L (2.27 W/mK of SiTe₂ and 1.62 W/mK of SnTe₂) at room temperature. Considering both acoustic and polar optical phonon scattering of the electronic relaxation time, the figure of merit (ZT) can achieve 0.46 at 600 K and 0.71 at 900 K for monolayer SiTe₂ and SnTe₂ respectively.