With the help of the Slater-Koster (SK) parametrization, we construct simplified force constant (FC) models to describe the phonons of several two-dimensional (2D) transition metal dichalcogenides (TMDs) (MoX2, X = S, Se or Te) by only considering the FCs to fourth-nearest-neighbor interactions. By fitting the phonon dispersions derived fromfirst-principles calculations, we find these models can well describe the symmetry characters and semimetal states of MoX2’s phonons. Combining the basis of the FC model and the theory of tensor representation, we derive the origin of the irreducible representations (Irreps) at the high symmetry pointsΓ, K and M. Moreover, by using the compatibility relation between high symmetry points and high symmetry lines, wefind the semimetal states of MoX2 are protected by vertical and horizontal mirrors. Our work provides a effective tool to further study the phonons of 2D TMDs.

Topological nodal‐line semimetals have received extensive attention in recent years, and they exhibit a variety of shapes under various crystallographic symmetry protections, including nodal rings, nodal chains, and nodal knots. By using symmetry analysis and an effective force constant model, it is proposed that a class of 2D hourglass nodal nets protected by nonsymmorphic symmetry can exist in the space group , where the glide operations play a crucial role in the formation of the nets. Further effective model analysis shows that the connect points of the nets are Dirac points that are forced by the symmetries. Based on first‐principle calculations, it is predicted that this nodal net can be realized in the phonon dispersion of 3D Sn. The Berry phase and nontrivial surface states further support its nontrivial topological characteristics. Herein, the studies expand the nodal structure in the study of nodal line in phononic systems and offer a solid foundation for subsequent experimental investigation on this challenging topological state.