Large change of interlayer vibrational coupling with stacking in Mo_1-xW_xTe₂

Stacking variations in quasi-two-dimensional materials can have an important influence on material properties, such as changing the topology of the band structure. Unfortunately, the weakness of van der Waals (vdW) interactions makes it difficult to compute the stacking dependence of properties, and even in a material as simple as graphite the stacking energetics remain unclear. Mo1-xWxTe2 is a material in which three differently stacked phases are conveniently accessible by temperature changes: 1T', T-d*, and the reported Weyl semimetal phase T d . The transitions proceed via layer sliding, and the corresponding interlayer shear mode (ISM) is relevant not just for the stacking energetics but also for understanding the relationship between Weyl physics and structural changes. However, the interlayer interactions of Mo1-xWxTe2 are not well understood, with wide variation in computed properties. We report inelastic neutron scattering of the ISM in a Mo0.91W0.09Te2 crystal. The ISM energies are generally consistent with the linear chain model, as expected given the weak interlayer interaction, though there are some discrepancies from predicted intensities. However, the interlayer force constants K-x in the T-d* and 1T' phases are substantially weaker than that of T-d at 75(3) and 83(3)%, respectively. Considering that the relative positioning of atoms in neighboring layers is approximately the same regardless of overall stacking, our results suggest that longer-range influences, such as stacking-induced electronic band-structure changes, may be responsible for the substantial change in the interlayer vibrational coupling and thus the C-55 elastic constant. These findings should elucidate the stacking energetics of Mo1-xWxTe2 and other vdW layered materials.