Doping dependence of the sliding-layer phase transitions in (Mo,W)Te₂

The electronic properties of materials composed of weakly-bound layers can change substantially depending on how the layers are stacked. For example, the monoclinic 1T’ phase of MoTe₂ is reported to become a Weyl semimetal when cooled below 250 K into its orthorhombic T_d phase, which differs from 1T’ mainly by in-plane displacements of the layers. Substituting Mo for W raises the transition temperature. In this talk, we will focus on the effect of W-doping on Mo_1-xW_xTe₂ crystals up to x=0.5, using neutron scattering to investigate structural changes. The transition temperature continues to increase to at least ~460 K, approximately linearly, as a function of W-doping (as estimated from the c-axis lattice constant). The monoclinic beta angle steadily decreases with doping, from 93.9 to 93.2 degrees. The T_d* phase (a thus far unreported phase which will be discussed in Yu Tao’s talk) was present on warming up to x~0.3, but was not seen above this point. Some MoTe₂ crystals had broad transitions, likely due to Te vacancies; the existence of a T_d* phase in these crystals could not be determined. We discuss these tendencies and their implications for finding new phases in materials with weakly-bound layers.