Novel structure in the Mo_1-xW_xTe₂ family

Mo_1-xW_xTe₂ belongs to the family of 2-dimensional transition metal dichalcogenides that are of wide interest because of their fascinating topological properties. Mo_1-xW_xTe₂ undergoes a structural phase transition from the monoclinic 1T^' phase at high temperatures, to the orthorhombic T_d phase at low temperatures through a first-order structural phase transition. Both phases consist of nearly-identical layers which differ primarily by the layers' in-plane positioning. With elastic neutron scattering, we study the transition between these two structures on single crystals of MoTe₂ with and without W-doping. Structural changes including changes in interlayer disorder were observed from the elastic scattering along (2, 0, L) on cooling and warming through the hysteresis. We observed a thus far unreported unit cell doubling phase, T_d^*, that emerges without disorder on warming from T_d, and deduced its layer stacking pattern as 'AABB' rather than the 'ABAB' and 'AAAA' orders for the 1T^' and T_d phases. We describe the transition in terms of a 1-dimensional Ising model whose interaction coefficients change with temperature. These results clarify in microscopic detail the nature of these phase transitions.