Local structure memory effects in the polar and nonpolar phases of MoTe₂

We use total scattering to study the reversible transition between the polar 1T′ and nonpolar T_d phases of layered MoTe₂ taking place at 240 K. Whereas, macroscopically, the transition appears to be first order, locally, it is not. In particular, a great deal of the stacking sequence of Te-Mo-Te layers characteristic of the polar 1T′ phase persists locally in the nonpolar Td phase, and vice versa, over a broad temperature range extending about 100 K both below and above the transition. The intermixing ratio for the two sequences evolves gradually across the transition temperature, consistent with a second-order transition behavior. The presence of coexisting local polar and nonpolar regions and the resulting variety of internal interfaces where the spatial inversion symmetry is broken may be behind some of the unusual electronic properties of Td-MoTe₂, including its putative type-II Weyl semimetal state.