Observation of a T_d-1T’ structural phase transition at ambient pressure in Weyl semimetal WTe₂

Layered transition metal dichalcogenides MoTe₂ and WTe₂ are suggested to be Weyl semimetals in their orthorhombic T_d phase. These materials exist in bulk forms as stacks of strongly in-plane bonded layers with weak van der Waals interlayer interaction and their properties often vary with stacking changes. For example, MoTe₂ exhibits a first order structural phase transition at ∼260K from a low-temperature orthorhombic T_d phase to a high-temperature monoclinic 1T' phase, with an intermediate pseudo-orthorhombic T_d* phase seen only on warming. In contrast, it is long believed that WTe₂ has only the T_d phase at ambient pressure since the material was first structurally characterized decades ago. We performed elastic neutron scattering measurements at the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory (ORNL) on a WTe₂ single crystal and observed a T_d-1T’ structural phase transition at ~565K. The transition proceeds without any hysteresis or intermediate phase. The observation of the 1T' phase in WTe₂ at ambient pressure adds details to the structural behavior of the Mo_1-xW_xTe₂ family, and gives new insights into re-examination of theories that lack a transition in WTe₂.