An Investigation of the Structural Phase Transitions in a Weyl Semimetal Using Neutron Scattering

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have been widely explored due to their exotic quantum behaviors that include a nontrivial band topology, extreme magnetoresistance (XMR), pressure-induced superconductivity, charge density waves and Mott physics. In the last decade or so, TMDs MoTe2 and WTe2 have garnered attention because their non-centrosymmetric orthorhombic phase is host to Weyl fermions. Bulk MoTe2 and WTe2 consist of van der Waals bound layers stacked along the c-axis following an A/B-type of stacking sequence, where their electronic band structures can be tuned through layer stacking order. Bulk MoTe2 exhibits a structural phase transition at around 260 K from the high temperature monoclinic 1T’ phase to the orthorhombic Td phase upon cooling. On the other hand, in WTe2, the orthorhombic phase is presumed to be the stable phase at all temperatures. The transition from 1T’ to Td breaks inversion symmetry and allows the Weyl quasiparticles to emerge. The Td-1T’ transition is accompanied by substantial stacking disorder, the effects of which are not well understood. To this end, the structural properties of MoTe2 and WTe2 and their solid solutions play a critical role in determining the topological properties of this system. A study of the structural mechanism leading to the nontrivial topology will provide insights into the nature of the Weyl electronic structure.

The structural phase diagram of Mo1-xWxTe2 has been explored. The Td-1T’ structural phase transition and associated stacking disorder across the phase boundary were investigated using elastic neutron scattering and X-ray diffraction (XRD). In MoTe2, a cell doubling structure Td*, present between Td and 1T’, was observed. The Td* phase appears only on warming from Td, whereas on cooling from 1T’, broad diffuse scattering was observed across the structural phase transition instead. The Td* structure consists of four layers in the unit cell and is constructed by an ‘AABB’ layer stacking sequence rather than the ‘AA’ and ‘AB’ sequences of the Td and 1T’ phases, respectively. Compared with Td, the Td* phase has additional Bragg peaks at half integer L, which appears to be orthorhombic. However, structural refinements showed that Td* is centrosymmetric with the same space group as the monoclinic 1T’ phase.

The composition dependence of the Td-1T’ transition in Mo1-xWxTe2 was additionally investigated. The Td* phase observed in MoTe2 appears only on warming from Td in Mo1-xWxTe2 in low W-substitution (x ≤ 0.21). Increasing the W fraction to x = 0.34 and beyond leads to phase coexistence of Td and 1T’ across the transition and the vanishing of the Td* phase. With W-substitution the structural phase transition temperature increases from 260 K in x = 0, to near 500 K by x = 0.54.

The pressure dependence of the Td-1T’ transition was also investigated. In a Mo0.8W0.2Te2 single crystal, the Td* phase appears only on warming from Td at pressures lower than 0.88 GPa and disappears by 1.20 GPa. Hydrostatic pressure suppresses the Td-1T’ transition, and only the 1T’ phase remains at 1.40 GPa and beyond. The structural transition temperature range remains roughly constant with increased W-substitution but broadens with pressure.

In WTe2, the monoclinic 1T’ phase has not been observed from previous studies. It was assumed that the system enters into the Td phase on cooling from the melt. However, in this work, a sharp Td-1T’ transition at ambient pressure was observed in a WTe2 single crystal near 565 K that proceeded without hysteresis. In WTe2 powder, however, the thermal transition from the Td to the 1T’ phase is much broader, and a two-phase coexistence was observed until 700 K. No Td* phase is present in WTe2. The observation of the Td* phase in MoTe2 and the 1T’ phase in WTe2 at ambient pressure explains the inversion symmetry breaking mechanism from 1T’ to Td in Mo1-xWxTe2.

This work has been supported by the Department of Energy, Grant No. DE-FG02-01ER45927. A portion of this research used resources at the High Flux Isotope Reactor and the Spallation Neutron Source, which are DOE Office of Science User Facilities operated by Oak Ridge National Laboratory. We acknowledge the support of the National Institute of Standards and Technology, US Department of Commerce, in providing the neutron research facilities used in this work. Utilization of the FEI Quanta LV200 Environmental Scanning Electron Microscope instrument within UVa’s Nanoscale Materials Characterization Facility (NMCF) was fundamental to this work, and we acknowledge the assistance of Richard White for equipment training and analysis of the data.