The structural phases and vibrational properties of Mo_1-xW_xTe₂ alloys

The structural polymorphism in transition metal dichalcogenides (TMDs) provides exciting opportunities for developing advanced electronics. For example, MoTe₂ crystallizes in the 2H semiconducting phase at ambient temperature and pressure, but transitions into the 1T′ semimetallic phase at high temperatures. Alloying MoTe₂ with WTe₂ reduces the energy barrier between these two phases, while also allowing access to the T_d Weyl semimetal phase. The Mo_1-x W_xTe₂ alloy system is therefore promising for developing phase change memory technology. However, achieving this goal necessitates a detailed understanding of the phase composition in the MoTe₂-WTe₂ system. We combine polarization-resolved Raman spectroscopy with x-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) to study bulk Mo_1-xW_xTe₂ alloys over the full compositional range x from 0 to 1. We identify Raman and XRD signatures characteristic of the 2H, 1T′, and T_d structural phases that agree with density-functional theory (DFT) calculations, and use them to identify phase fields in the MoTe₂-WTe₂ system, including single-phase 2H, 1T′, and T_d regions, as well as a two-phase 1T′ + T_d region. Disorder arising from compositional fluctuations in Mo_1-xW_xTe₂ alloys breaks inversion and translational symmetry, leading to the activation of an infrared 1T′-MoTe₂ mode and the enhancement of a double-resonance Raman process in 2H-Mo_1-x W_xTe₂ alloys. Compositional fluctuations limit the phonon correlation length, which we estimate by fitting the observed asymmetric Raman lineshapes with a phonon confinement model. These observations reveal the important role of disorder in Mo_1-xW_xTe₂ alloys, clarify the structural phase boundaries, and provide a foundation for future explorations of phase transitions and electronic phenomena in this system.