Evolution of Raman spectra in Mo$_{1-x}$W$_x$Te$_2$ alloys

The structural polymorphism in transition metal dichalcogenides (TMDs) provides exciting opportunities for developing advanced electronics. For example, MoTe $_{2}$ crystallizes in the 2H semiconducting phase at ambient temperature and pressure, but transitions into the 1T $^{'}$ semimetallic phase at high temperatures. Alloying MoTe $_{2}$ with WTe $_{2}$ reduces the energy barrier between these two phases, while also allowing access to the T $_{d}$ Weyl semimetal phase. The MoWTe $_{2}$ 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 MoTe $_{2}$ -WTe $_{2}$ system. We combine polarization-resolved Raman spectroscopy with X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) to study MoWTe $_{2}$ alloys over the full compositional range x from 0 to

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 MoTe $_{2}$ -WTe $_{2}$ 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 MoWTe $_{2}$ alloys breaks inversion and translational symmetry, leading to the activation of an infrared 1T $^{'}$ -MoTe $_{2}$ mode and the enhancement of a double-resonance Raman process in 2H-MoWTe $_{2}$ 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 MoWTe $_{2}$ alloys, clarify the structural phase boundaries, and provide a foundation for future explorations of phase transitions and electronic phenomena in this system.