Effect of thermally excited lattice vibrations on the thermodynamic stability of tungsten ditellurides WTe2 under high pressure: A first-principles investigation

The thermodynamic stability as a function of pressure and temperature of three WTe2 polytypes, i. e., Td, 1T , and 2H phases, is examined using first-principles calculations, where the temperature-dependent contributions, arising particularly from the lattice vibrations, are derived from the quasiharmonic approximation. We find that the critical transition pressure to the 2H phase increases almost linearly with increasing temperature. Through the inspection of the phonon density of states, 2H-WTe2 is dynamically stable not only at high pressure, but also at zero pressure, indicating possible existence of 2H-WTe2 also at ambient conditions. On the other side, our results demonstrate that, at relevant temperature and pressure, the thermodynamic stability of 1T-WTe2 is comparable to that of Td-WTe2, and further analysis reveals substantial similarities in terms of structural behavior between Td-WTe2 and 1T-WTe2. These findings suggest not only that the two polytypes are likely to coexist in practical samples of WTe2 due probably to grains/regions either with tiny difference in stress, for example, or with different growth history, but also that the designation of 1T , having been regularly used to described the compound under pressure in the literature, might actually be a mixture of Td and 1T polytypes, whose WTe2 layers laterally slide via low transition barrier induced probably by the applied pressure, resulting in the splitting of (101) and (113) peaks as observed in the diffraction experiments.