Stabilization of 1T′ phase WTe2 by scalar relativistic effect

Among semiconducting hexagonal (1H), metallic octahedral (1T), and semimetallic distorted octahedral (1T') phases that group-VIB transition metal dichalcogenides (TMDCs) may have, the thermodynamically stable structure is known to be the 1H phase. The exception is WTe2, for which it is 1T' phase. However, the origin of the stable 1T' phase WTe2 is unknown. Here, we study the electronic property of group-VIB TMDCs using first-principles calculations to find the origin of the 1T' phase of WTe2. We find that the scalar relativistic effect in W stabilizes the 1T' phase WTe2 against 1H-WTe2. The Fermi surface of the electron pocket of WTe2 is sensitively changed as the relativistic effect is turned on and off in W pseudopotentials. Calculated phonon dispersion exhibits drastic softening in the phonon mode at the M point, which corresponds to the nesting vector q(M) on the Fermi surface. We show that difference in occupation of s-and d-orbitals is a key parameter that determines the shape and size of the electron pocket, and thus the stable phase of group-VIB TMDCs between 1H or 1T' phases. Published by AIP Publishing.