Unraveling chemical bonding and Born charge in 1T-HfS₂

We combine infrared absorption and Raman scattering spectroscopies to explore the properties of 1T-HfS₂- a heavy transition metal chalcogenide with strong spin-orbit coupling due to incorporation of the 5d center. We employ the LO-TO splitting of the E_u mode along with a reevaluation of mode mass, unit cell volume, and dielectric constant to reveal the Born effective charge. We find Z_B*= 5.33e, in excellent agreement with complementary first principles calculations. In addition to resolving controversy over the nature of chemical bonding in this system, we decompose the Born charge into polarizability and local (ionic) charge. We find α= 5.07 ų and Z*= 5.19e, respectively. In order to understand how Z_B* relates to the nominal 4+ charge of the Hf center, we decompose the theoretical Born effective charge into band-by-band contributions, and find that polar displacement-induced charge transfer from sulfur p to hafnium d orbitals is responsible for the enhancement of Born charge. 1T-HfS₂ is thus an ionic crystal with strong and dynamic covalent effects. Taken together, our work places the vibrational properties of 1T-HfS₂ on a firm foundation and opens the door to intercalation and doping studies, growth of nanotubes, and sheet exfoliation.