  We combine infrared absorption and Raman scattering spectroscopies to explore
the properties of the heavy transition metal dichalcogenide 1T-HfS$_2$. We
employ the LO-TO splitting of the $E_u$ vibrational mode along with a
reevaluation of mode mass, unit cell volume, and dielectric constant to reveal
the Born effective charge. We find $Z^*_{\rm{B}}$ = 5.3$e$, in excellent
agreement with complementary first principles calculations. In addition to
resolving controversy over the nature of chemical bonding in this system, we
decompose Born charge into polarizability and local charge. We find $\alpha$ =
5.07 \AA$^3$ and $Z^{*}$ = 5.2$e$, respectively. Polar displacement-induced
charge transfer from sulfur $p$ to hafnium $d$ is responsible for the enhanced
Born charge compared to the nominal 4+ in hafnium. 1T-HfS$_2$ is thus an ionic
crystal with strong and dynamic covalent effects. Taken together, our work
places the vibrational properties of 1T-HfS$_2$ on a firm foundation and opens
the door to understanding the properties of tubes and sheets.
