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Abstract 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 $${Z}_{{\rm{B}}}^{* }$$ Z B *  = 5.3 e , in excellent agreement with complementary first-principles calculations. In addition to resolving the controversy over the nature of chemical bonding in this system, we decompose Born charge into polarizability and local charge. We find α  = 5.07 Å 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.