  We report the Pockels electro-optic properties of ZrO2 and HfO2 orthorhombic
Pbc21 and rhombohedral R3m ferroelectric phases, and we compare them to the
well-known LiNbO3 Pockels material from density functional theory calculations
using the CRYSTAL suite of programs. Specifically, three essential processes
are explicitly investigated: The electronic, the ionic (or vibrational), and
the piezoelectric contributions. Our calculations reveal that the ionic part
coming from the low frequency phonon modes contributes the most to the
electro-optic coefficients of rhombohedral LiNbO3 and of orthorhombic ZrO2 and
HfO2. While these low-frequency modes show zero contribution to Pockels
coefficients for the rhombohedral phase of the latter compounds.
