  Optical whispering gallery mode (WGM) resonators have been very attracting
platforms for versatile Kerr frequency comb generations. We report a systematic
study on the material dispersion of various optical materials that are capable
of supporting quality factors above $10^9$. Using an analytical approximation
of WGM resonant frequencies in disk resonators, we investigate the effect of
the geometry and transverse mode order on the total group-velocity dispersion
($GVD$). We demonstrate that the major radii and the radial mode indices play
an important role in tailoring the $GVD$ of WGM resonators. In particular, our
study shows that in WGM disk-resonators, the polar families of modes have very
similar $GVD$, while the radial families of modes feature dispersion values
that can differ by up to several orders of magnitude. The effect of these giant
dispersion shifts are experimentally evidenced in Kerr comb generation with
magnesium fluoride. From a more general perspective, this critical feature
enables to push the zero-dispersion wavelength of fluorite crystals towards the
mid-infrared (mid-IR) range, thereby allowing for efficient Kerr comb
generation in that spectral range. We show that barium fluoride is the most
interesting crystal in this regard, due to its zero dispersion wavelength
($ZDW$) at $1.93 \rm{\mu m}$ and an optimal dispersion profile in the mid-IR
regime. We expect our results to facilitate the design of different platforms
for Kerr frequency comb generations in both telecommunication and mid-IR
spectral ranges.
