  We demonstrate transition between the fine structure splitting of the ground
state of triply ionized zirconium (Zr IV) is suitable for a terahertz (THz)
atomic clock. Its transition frequency is about 37.52 THz and is mainly guided
by the magnetic dipole (M1) transition and can be accessible by a readily
available laser. We suggest to consider stable even isotopes of Zr and $M_J=
\pm 1/2$ sublevels (i.e. $|4D_{3/2},M_J=\pm 1/2\rangle \rightarrow
|4D_{5/2},M_J=\pm 1/2\rangle$ clock transition) for the experimental advantage.
By performing necessary calculations, we have estimated possible systematics
due to blackbody radiation, ac Stark, electric quadrupole and second-order
Zeeman shifts along with shifts due to the second-order Doppler effects. The
proposed THz atomic clock can be very useful in quantum thermometry and
frequency metrology.
