  Due of its polymorphism, Mn$_2$FeSbO$_6$ can be synthesized at high pressures
and temperatures as a ferrimagnetic ilmenite or an antiferromagnetic
perovskite. The structural phase transformation is discussed in detail, and
magnetic structures are proposed for both phases. The high-pressure
Mn$_2$FeSbO$_6$ polymorph is a rare example of $A_2 B'B"$O$_6$ perovskite with
solely Mn cations on the $A$-site. Fe and Sb cations are ordered on the
$B$-sites. Theoretical calculations for the perovskite phase suggest a complex
magnetic structure, holding an electronic polarization.
