  Widespread adoption of superconducting technologies requires the discovery of
new materials with enhanced properties, especially higher superconducting
transition temperatures T$_{c}$. The unexpected discovery of high T$_{c}$
superconductivity in cuprates and in materials as diverse as heavy fermions,
organic conductors, and endohedrally-doped fullerenes suggests that the highest
T$_{c}$s occur when pressure or doping transform the localized and
moment-bearing electrons in antiferromagnetic insulators into itinerant and
weakly magnetic metals. The absence of this delocalization transition in
Fe-based superconductors may limit their T$_{c}$s, but even larger T$_{c}$s may
be possible in their isostructural Mn analogs, which are antiferromagnetic
insulators like the cuprates. It is generally believed that prohibitively large
pressures would be required to suppress the strong Hund's rule coupling in
these Mn-based compounds, collapsing the insulating gap and enabling
superconductivity. Indeed, no Mn-based compounds are known to be
superconductors. The electronic structure calculations and x-ray diffraction
measurements presented here challenge these long held beliefs, finding that
only modest pressures are required to transform LaMnPO, isostructural to
superconducting host LaFeAsO, from an insulating tetragonal structure with a
large Mn moment to a gapless orthorhombic structure with a small Mn moment.
Proximity to this electronic delocalization transition in LaMnPO results in a
highly interacting metallic state, the familiar breeding ground of
superconductivity.
