  We show using a combination of powder X-ray and neutron diffraction, first
principles calculations, temperature- and field-dependent magnetization, heat
capacity and resistivity data that the superconducting behavior of
`Sr$_4$V$_2$O$_6$Fe$_2$As$_2$' is dependent on synthesis conditions,
particularly, heating profiles result in unintentional chemical doping. This
compound can be tuned from a state in which the vanadium electrons are
itinerant with a high electronic density of states, to a state where the
vanadium-oxide layers are insulating and presumably antiferromagnetic.
