  Electronic structure calculations indicate that the Sr2FeSbO6 double
perovskite has a flat-band set just above the Fermi level that includes
contributions from ordinary sub-bands with weak kinetic electron hopping plus a
flat sub-band that can be attributed to the lattice geometry and orbital
interference. To place the Fermi energy in that flat band, electron doped
samples with formulas Sr2-xLaxFeSbO6 (0 < x < 0.3) were synthesized and their
magnetism and ambient temperature crystal structures determined by
high-resolution synchrotron X-ray powder diffraction. All materials appear to
display an antiferromagnetic-like maximum in the magnetic susceptibility, but
the dominant spin coupling evolves from antiferromagnetic to ferromagnetic on
electron doping. Which of the three sub-bands or combinations is responsible
for the behavior has not been determined.
