  In this letter, we report the results of detailed studies on Mn- and
Cu-substitution to Fe-site of beta-FeSe, namely MnxFe1-xSe0.85 and
CuxFe1-xSe0.85. The results show that with only 10 at% Cu-doping the compound
becomes a Mott insulator. Detailed temperature dependent structural analyses of
these Mn- and Cu-substituted compounds show that the structural transition,
which is associated with the changes in the building block FeSe4 tetrahedron,
is essential to the occurrence of superconductivity in beta-FeSe.
