Abstract
Coexistence of two phases, having different in-plane dimensions, is found in the planar Sr
2−
x
La
x
MnO
4 for 0 <
x ≤ 0.38 at room temperature. Furthermore, charge ordering with modulation wave vector (
π
2
,
π
2
) is discovered for
x=0.5 below ∼ 250 K. Together with transport and magnetic measurements, these results indicate that the phase separation is due to the competition between the single and the double occupancies of the Mn 3
d orbitals by the doped electrons. The doubly-occupying electrons form on-site triplets which are mobile at high temperatures and form a bipolaron lattice below ∼ 250 K.