  We study a two-leg spin-1/2 ladder with isotropic exchanges and biquadratic
interactions in the basic plaquettes. It is shown that for the extremely
frustrated case, the system exhibits a self-organized phase separation. In some
parameter regions, the singlet rungs form a Wigner-like lattice in the
triplet-rung host. There are three types of elementary excitations in this
modulation phase, i.e., the spinons in a triplet domain, the broken singlet
rungs and the deformation of the Wigner-like lattice. The flux phase induced by
an external magnetic field in the rung-dimerized phase is also discussed.
