  The electronic properties of quarter-filled organic materials showing
spin-Peierls transition are investigated theoretically. By studying the
one-dimensional extended Peierls-Hubbard model analytically as well as
numerically, we find that there is a competition between two different
spin-Peierls states due to the tetramized lattice distortion in the strongly
correlated regime. One is accompanied by lattice dimerization which can be
interpreted as a spontaneous Mott insulator, while the other shows the
existence of charge order of Wigner crystal-type. Results of numerical density
matrix renormalization group computations on sufficiently large system sizes
show that the latter is stabilized in the ground state when both the on-site
and the inter-site Coulomb interactions are large.
