Interplay of superconductivity with structural phases in a generalized t-Jmodel

The phase diagram of the t - J - V model is discussed using a 1/N expansion in terms of X operators. It is shown that a flux phase of d-wave symmetry is stabilized by the Coulomb interaction at intermediate dopings and compet es with d-wave superconductivity. Since the flux wave instability is strong er than the superconducting one optimal doping is essentially determined by the onset of the flux phase at zero temperature at the doping delta (c) si milar to 0.14 for J/t = 0.3. For delta < <delta>(c) the flux phase coexists with superconductivity at low and exists as a single phase at higher tempe ratures. Due to the different origins of the two instabilities the flux pha se boundary and especially delta (c) is much less sensitive to impuritiy sc attering than the boundary for superconductivity in agreement with experime nts in Zn doped La - 214 and (Y, Ca)- 123.