Stability of the orthorhombic $Fddd$ phase in diblocks using Landau theory of weak crystallization

Recent numerical SCFT caculations by Tyler and Morse [{\em Phys. Rev. Lett.}, {\bf 94}, 208302, 2005] predict a stable orthorhombic network phase with space group $Fddd$ in weakly segregatd diblocks. In this work, we examine the stability of the $Fddd$ phase using Landau theory. Our analysis and results suggest that $Fddd$ structure with a special unit cell is expected to be a stable phase not only in weakly segregated diblocks but in any other weakly ordered material.