[en] Samples with a nominal composition of Bi_2-xSb_xSr₂CaCu₂O_y were synthesized by the standard solid state reaction. In this paper, a new phase in these samples is identified by selected-area electron diffraction and the X-ray energy dispersive spectrum. The new phase, with composition Bi₆Sr₅(Ca,Sb)₅O_z, belongs to the monoclinic system, A-base centred lattice, with cell dimensions a=21.73 AA, b=5.76 AA, c=19.32 AA, β=95.0 degrees . The influence of the monoclinic phase on the improvement of superconductivity is also discussed. (author)