A new series of mixed oxide superconductors with the stoichiometric composition La_2−xDy_xCa_yBa₂Cu_4+yO_z(x=0.0−0.5,y=2x) has been studied for structural and supercon-ductiong properties. Our earlier studies on La_2−x(Y/Er)_xCa_yBa₂Cu_4+yO_z series, show a strong de-pendence of Tc on hole concentration (psh ). In the present work, the results of the analysis of the neutron diffraction measurements at room temprerature on x=0.3 and 0.5 samples are reported. It is interesting to know that Ca substitutes for both La and Ba site with concomitant displacement of La onto Ba site. Superconductivity studies show that maximum T_c is obtained for x=0.5,y=1.0 sample (Tc∼75 K), for La_1.5D_^y0.5Ca₁Ba₂Cu₅O_z(La-2125).