Synthesis, $P-T$ phase diagram, and $T_c$ of $R_2Ba_4Cu_7O_{15}$ (R= Dy,Y, DyY, GdY )*

The oxygen pressure - temperature phase diagrams of $R_2Ba_4Cu_{15}$ (R=Dy,Y, Er, DyY, GdY, and EuY) superconductors have been investigated in the temperature range between $850$ and $1025^{\circ}C$ and the pressure range between $1\,$ and $50\,atm.O_2$. The relative fraction of the phases: 123, 124, and 247, was determined by studying the intensity of the xray diffraction peaks of each phase. The condition at which the phase pure 247 exists was determined to be $P=10 \;atm$ and $T=1025^{\circ} C$. Under these conditions samples with larger size R = LaY, NdY, SmY, and Eu fail to form pure 247 phase. Annealing at $P=200 \; atm. O_2$ and $400 ^{\circ} C$ was used to increase oxygen content of the as-synthesized materials and to induce superconductivity. The highest transition temperatures of $70 K$ were observed for R=Y compositions.