Eight new high-temperature superconductors with the 1:2:4 structure

New high-temperature superconducting compounds RBa₂{Cu}₄O₈ (R=Nd, Sm, Eu, Gd, Dy, Ho, Er, and Tm) which share the 1:2:4 structure of YBa₂{Cu}₄O₈ are bulk synthesized using a new high-pressure oxygen technique. All compounds show substantial Meissner diamagnetism [(3--25)×{}10^-4 emu/g at 20 K]. Critical temperatures (57--81 K) vary inversely with ion size (opposite to 1:2:3), and directly with orthorhombic distortion (which is much smaller than in corresponding 1:2:3 compounds). R₂{Ba}₄Cu₇{O}ₓ$ compounds (with alternating blocks of 1:2:3: and 1:2:4) were also prepared. The variable oxygen stoichiometry of 1:2:3 is not shared by the 1:2:4 compounds.