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

New high-temperature superconducting compounds RBa2Cu4O8 (R=Nd, Sm, Eu, Gd, Dy, Ho, Er, and Tm) which share the 1:2:4 structure of YBa2Cu4O8 are bulk synthesized using a new high-pressure oxygen technique. All compounds show substantial Meissner diamagnetism [(325)×10-4 emu/g at 20 K]. Critical temperatures (5781 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). R2Ba4Cu7Ox 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.