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

New high-temperature superconducting compounds R${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$ (R=Nd, Sm, Eu, Gd, Dy, Ho, Er, and Tm) which share the 1:2:4 structure of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$ are bulk synthesized using a new high-pressure oxygen technique. All compounds show substantial Meissner diamagnetism [(3--25)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ 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). ${R}_{2}$${\mathrm{Ba}}_{4}$${\mathrm{Cu}}_{7}$${\mathrm{O}}_{\mathrm{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.