Single-Crystal Growth, Alkali Metal Ordering, and Superconductivity in La2-xMxCuO4 (M = Na, K)

Abstract

Single crystals of La{sub 2-x}M{sub x}CuO{sub 4}, where M is Na or K, have been prepared by precipitation from molten NaOH at 320{degrees}C or from molten KOH at 450{degrees}C. The composition of several individual crystals as determined by wavelength-dispersive X-ray fluorescence analysis averaged to La{sub 1.88}Na{sub 0.12}CuO{sub 4} and La{sub 1.79}K{sub 0.21}CuO{sub 4}. Iodometric titrations of polycrystalline samples with identical compositions showed that the formal oxidation states for the Cu atoms are +2.19 and +2.31, respectively. On the basis of the analysis of single-crystal X-ray diffraction data, it has been determined that the alkali metal doped lanthanum copper oxides crystallize in a new structure type that this distinct from the analogous phases containing alkaline earth metal ions. The space group is Pr/nmm, and lattice parameters are a = 3.766(1) {angstrom}, c = 13.215(5) {angstrom} for the Na-doped material and a = 3.7708(7) {angstrom}, c = 13.321(2) {angstrom} for the K-doped material. In these phases, the alkali metal ions are ordered in the rock-salt blocks, and the Cu atoms are displaced out of the plane toward the alkali metal ions. Despite this distortion of the CuO{sub 2} planes, both phases exhibit superconductivity, near 26 and 6 K, respectively.