W.J. Zhu

Synthesis and crystal structure of new rare-earth copper oxyselenides: RCuSeO (R=La, Sm, Gd and Y)

A new series of copper oxyselenides RCuSeO (R=La, Sm, Gd and Y) have been prepared and studied by X-ray powder diffraction. The Sm-containing phase was determined to have a tetragonal crystal structure with the lattice parameters a=3.9512(1)A and c=8.7157(2)A, the space group of which is P4/nmm. Its structure is built of a fluorite type layer [Sm2O2] and an anti-fluorite type layer [Cu2Se2] stacking alternatively along c direction. The similarity of the diffraction patterns for the samples with R=La, Gd and Y to that of SmCuSeO suggests their assuming the same crystal structure.

(B,Cu)Sr2YCu2O7, a new layered copper-oxide based on the boron-oxygen group

Abstract New layered copper-oxides (B,Cu)Sr 2 YCu 2 O 7 with various buron contents have been synthesized. Their crystal structures resemble that of YBa 2 Cu 3 O 7−δ , but contain boron-oxygen groups connecting (CuO 2 ) sheets beside Y layers. (B x Cu 1− x ) forms a solid solution with an approximate range of x =0.1-0.9. The compound of idealized composition (B 0.5 Cu 0.5 )Sr 2 YCu 2 O 7 crystallizes in the tetragonal space group P4/mmm ( a=3.818, c=10.96 A ). Electrical resistivity measurements indicate metallic or semiconducting characteristics for samples with different boron contents. No superconducting transition was observed for these samples. In the (B 0.5 Cu 0.5 )Sr 2− x Ba x YCu 2 O 7 series, Ba doping at the Sr site induces superconductivity, with the optimum T c of 51 K for (B 0.5 Cu 0.5 )Sr 0.8 Ba 1.2 YCu 2 O 7 . These samples remain one single phase up to the Ba context fx=16 .

Synthesis and crystal structure of barium copper fluochalcogenides:[BaCuFQ (Q=S,Se)]

Abstract Two new copper fluochalcogenides BaCuFQ (Q=S,Se) have been synthesized. They crystallize in a tetragonal crystal structure with space group P4/nmm and lattice parameters a =4.1230(1) A , c =9.0327(2) A for BaCuFS and a =4.2391(1) A , c =9.1217(2) A for BaCuFSe. The structure can be viewed as the intergrowth of a fluorite type layer [Ba 2 F 2 ] and an anti-fluorite type layer [Cu 2 Q 2 ].

Synthesis and crystal structure of the new layered cuprate CaBa2Sm2Cu2Ti3O14

Abstract A new layered cuprate CaBa 2 Sm 2 Cu 2 Ti 3 O 14 has been prepared and studied by X-ray powder diffraction. The structure can be viewed as the intergrowth of copper-oxygen square pyramids and titanium-based perovskite layers with space group P4/mmm and lattice parameters a=3.8885(2)A˚, c=19.577(1)A˚. The atom stacking sequence along c axis is Ca(CuO 2 )(BaO)(TiO 2 )(SmO)(TiO 2 )(SmO)(TiO 2 )(BaO)(CuO 2 )Ca.

Rare-earth ionic size effect on the Pr-induced TC depression in the series R1-xPrxBa2Cu3O7-δ (R=Sm, Gd, Y, Er, Tm)

Abstract The rare-earth ionic size effect on the depression of superconducting transition temperature T c by Pr substitution has been studied in the R 1- x Pr x Ba 2 Cu 3 O 7- δ (R=Sm, Gd, Y, Er, Tm) series. For each system, T c decreases with increasing Pr content. The larger the ionic size difference of R and Pr ions, the less T c decreases and hence the smaller the critical Pr concentration for destruction of superconductivity. In contrast to the hydrostatic pressure effect on T c , this anomalous behaviour can be explained by considering that the large size difference of R and Pr ions will lead to the high concentration of the neighbouring relation R-R, and thus the relatively high T c for the sample.

Synthesis of the superconductors HgBa2CaCu2O6+δ and HgBa2Ca2Cu3O8+δ

Abstract The superconductors HgBa 2 CaCu 2 O 6+δ and HgBa 2 Ca 2 Cu 3 O 8+δ (denoted 1212 and 1223) were prepared at temperatures below 750°C from simple oxides without using precursors. The latter one forms at a higher temperature and needs a longer calcinating time. After oxygen annealing the T c for the 1223 phase increases with about 15 K from 118 to 133 K, while that of the 1212 phase changes only little.

Synthesis and crystal structure of the new layered cuprate (Sm1.1Ce0.9)Ba2Sm2Cu2Ti2GaO16

A new layered cuprate (Sm{sub 1.1}Ce{sub 0.9})Ba{sub 2}Sm{sub 2}Cu{sub 2}Ti{sub 2}GaO{sub 16} has been prepared and studied by X-ray powder diffraction. It assumes a body-centered crystal structure with space group 14/mmm and lattice parameters a = 3.8864(2){angstrom}, c = 44.761(2){angstrom}. Atom stacking sequence along c-axis is [(Sm,Ce){sub 2}O{sub 2}](CuO{sub 2})(BaO)(TiO{sub 2})(SmO)(GaO{sub 2})(SmO)(TiO{sub 2})(BaO)(CuO{sub 2})[(Sm,Ce){sub 2}O{sub 2}], where[(Sm,Ce){sub 2}O{sub 2}] is the fluorite-type layer.

Superconductivity at 30 K in the Nd2CuO4-type cuprate Tm1.83Ca0.17CuO4

Abstract The Nd 2 CuO 4 -type Ca-doped rare-earth cuprates R 1.83 Ca 0.17 CuO 4 (R=Gd, Dy or Tm) have been synthesized under high pressure. Only that with the small ion Tm 3+ and thus the short Cuue5f8O bonds (1.916 A) was superconducting (at about 30 K). This was considered to be the first example of p-type superconductivity in the purely square-planar CuO 2 layers without apical oxygens.

Synthesis and crystal structure of copper oxybromides [M2Cu3O4Br2 (M=Sr,Ba)]

Abstract Two new members of copper oxybromides M 2 Cu 3 O 4 Br 2 (M=Sr,Ba) have been prepared and studied by X-ray powder diffraction. They were determined to assume a tetragonal crystal structure with space group 14/mmm and lattice parameters a =5.4840(2) A , c =13.297(1) A for Sr 2 Cu 3 O 4 Br 2 and a =5.5386(2) A , c =14.624(1) A for Ba 2 Cu 3 O 4 Br 2 . The structure is built of the sheets [Cu 3 O 4 ] and the fluorite-type layers [M 2 Br 2 ] stacking alternatively along [001]. The [Cu 3 O 4 ] layer can be described as the well-known sheets [CuO 2 ] in cuprate superconductors with the addition of Cu to the centers of the square units [CuO 2 ] at a period of √2a p ×√2 a p . In the alkaline-earth copper oxyhalide systems, M 2 CuO 2 X 2 can be obtained only with the low ionic radius sum r M + X of M 2+ and X − , while M 2 Cu 3 O 4 X 2 was got only with a high r M + X where M=Ca,Sr,Ba and X=Cl,Br.

Possible origin of the sharp drops of resistivity around 250 K in superconducting Hg-compounds

Abstract The resistivity transitions and sometimes the loss of resistance at about 235 K of the superconducting Hg-based cuprates synthesized under high pressure reported by Tholence et al. [Phys. Lett. A 184 (1994) 215] can be considered to come from the freezing of traces of mercury in the samples when the solid mercury forms a current percolation net which gives a very low measured resistivity that lies within the conventional experimental errors.