Z.X. Zhao

Raman scattering on superconducting crystals of Bi2(Sr1−xCax)n+2Cun+10(6+2n)+δ (n = 0, 1)

Abstract We report Raman spectra of highly oriented crystals of Bi2Sr2CuO6+δ and Bi2Sr2CaCu2O8+δ. A factor group analysis of the k = 0 phonons is also given. Most phonons observed have Ag symmetry. A mode of B1g symmetry is observed at 282 cm−1 which is equivalent to the 337 cm−1 mode of YBa2Cu3O7.

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 ].

The crystal structure and superconductivity of new superconducting phase TlSr2Ca0.5Sm0.5Cu2O6.75

Abstract The crystal structure of superconducting phase TlSr2Ca0.5Sm0.5Cu2O6.75 with Tc(0) = 37.6K in Tl-Sr-Ca-Sm-Cu-O system has been determined by means of X-ray powder diffraction method. It belongs to primary tetragonal cell with a = 3.817, c = 12.054 A . Its space group is D14h-P4/mmm. Each unit cell contains one formula unit. The positions of ions in unit cell are as following: 0.5 Ca and 0.5 Sm cations disorderly occupy 1(d) equivalent point position. The occupation factor for each of them is 0.5. Tl cation occupies equivalent point position 1(a). 2 Cu cations occupy 2(g) with z = 0.365. 2 Sr cations occupy 2(h) with z = 0.210. 7 O anions respectively occupy 1(c), 2(g) (z = 0.165) and 4(i) (z = 0.365) equivalent point positions. The crystal structure of TlSr2Ca0.5Sm0.5Cu2O6.75 is very similar to that of TlBa2CaCu2O6.5 in Tl-Ba-Ca-Cu-O system or RBa2Cu3O7 in R-Ba-Ca-O system. In the superconducting phase with oxygen-deficient pseudo-perovskite structure, the similarity of cation-oxygen coordination polyhedron is important for the substitution among the cations.

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.

Structural phase transitions of superconducting materials Bi1−xPbxSrCaCu2Oy (x=0, 0.2)

Abstract The structural phase transition process of superconducting phase in Bi 1−xPb x SrCaCu 2 Oy ( x =0, 0.2) samples has been investigated from - 190 °C to 800 °C by electron diffraction. Similar incommensurate modulated structure has been also found in Bi 0.8 Pb 0.2 SrCaCu 2 Oy sample, but the action of doping Pb reduced the intensity of the incommensurate modulation. Two structural phase transitions have been found at the temperature about 296 °C and 490 °C for BiSrCaCu 2 Oy sample and at about 310 °C and 470 °C for Bi 0.8 Pb 0.2 SrCaCu 2 Oy sample respectively. After second transition companied by disappearance of modulation and appearance of crystallite a new structural phase has been formed.

Superconducting and microstructural properties of Fe-doped Tl-1223 cuprates

Abstract The Fe-doping effects on superconductivity, transport and microstructure are investigated by means of Hall coefficient measurements, thermogravimetric analysis, Mossbauer spectroscopy and the calculation of the point-charge-effective-valence model (PCEV model) in Fe-doped Tl 0.5 Pb 0.5 (Sr 0.8 Ba 0.2 ) 2 Ca 2 (Cu 1− x Fe x ) 3 O y (Tl-1223) superconductors. In the doping level of Fe ( x =0–0.05), both zero-resistance temperature T co and carrier concentration n H decrease linearly with Fe dopants increasing. The striking linear behavior of T co and n H suggests the potential effects of charge localization on the transport properties and also shows that Fe dopants occupy different Cu sites in a stable manner with Fe content increasing. The thermogravimetric analysis shows the Fe 3+ ions substituting for Cu 2+ ions can bring excess oxygen atoms into lattice and form some Fe–O defect clusters. Mossbauer measurements are performed to investigate the microstructure of Fe–O defects. In addition, the atomic position of Fe dopant and the electric field gradient (EFG) on Fe sites are investigated under the PCEV model. We found that the signs of the EFG on Fe sites are positive. According to quadrupole splittings deduced from Mossbauer spectroscopies and results of model calculation, it is further confirmed that there is a small shift of Fe ion relative to the original center of the CuO 2 network, and in some different kinds of Fe–O defect clusters, Fe dopants can lead to excess oxygen atoms entering lattice. These results provide a further support for the interpretation about the displacement of Fe dopant and the assignment of Fe site. Our discussion suggests that the microstructural distortion and the excess oxygen defect induced by Fe doping encourage in the localization of cruising carrier on CuO 2 planes, which is one of the reasons to explain the decrease of T c and carrier concentration.

Mössbauer studies on novel high-Tc (Sn,Sr)-doped La2−2xSr2xCu1−xSnxO4 superconductors

Abstract For La 2−2 x Sr 2 x Cu 1− x Sn x O 4 ( x =0.075, 0.090 and 0.110) superconductors, the effects of Sn-doping on local microstructure is studied by means of Mossbauer spectroscopy. The results show that Sn ions in the 4+ valence state dominantly occupy Cu sites rather than La(Sr) site. In the view of the effective oxygen content (or excess oxygen), the dual role of Sn dopant on the carrier concentration is discussed. We attempt to explain the high transition temperature T c in (Sr,Sn)-doped La 2−2 x Sr 2 x Cu 1− x Sn x O 4 samples under the model of amphoteric effect of Sn-doping on the concentration.