Z.Q. Mao

Electron crystallographic study of Bi2(Sr0.9La0.1)2CoOy

The superstructure in Bi-2(Sr0.9La0.1)(2)CoOy has been studied by the two-stage image-processing technique consisting of image deconvolution and phase extension; and based on the combination of high resolution electron microscopy and electron diffraction. All unoverlapped atoms including oxygen are resolved in the final projected potential map of resolution of about 1 Angstrom. The structure modulation is seen clearly. It is caused by the fluctuation of both atomic positions and occupancy

Transport properties of the Mn-doped La214 system

The normal-state transport properties of the La1.85-xSr0.15+xCu1-xMnxO4 system have been investigated by means of resistivity and thermoelectric power. The experimental results indicate that Mn doping induces a strong lattice deformation and leads to a gradual localization of the holes. The temperature-dependent resistivity and the broad peak in the S-T curve as well as the shift of the peak are discussed in terms of the change of crystal structure and d-p hybridization state.

Microstrains in La1.85Sr0.15Cu1−yMyO4 (M=Zn, Ni, Mg) cuprates with y≤0.30

Abstract La 1.85 Sr 0.15 Cu 1− y M y O 4 ceramic cuprates with M=Zn, Ni, Mg, and y ≤0.30 have been synthesized. The superconducting critical temperatures and the room temperature resistivities for M doping have been determined by a standard four-probe method. The width of each reflection of X-ray diffraction for each sample is analyzed by Rietveld refinements which is related to the average grain size and the average lattice microstrain. In this way we attempt to explain the stronger suppression for non-magnetic doping such as Zn or Mg than that for magnetic doping such as Ni.

Structure studies of La1.85Sr0.15CuO4 doped with mg at high doping level

Ceramic cuprates of La1.85Sr0.15Cu1-yMgyO4 with 0.00≤y≤0.30 are synthesized. The resistivity dependence of temperature is obtained up to room temperature. The metal-insulator transition at the range of higher doping level (y> 0.10) is observed. There is no impurity phase detected in the entire Mg-doped region. The structure of these Mg-doped samples is characterized by X-ray diffraction studies. The atomic structural parameters are obtained by Rietveld refinements for the Mg-doped samples with y≤0.30. No phase transition is detected within the accuracy of our experiments. The Mg doping reduce the local Jahn-Teller distortion of the CuO6 octahedra. Some meaningful bond distances are determined according to the refined results. According to the variations of the lattice parameters with y, the whole doping range could be divided into two regions: low doping level (LDL) and high doping level (HDL). The bond length between the two apical oxygen atoms in the CuO6 octahedra for the Mg-doped samples increases with increasing content of Mg in the LDL, and high doping level (HDL). The bond length between the two apical oxygen atoms in the CuO6 octahedra for the Mg-doped samples increases with increasing content of Mg in the LDL, and decreases in the HDL.

Electronic structure of the Ba-doped (Pb,Cd)-1212 cuprates

Abstract Changes in the electronic structure for the (Pb 0.5 Cd 0.5 )(Sr 2− x Ba x )(Ca 0.5 Y 0.5 )Cu 2 O 7 (0 ≤ x ≤ 0.5) system associated with an semiconductor-superconductor transition have been investigated by means of X-ray photoelectron spectroscopy. The main line of Cu 2 p 3 2 core level and the valence-band peak in the range of 0∼8 eV for the samples are broadened with Ba doping, which suggest that there are two charge-transfer mechanisms; in-plane O 2p x , y → Cu 3d x 2 − y 2 and apical O 2p z → Cu 3d 3 z 2 − r 2 , in the system. For the higher doped ( x =0.4 and 0.5) superconductive samples, the latter charge transfer seems to be more pronounced than that for the semiconductor samples ( x ≤ 0.3). Pb4f and O ls core-level spectra show that as the Ba doping content increases the oxidation state of Pb increases. The state of density in the Fermi level increases slightly, and a new state appears more clearly just below the Fermi level with the partial substitution of Ba for Sr, which can be attributed to the presence of apical O 2p z derived holes in the (Sr,Ba)O layers. The evidence suggests that the appearance of superconductivity in the (Pb,Cd)-1212 system has a relation with the changes in electronic structure, especially the apical O 2p z → Cu 3d 3 z 2 − r 2 hybridization mechanism.

Effects of Ba doping on Tc and vibration features of (Pb, Cd)-1212 cuprates

Abstract Samples of nominal composition (Pb 0.5 Cd 0.5 ) (Sr 2− x Ba x ) (Ca 0.5 Y 0.5 )Cu 2 O y (0⩽ x ⩽0.5) synthesised at 920°C in air are investigated. XRD shows the samples to exhibit the tetragonal 1212-phase. The lattice parameters, the width of the Raman peak for the apical-0(2) vibration mode and the infrared absorption peak for the 0(3) vibration mode, all increase with increasing Ba doping level. In the doping range (0.0⩽ x ⩽0.5), there is a transition from insulator to superconductor. The Raman shift and the wavenumber of the infrared absorption for the phonon peaks appear to change regularly.

Structural modulation in Bi2(Sr0.9La0.1)2Cu1−xCoxOy (x=0.2, 0.6, 1.0)

Abstract Three specimens of Bi 2 (Sr 0.9 La 0.1 ) 2 Cu 1− x Co x O y with x =0.2, 0.6, 1.0, respectively, were investigated by transmission electron microscopy to explore the influences of Co substitution for Cu on the structural modulation. They have in common orthorhombic modulated structures. But the modulation periodicity decreases gradually with the increase of Co content. Its ratio to the corresponding lattice parameter of basic structure equals 4.28 for x =0.2, 4.18 for x =0.6 and 4.00 for x =1.0. Consequently, the modulated structures of the former two phases are incommensurate, while it is commensurate for the last one. The stacking disorder due to the weak bonding connection between two adjacent Bi–O layers has been observed in the two incommensurate modulated phases.

X-ray diffraction studies on La1.85Sr0.15CuO4 doped with Mg in high doping level

Abstract The nominal composition of La 1.85 Sr 0.15 Cu 1−y Mg y O 4 superconductors with 0.00≤y0.30 are synthesized by the solid state reaction techniques. The structure of these Mg doped samples is characterized by X-ray diffraction studies. The atomic structure parameters are obtained by Rietveld refinements. Some meaningful bond distances are determined according to the refined results. The distance between the two apical oxygen atoms in the CuO 6 octahedra changed with the Mg content. The room temperature resistivity for each sample is measured.

Characterization of the (Bi2−xSbx)Sr2CaCu2Oy system

Abstract Samples of nominal composition (Bi2−xSbx)Sr2CaCu2Oy (0.1≤x≤0.3) are investigated. X-ray diffraction and electron diffraction show that the (Bi2−xSbx)Sr2CaCu2Oy system has a monoclinic phase with the lattice parameters a=21.73A, b=5.76A, c=19.32Aandβ=95.0°, except for the tetragonal Bi-2212 phase. The results of energy dispersive X-ray spectroscopy reveal that Sb does not substitute for Bi in the Bi-2212 main structure, but forms the monoclinic phase of Bi4Sr4Ca2SbOy. The superconducting transition temperature, which is examined by electric resistance and AC magnet susceptibility, is constant at about 87 K. The reason why Sb cannot substitute for Bi in Bi2O2 layers has been discussed from the point of view of coordination.

The relations of specific heat anomaly with variation of oxygen content in BiSrCaCuO(2212)

Abstract We have investigated the phase transition of Bi 1.8 Pb 0.2 Sr 2 CaCu 2 O 8 samples with specific heat in low temperature. The phase transition temperatures depend on the oxygen contents. There is a relation between the specific heat anomaly and the oxygen contents in these samples. The specific heat anomaly peak is the largest for the optimum doped sample, the anomaly peaks are decreased for the underdoped samples. There are the phase segregation phenomenon when the oxygen is removed content in these Bi2212 system samples, their transition temperatures are about 60K and 90K. The peak of specific heat anomaly for the overdoped sample (Tc=60K) does not appeared. The resistance measurement shows that the phase transition temperature of these samples are the same as that of the specific heat measurement. The electronic specific heats C e and the Debye temperature θ D are calculated.