R.L. Hitterman

Pressure-induced structural changes in superconducting HgBa2Can−1CunO2n+2+δ (n = 1, 2, 3) compounds

Abstract The crystal structures of superconducting HgBa 2 CuO 4+δ and HgBa 2 CaCu 2 O 6+δ have been investigated with a pressure up to 0.6 GPa and HgBa 2 Ca 2 Cu 3 O 8+δ to 9.2 GPa by neutron powder diffraction. The compressibility along the c -axis is nearly the same for the three compounds and up to two times larger than the compressibility along the a -axis. The one-layer compound, HgBa 2 CuO 4+δ , shows the largest a -axis compressibility, while HgBa 2 Ca 2 Cu 3 O 4+δ shows the smallest compressibility. The bond compressibilities of HgBa 2 CuO 4+δ and HgBa 2 CaCu 2 O 6+δ are significantly different from HgBa 2 Ca 2 Cu 3 O 8+δ bond compressibilities. In the one- and two-layer compounds the largest bond compressibility was the Cu-O2 (apical) bond distance, while for the three-layer compound it was the Hg-O2 bond distance.

Structural study of Sr2CuO3+δ by neutron powder diffraction

Abstract Average crystal structures of superconducting Sr 2 CuO3+ δ synthesized under high pressure and nonsuperconducting Sr 2 CuO3+ δ synthesized at ambient pressure from a hydroxometallate precursor were refined from neutron powder diffraction data. A simplified model was used to fit the modulated superstructures. Both compounds have an oxygen-deficient La 2 CuO 4 -type tetragonal T structure with oxygen vacancies located in the CuO 2 planes, not in the Sr 2 O 2 layers. This result raises important questions regarding the nature of superconductivity in Sr 2 CuO3+ δ reported to be a 70 K superconductor.

Dependence of superconducting transition temperature on doping and structural distortion of the CuO2 planes in La2−xMxCuO4 (M=Nd, Ca, Sr)

We have separated the structural and doping effects on the superconducting transition temperature in La2−xMxCuO4 (M=Nd, Ca, Sr). At a fixed doping level, the highest Tc is found for flat and square CuO2 planes in the tetragonal structure. Tc is reduced proportionally to the structural distortions of the CuO2 planes in the orthorhombic structure. The local minimum of Tc vs doping is observed for both orthorhombic Ca- and Sr-substituted materials around x∼0.12. This minimum may be related to the intrinsic dependence of the electronic density of states at EF due to a weakly split singularity of DOS.

Residual stress and stress distribution in a WC-Ni composite

Abstract The differential thermal stress state in a WC-15.6wt.%Ni cemented carbide composite has been studied using neutron powder diffraction. Absolute stress values have been obtained from the difference in WC cell parameters in a stress-free, loose WC powder and the WC phase in the composite over the temperature range 20–700 K. In addition, the stress distribution in the WC phase was determined by analysis of WC peak breadths in the reference powder and the composite. The peak breadths were corrected for particle size effects using a procedure based on the integral peak breadth method of particle size-strain analysis. The resultant full width at half-maximum values of the elastic strain distribution indicate that a broad range of strain, and thus stress, is present in the carbide phase of the composite. The distribution is centered on an average compressive stress and ranges from very compressive to very tensile values. The stress distribution results are consistent with prior results in other WC-Co and NbC-Co cemented carbides.

The site preference of Cu substituted Zn and Ni in YBa2Cu3O7−γ

Abstract Neutron diffraction measurements were performed at room temperature on samples of YBa2(Cu1−xMx)3O7−γ (M = Zn, Ni) with x = 0.05. The commonly used full pattern analysis is found to be insensitive to whether M substitutes for Cu in the chains or in the planes. The few neutron reflections which have the higher sensitivity to the M position have been studied. It is found that both Zn and Ni substitutes for Cu in the planes. A discussion, which also includes Fe and Co substitutions, on the relation of this result to the structural transport, and magnetic properties of these mixed phases is presented.

Thermal expansion measurements using neutron diffraction of Bi2CaSr2Cu2Ox

Abstract The differential thermal expansion of Bi 2 CaSr 2 Cu 2 O x , Bi-(2122), has been measured using high-temperature neutron powder diffraction in air, 0.980% oxygen in nitrogen, and pure argon. Thermal expansion in the c -axis direction is greater by a factor of about 1.4 than that in either the a - or b -axis direction for all sample environments. The neutron results for Bi-(2122) in air are compared with results from similar X-ray powder diffraction measurements.

Dependence of superconducting transition temperature on doping and structural distortion of the CuO{sub 2} planes in La{sub 2{minus}{ital x}}M{sub {ital x}}CuO{sub 4} (M=Nd, Ca, Sr)

By comparing structural and superconducting properties of La{sub 2{minus}{ital x}{minus}{ital y}}Ca{sub {ital x}}Nd{sub {ital y}}CuO{sub 4} to La{sub 2{minus}{ital x}}Sr{sub {ital x}}CuO{sub 4} we have separated the effects of structure and doping on the superconducting {ital T}{sub {ital c}}. At a fixed doping level, the highest {ital T}{sub {ital c}} is found for flat and square CuO{sub 2} planes in the tetragonal structure. {ital T}{sub {ital c}} is reduced by the structural distortions of the CuO{sub 2} planes in the orthorhombic structure. The local minimum of {ital T}{sub {ital c}} vs doping observed around {ital x}{approximately}0.12 indicates the presence of a weakly split singularity of the electronic density of states for the orthorhombic material. {copyright} {ital 1996 The American Physical Society.}

Pressure-induced structural changes and charge transfer in Tl2Ba2Cu6+z

Abstract Tl2Ba2CuO6+z has negative coefficients of Tc, dTc/dp, that vary appreciably with oxygen content. We refined the structure parameters for three samples of Tl2Ba2CuO6+z with Tcs of 0, 46 and 79 K from time of flight neutron powder diffraction data measured at 0–0.622 GPa and room temperature. The lattice parameters of the tetragonal unit cells and the unit-cell volume decreased linearly with increasing pressure. The three samples have similar bulk moduli K = 82.2–87.6 GPa, which are 29–43% smaller than those of other hole-doped superconductors (118–147 GPa). An axial CuO(2) bond, and to a lesser extent an equatorial CuO(1) bond, within a [CuO6] octahedron are compressed pronouncedly under high pressure in Tl2Ba2CuO6+z. We propose that this marked shrinkage of the two CuO bonds at high pressure is caused by the transfer of electrons from the CuO2 conduction sheets to the charge reservoirs (BaOTlOTlOBaO sheets0, lowering the Tc of this superconductor that is overdoped with holes. This charge-transfer model is consistent with the results of recent Hall-coefficient measurements under high pressure.

Structural properties of YBa2Cu3O6+x at elevated temperatures in controlled oxygen atmospheres

Abstract The structural properties of YBa2Cu3O6+x have been determined at elevated temperatures in controlled oxygen partial pressure by in situ neutron powder diffraction. Diffraction data were obtained as a function of the oxygen partial pressure at constant temperatures of 490°C and 440°C. These data, taken in thermodynamic equilibrium, provide information that cannot be obtained from metastable oxygen-deficient samples studied at room temperature. For example, the orthorhombic-to-tetragonal phase transition is observed to be continuous, as expected for a second order phahe transition. Sequential diffraction measurements following each change in experimental conditions were used to monitor the approach to equilibrium by observing changes in lattice constants. A considerable increase in equilibrium time (following a step-like change in oxygen partial pressure) was found as the orthorhombic-to-tetragonal phase transition was approached. No phases other than the high-temperature orthorhombic (ortho-I) and tetragonal phases were observed.

Anomalous behavior of the pressure dependence of lattice constants in Tl2Ba2CuO6+x

Abstract The crystal structure of Tl 2 Ba 2 CuO 6+ x has been investigated as a function of pressure and temperature under hydrostatic conditions using neutron powder diffraction. The crystal structure at 0.609 GPa and 60 K depends systematically on the path in pressure-temperature space by which these conditions are achieved. Applying pressure at room temperature and then cooling produces different changes in the structure than cooling first and then applying pressure. Our results support those of Sieburger and Schilling [1] who reported that the T c at high pressure for this compound depends markedly on whether the sample is pressurized at room temperature or low temperature. We speculate that this unusual behavior may be associated with differences in the mobility of interstitial oxygen defects between room temperature and 60 K and that applying pressure at room temperature changes the state of the sample through such processes as defect ordering, while changing the pressure at low temperature does not.