Darl H. McDaniel

Signature of twin planes in Mössbauer spectroscopy of cuprate superconductors

One of the four quadrupole doublets (A, B, C, and D) observed in Mössbauer spectra of orthorhombic (Y or RE)1Ba2(Cu1-xFex)307-δ(δ≈0) samples is identified with Fe dopant replacing a Cu site in twin planes. The site in question, labelled as D, is characterized by a quadrupole splitting ΔD in the range 1.6±0.1 mm/s. The D-site intensity is found to approach zero when the 1-2-3, material is driven tetragonal, either upon oxygen desorption, upon oxygen loading, or upon increasing the Fe-dopant concentration.

Fe-site assignments in Y1Ba2Cu3O7 revisited

A new symmetric Fe3+ site (labelled E) (δ=0.28(2) mm/s and Δ=0.15 mm/s) is observed in the R1Ba2Cu3O7 family of superconductors for R=Y and Eu. The decay and growth of site E are established in two seemingly unrelated experiments, one as a function of room temperature aging for R=Y, and the other as a function of oxygen loading at elevated temperatures and pressures for R=Eu. In each experiment, the results show that site E is genetically related to the well documented chainsites A(Δ=1.9 mm/s), C(Δ=1.1 mm/s) and D(Δ=1.6 mm/s). We propose that site E represents a quasi-octahedral chain-site having pairs of O(1), O(4) and O(5) nearestneighbor oxygen sites. The two remaining doublets, B(Δ≈0.4 mm/s) and B′(Δ=0.78 mm/s) represent Fe3+ dopant sites present in the all-important CuO2 planes, possessing coordination numbers of 6 and 5, respectively.

Metal (M) dopant centred local structures, high-pressure synthesis and bulk superconductivity in : M = Fe, Co, Ni

Depression of by the dopants Fe or Co, but not Ni, is spectacularly reversed by synthesis of the titled cuprates at high P instead of ambient P of oxygen with enhanced from 32 to 82 K for M = Fe at x = 0.10. With Fe dopant, Mossbauer spectroscopy reveals a conversion of a tetrahedral (A) into a trigonal bipyramidal coordinated (C) Fe chain site by oxygen addition in the chains upon high-P processing. The A C site transformation is elucidated and extended to the case of Co dopant, and its consequences for superconducting behaviour discussed.

Ferromagnetism in compounds containing alkaline earth cuprates

Abstract A ferromagnetic series of alkaline earth cuprates has been synthesized utilizing sintering temperatures lower than those normally used. Curie point determinations for these compounds have been established and range from room temperature to 700°C.

SPONTANEOUS FERROMAGNETISM AT ROOM TEMPERATURE IN OXYGEN-RICH DICALCIUM CUPRATE CA2CUO3+DELTA

Stoichiometric Ca 2 CuO 3 , having square-planar Cu-chains [C. L. Teske and H. Muller-Buschbaum, Z. Anorg. Allg. Chem. 379 , 234 (1970); M. Hjorth and J. Hyldtoft, Acta Chem. Scand. 44 , 516 (1990)], is expectedly antiferromagnetic due to 1D intrachain superexchange [K. Okuda, S. Noguchi, K. Konishi, H. Deguchi, and K. Takeda, J. Magn. Magn. Mater. 104–107, 817 (1992)]. Nonetheless, we report remarkable spontaneous ferromagnetism at 293 K after sintering, prominently in oxygen. This apparently introduced excess oxygen at vacant quasioctahedral sites, promoting spin-flip and ferromagnetic interchain coupling. Thermogravimetry (TGA) revealed excess oxygen, ≈0.17 O/Cu. X-ray diffraction (XRD) yielded a comparatively smaller unit cell. Ferromagnetism disappeared by oxygen depletion, mimicking reported nonmagnetism of Ca 2 CuO 3− δ [Okuda et al. (1992)]. Elemental analysis showed insignificant magnetic impurity traces. T c ranged between 680 K and 723 K, depending on freshness and purity. Saturation magnetization varied with processing, optimally 0.30 A · m 2 /kg at 1.0 T applied field. Coercivity and remanence varied with purity.

Oxygen Evolution from Yba 2 cu 3 o 7‐δ Superconducting Powders Generated by Aerosol Routes

The oxygen desorption kinetics of the YBa 2 Cu 3 O 7‐δ powders generated by an aerosol route was investigated and compared to that of solid state reacted powders. The oxygen content of the aerosol sample was determined using a thermogravimetric analyzer (at 990 C) and wet chemical methods, and δ was calculated to be ‐0.1. The activation energy and pre‐exponential factor were determined by regression for two peaks at 540 C and 850 C. Other activation parameters were also calculated. The calculated values of these parameters for aerosol samples and those reported for solid state reacted powders are in good agreement.

Progress in Mössbauer spectrosccopy of high-temperature superconductors

Abstract57Fe dopant site assignments in several families of cuprate superconductors are reviewed. Evidence for motional broadening of the 14.4 keV resonance atT≤Tc for Fe sites localized in the all-important CuO2 planes in Y1Ba2Cu4O8 is presented.

Fe Doping Effects in Aerosol Processed YBa 2 (Cu 1-x Fe x ) 3 0 7–δ Submicrocrystalline Grains

Magnetization and Mossbauer spectroscopy results on aerosol processed YBa 2 (Cu 1-x Fe x ) 3 0 7–δ samples are presented. Systematics of T c (x), x (x) and Fe dopant site occupancies I n /I(x) as a function of dopant concentration ‘x’ are deduced and compared to those established on analogous solid-state- reacted samples. The absence of twinning in the aerosol processed grains leads to a random replacement of Cu for Fe in chains and planes of the host structure.