Microsc. Microanal. 23, 414–424, 2017 doi:10.1017/S1431927616012757
© MICROSCOPY SOCIETY OF AMERICA 2017
Nanoscale Stoichiometric Analysis of a High-Temperature Superconductor by Atom Probe Tomography
Stella Pedrazzini,1,* Andrew J. London,1 Baptiste Gault,1,2 David Saxey,3 Susannah Speller,1 Chris R. M. Grovenor,1 Mohsen Danaie,1 Michael P. Moody,1 Philip D. Edmondson,4 and Paul A. J. Bagot1
1Department of Materials, University of Oxford, Parks Road, , Oxford OX1 3PH, UK 2Max Planck Institute für Eisenforschung, Max-Planck-Straße 1, 40237 Düsseldorf, Germany 3Geoscience Atom Probe, Advanced Resource Characterisation Facility, John de Laeter Centre, Curtin University, Perth,
WA 6102, Australia 4Oak Ridge National Laboratory, Materials Science & Technology Division, 1 Bethel Valley Road, Oak Ridge, TN 37831, USA
Abstract: The functional properties of the high-temperature superconductor Y1Ba2Cu3O7−δ (Y-123) are closely correlated to the exact stoichiometry and oxygen content. Exceeding the critical value of 1 oxygen vacancy for every five unit cells (δ>0.2, which translates to a 1.5 at% deviation from the nominal oxygen stoichiometry of Y7.7Ba15.3Cu23O54 −δ) is sufficient to alter the superconducting properties. Stoichiometry at the nanometer scale, particularly of oxygen and other lighter elements, is extremely difficult to quantify in complex functional ceramics by most currently available analytical techniques. The present study is an analysis and optimization of the experimental conditions required to quantify the local nanoscale stoichiometry of single crystal yttrium barium copper oxide (YBCO) samples in three dimensions by atom probe tomography (APT). APT analysis required systematic exploration of a wide range of data acquisition and processing conditions to calibrate the measurements. Laser pulse energy, ion identification, and the choice of range widths were all found to influence composition measurements. The final composition obtained from melt-grown crystals with optimized superconducting properties was Y7.9Ba10.4Cu24.4O57.2.
Key words: Atom probe tomography, YBCO, superconductor, stoichiometry INTRODUCTION
Yttrium Barium Copper Oxide (YBCO) is one of the most commonly studied high-temperature superconductors with potential applications in large-scale power generators (Takeuchi et al., 2011) and AC power transmission cables (Li et al., 2011). It has also been proposed as a candidate material for the magnetic confinement of plasma in Tokamak reactors (Sykes et al., 2014). Its chemical formula is Y1Ba2Cu3O7 −δ (Y-123), where δ is a parameter describing the formation of vacancies on the oxygen sub-lattice con- trolled by the valence state of the metallic Cu (II)/Cu (III) ions (Jorgensen et al., 1991). The value of δ can be used to predict the functional properties of the material. When δ = 0.5 the triple-layer perovskite phase has tetragonal symmetry (Kreisler et al., 2012; Degardin et al., 2014). A tetragonal-to-orthorhombic phase transition occurs at δ ~0.2 (i.e., no >1 oxygen site for every five unit cells can be vacant for the orthorhombic crystal structure to be stable). Superconductivity is exhibited by this orthorhombic phase (Benzi et al., 2004). As the value of δ decreases further, between 0.2≥δ≥0, the superconducting transition
*Corresponding author.
stella.pedrazzini@
materials.ox.ac.uk Received June 12, 2016; accepted December 15, 2016
temperature increases from ~60K up to a maximum of 90K (Benzi et al., 2004). Therefore the desired crystal structure and functional properties of YBCO depend on an extremely delicate balance of oxygen stoichiometry and vacancies on the oxygen sub-lattice. Nonstoichiometry in the oxygen sub-lattice is of crucial
importance to the electrical properties of a wide range of complex functional oxide materials, but has proved extremely difficult to quantify on the nanoscale (Jia et al., 2003; Seh et al., 2007; Tuller & Bishop, 2011; Chen et al., 2013; Badwal et al., 2014). Finding a reliable way to quantify local oxygen stoichiometry is essential for the prediction of the properties of functional oxides, for example strontium titanate (STO), materials such as YBCO and other high- temperature oxide superconductors in real service environ- ments. Previous studies have attempted to measure (or to estimate) the oxygen content of YBCO using a variety of experimental approaches. Some estimates rely on indirect methods, such as the measurement of the c axis unit cell dimension, which depends on the value of δ (Degoy et al., 1996;Wu et al., 1998; Benzi et al., 2004). Alternatively, bulk oxygen measurements are relatively quick and easy, providing average values of δ. Most such methods involve the extraction of oxygen from the sample using dissolution and a chemical method to calculate the oxygen content.
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