Structural disorder and superconductivity suppression in NdBa2Cu3Oz (z∼7)

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Abstract

Structural features of a low-Tc nearly stoichiometric Nd1+xBa2−xCu3Oz phase (x⩽0.05, z∼6.9, a=3.897–3.899Å, b=3.902–3.908Å, c=11.707–11.719Å) have been investigated for the first time by XRD, EXAFS structure refinement techniques and Raman spectroscopy. It has been found that the abnormal lattice parameters and the low superconductivity transition temperature (55 K) of such a phase are related to structural disorder in the Ba and Nd sites resulting in oxygen disordering which cannot be avoided by a standard oxidation procedure. It is shown that longer annealing at high temperatures is an important factor for reducing the oxygen disordering and hence enhancing superconducting properties after oxidation.

Introduction

The NdBa2Cu3Oz superconductor (Nd123) appears destined for wide use in manufacturing bulk materials with enhanced stability of critical current density in high magnetic fields, a higher superconductivity transition temperature (Tc) and better chemical stability [1], [2]. The commonly accepted advantage of such materials over the classical superconductor YBa2Cu3Oz (Y123) is connected directly with an anomalous peak effect caused by nanoscale compositional fluctuations [1], [2], [3]. These fluctuations become possible because, unlike the Y123 phase, Nd123 is known to admit a wide range of solid solutions Nd1+xBa2−xCu3Oz (Nd123ss, x=0.0–1.0 [4], [5]) with superconducting properties suppressed by Nd/Ba heterovalent substitution. This extra degree of freedom in solution demixing probably leads to formation of semicoherent inhomogeneity regions resulting in strong flux pinning [1].

At the same time, it should be noted that the existence of a range of solid solutions makes it more difficult to control the physical properties of materials based on Nd123 since, in addition to the favorable possibility of pinning, some negative side effects may be expected including cation and oxygen disorder in the lattice, which can lead, in general, to “anomalous” samples with suppressed superconducting properties unrecoverable by a complete oxygenation procedure [6]. Such samples of Nd123 phases with low Tc and small orthorhombic distortion are most often ignored or attributed to a form of experimental error (e.g. instrumental error or impurity contamination). In other words, achieving the maximal possible Tc=95 K can be difficult. This problem has been solved partly by application of sophisticated preparation methods such as chemical homogenization of precursors [7], [8], [9], [10], [11], [12], [13] followed by a precisely optimized heat treatment [14], melt-solidification under low-pO2 atmosphere [2], or liquid composition control during crystal growth [15], [16], [17]. Nevertheless, “anomalous” cases of low-Tc Nd123 phases still remain common, requiring extensive analysis of the underlying causes so that a simple and reproducible method for commercial production can be found.

As the most important specific tasks, searching for preparation conditions followed by structural studies of the “anomalous” samples of Nd123 have to attract special interest because of the consequences that phase transitions and lattice distortions may have on the superconducting properties. Being a transient state to the perfectly ordered Nd123 phase with well-known “ideal” structure, such samples are expected to have local structural deformations rather than long-range changes of their lattice. In this context, extended X-ray absorption fine structure (EXAFS) studies can play a significant role due to the sensitivity of EXAFS to short-range local structure, while Raman scattering spectroscopy brings information about local symmetry and force constants of particular chemical bonds by monitoring the vibrational frequencies and symmetries of observed modes. Both the techniques can examine disorder of light atoms such as oxygen and are complementary to the long-range structure refinement from the X-ray diffraction (XRD) data, which fails to refine oxygen structural parameters adequately and also cannot distinguish Nd and Ba atoms because of their similar scattering factors. Thus, in the present work, we performed a combined study of low-Tc samples of fully oxygenated nearly stoichiometric Nd1+xBa2−xCu3Oz phases by means of XRD, EXAFS, and Raman scattering spectroscopy.

Section snippets

Sample preparation

The NdBa2Cu3Oz phase was prepared via two different types of precursors. First, the polymerized complex method [18] was used to obtain highly homogeneous samples. In this method, Nd2O3 preliminarily annealed at 900°C was dissolved in 20% excess of HNO3. Citric acid and ethylene glycol were added and the solution was stirred at 80°C until it became transparent. Afterwards, stoichiometric amounts of BaCO3 and Cu2(OH)2CO3 (with known Cu content) were dissolved in the solution. The temperature was

Preparation conditions and superconducting properties

An efficient, if not the only, way to control stoichiometry, impurity levels and compositional uniformity of multicomponent compounds is connected with chemical homogenization. Indeed, soft chemistry methods have been successfully used for mixing of starting materials in order to obtain high-quality single-phase superconducting compounds [13]. In this case, the reaction rate between oxides increases remarkably due to the small grain size of the intermediate fine precursors. At the same time,

Conclusion

In summary, low-Tc “nearly stoichiometric” Nd1+xBa2−xCu3Oz samples (x⩽0.05, z∼6.9, a=3.897–3.899Å, b=3.902–3.908Å, c=11.697–11.719Å) were prepared by the polymerized complex method and from carbon-free precursors. It was found by both XRD and EXAFS structure refinement techniques as well as by Raman scattering that the small orthorhombicity of the samples in a fully oxygenated state and their low superconductivity transition temperature (∼55 K) are possibly related to antisite disorder of Ba

Acknowledgements

This work was supported by the Grand-in-Aid for Scientific Research 11450246, RFBR (grant 98-03-32575, 96-15-97385), the Russian Universities Program (grant 98-06-5057) and the Program for Actual Problems of Physics of Condensed Matter (Superconductivity, grant 96079). Part of this work was performed at the Australian National Beamline Facility with support from the Australian Synchrotron Research Program, which is funded by the Commonwealth of Australia under the Major National Research

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