Novel techniques of neutron powder diffraction and their applications to superconducting oxides
Introduction
We have been exploiting our own technology of data collection and structure refinement for neutron powder diffraction and applying it to various inorganic compounds such as superconductors, ferroelectrics, cathode materials for lithium-ion secondary batteries, and zeolites. The present paper reports applications of the following three powder-diffraction techniques to superconductors and related compounds: (a) time-of-flight (TOF) neutron powder diffractometer Vega equipped with arrays of one-dimensional position-sensitive detectors (PSDs) [1], (b) Rietveld analysis under partial profile relaxation [2], [3], [4], and (c) whole-pattern fitting connected with a maximum-entropy method (MEM) [3], [5]. Methods (a) and (b) surpass the conventional Rietveld method in goodness-of-fit and representation of detailed structures, respectively. We shall exemplify some successful structure refinements with these innovative techniques.
Section snippets
Crystal structure of the superconducting perovskite (K0.87Bi0.13)BiO3
Vega [1] has been operated at the KENS pulsed neutron source since 1993 in place of the previous diffractometer, HRP. Vega has a backward bank containing an array of one-dimensional 3He PSDs whereby a wide solid angle is covered two-dimensionally. This unique geometry makes it possible to enhance the efficiency of data collection without sacrificing resolution (Δd/d≈2×10−3 for the backward bank). Vega is therefore suited for collecting neutron diffraction data of small amounts of samples
Rietveld refinement under partial profile relaxation
Profile functions for use in the Rietveld, Pawley, and Le Bail methods generally contain two levels of profile parameters: primary profile parameter (PPP) and, in its turn, secondary profile parameter (SPP). The dependence of PPPs on θ (angle-dispersive diffraction) or d (TOF neutron diffraction) is represented empirically or with physical foundations to afford equations including SPPs. For example, in the equation of Caglioti et al.,the full-width-at-half-maximum, Hk,
Nuclear-density distribution in HgBa2CuO4+δ
We have recently developed a state-of-the-art system for structure refinement, REMEDY [3], comprising a Rietveld-analysis program RIETAN-98 [4] and an MEM program MEED [5]. With REMEDY, we alternate MEM analysis and whole-pattern fitting of angle-dispersive powder diffraction data. In the latter, the diffraction pattern calculated from structure factors, Fc, derived by the Fourier transform of nuclear (scattering length) or electron densities is fit to the observed one to refine parameters
Acknowledgments
The authors are grateful to T. Ikeda, K. Oikawa, and S. Torii for their technical help. This work was partly supported by the New Energy Development Organization.
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