The origin of T c suppression due to substitution of rare-earth elements (R) is investigated for the samples of ͑La 1Ϫx R x ͒ 1.82 Sr 0.18 CuO 4 , ͑RϭEu,Nd,Pr͒ through the measurements of electrical resistivity and lattice parameters. With increasing x, three phases, T, T*, and TЈ appear successively and the critical concentration for disappearance of superconductivity exists in the T-phase region, each of which is xϭ0.12, 0.4, and 0.6 for RϭEu, Nd, and Pr, respectively, indicating that the smaller the ionic radius of the substituted R element, the stronger the T c suppression. Temperature dependence of the conductivity is described by ϰlnT above xϭ0.08 and 0.1 for RϭEu and Nd, respectively, but not for RϭPr. The origin of T c suppression is discussed in terms of two contributions; the reduction of the Madelung site potential at apical oxygen sites and the random potential scattering on the CuO 2 planes which is effective for pair breaking in d-wave superconductors, both of which are caused by displacement of apical oxygens due to substitution of R elements.
The identification of rock-forming and clay minerals contained in the reference material of “Pond Sediment” (Reference Material NIES No. 2 from the National Institute for Environmental Studies, Japan) has been made with X-ray diffraction method. It was found that the sediment contained α-quartz and feldspars as the principal components along with small quantity of pyroxenes, amphiboles, and some clay minerals. The sediment sample was treated chemically for extraction, and also separated with density and with grain sizes. Elemental compositions of such extracted or separated fractions of the sediment were determined by inductively coupled argon plasma emission and atomic absorption spectrometry. The identified minerals and the elemental distributions in the separated fractions helped to characterize the pond sediment.
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