A Cr(IV) based 1212-type cuprate
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A Cr(IV) based 1212-type cuprate Rocío Ruiz-Bustos, Myriam H. Aguirre, M.Á. Alario-Franco* Laboratorio de Química del Estado Sólido, Facultad de Química Universidad Complutense, 28040, Madrid Spain (EU) (* [email protected])
Abstract By means of high pressure and high temperature we have prepared a new material similar to the classic YBa2Cu3O7 (Ybacuo) superconductor but having chromium in the so-called charge reservoir layer: CrSr2YCu2O8. It has been characterised by different techniques such as X-ray diffraction, TEM, ED, EDS, and magnetic susceptibility. The new material is isostructural with the Ru equivalent compound (RuSr2YCu2O8). An antiferromagnetic transition is observed on cooling below 130 K. The as prepared material is not superconducting down to 4 K.
Introduction The so-called Charge Reservoir Layer (CRL) of the cuprate superconductors has been the basic playground to innovate in the search of HTSC materials [1,2]. Even if there have also been some variations in the sometimes called “perovskite skeleton” or “intermediate layer” in which one can have either a NaCl-type layer or a fluorite-like region, originating different global stoichiometries e.g.1212 or 1222 in abbreviated form, the CRL has been modified in almost all possible substitutional ways [3]. Once that the room pressure synthesis possibilities have been worked out, the use of high pressures and temperatures has opened up a new venue in the search of yet more substitutions. In this way, elements as different as C, S, Ga, P, or Fe, to quote but a few, have been introduced in the CRL. This can happen with the particular element in its own, i.e. as an oxide layer, such is the case of Carbon as [C-O3] or Sulfur as [S-O4]; but it is rather more common with the element combined with another “metal” such as Cu in the (Cu,C), or (Hg,V) based cuprates and related materials [4,5] . The case of the element chromium is particularly interesting in view of the wide possibilities it offers in both oxidation states and coordination numbers [6]. Indeed, Cr(III), an octahedral-only t2g3 ion, is very stable due to its high Octahedral Crystal Field (OCF) stabilisation energy (∼ 165 kjul/mol as [Cr(III)-O6] ) ; in the other extreme, Cr(VI), a d0 ion, has no common coordination but the tetrahedral one: [Cr(VI)-O4] and no OCF stabilisation. Yet Cr(VI) has given [7,8] the widest series of cuprates known so far: (Cu,Cr) Ba2Can-1CunO3n+2, some of which are superconducting. But, Chromium has, indeed, some other oxidation states and, in the present work, we have made a new cuprate with just Cr(IV) in the CRL: CrSr2YCu2O8. Cr(IV) oxides, like CrO2 and SrCrO3 are rather unstable towards decomposition, so that High Pressure is required for the synthesis. Once prepared, however, CrSr2YCu2O8 is rather stable and does not decompose in air below ∼900 K, some 250 degrees higher than CrO2 itself [9].
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Experimental CrSr2YCu2Os was prepared by a high pressure synthesis procedure in a Belt type apparatus, according to the reaction: CrO2 + ½ Y2O3 + 2 SrCuO2
⇒ Cr
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