High-Tc Superconducting Cuprates, (Ce,Y)sO2s-2Sr2(Cu2.75Mo0.25)O6+δ: Tc-increase with apical Cu-O decrease at constant Cu-O planar distance

Evidence for high-Tc cuprate superconductivity is found in a region of the phase diagram where non-superconducting Fermi liquid metals are expected. Cu valences estimated independently from both XANES measurements and bond valence sum calculations are greater than 2.25 and are in close agreement with each other for structures of the homologous series given in the title with s = 1, 2, 3, 4 and 5. Two questions arise from the present perspective: 1) Is all the action in the CuO2 layers? 2) Is there superconductivity beyond the usual dome? The record short apical oxygen distance found in the homologous series especially in the s = 1 member, at odds with the current theory, suggests the possibility of a new pairing mechanism. The apical Cu-O distance in the s = 1 member decreases upon oxygenation from 2.29 to 2.15 Å while the Cu valence increases to 2.45 Å. Between 2004 and 2010 the researchers at Tokyo Institute of Technology (Japan) and Aalto University in Helsinki (Finland) synthesized the first 5 members of the cuprate homologous series 1-6 : (Ce,Y)sO2s-2Sr2(Cu2.75Mo0.25)O6+δ, s = 1, 2, 3, 4, and 5. None of the asprepared samples (AS) were found to become superconducting, but when the samples were treated under high oxygen pressure (5 GPa, 500° C in the presence of KClO3 or Ag2O2) the s = 1 member became superconducting at 88 K, while the s = 2, 3, 4 and 5 members exhibited a constant Tc of about 56 K. The chemical formula and the structural arrangement of the five compounds were determined by conventional x-ray powder diffraction analysis, electron diffraction and high-resolution transmission electron microscopy. The superconducting transition temperatures were determined by FC and ZFC magnetization measurements. Later Table 1 :Cu valences for the AS and HPO samples estimated from the Cu L2,3 absorption edge and the hole concentration ρ on the CuO2 layers estimated from the O K absorption edge v(Cu)* AS v(Cu)* HPO ρ(CuO2)** Tc s = 1 2.16*** 2.46 0.53 88 s = 2 2.13 2.24 0.28 56 s = 3 2.14 2.25 0.26 56 s = 4 2.14 2.26 0.27 56 *Values estimated from the Cu L2,3 absorption edge. ** Values estimated from the O K absorption edge. ***Value obtained also by iodometric titration. 11th European Conference on Applied Superconductivity (EUCAS2013) IOP Publishing Journal of Physics: Conference Series 507 (2014) 012031 doi:10.1088/1742-6596/507/1/012031 Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd 1 It is interesting to note that the Cu valence of the cations belonging to the CuO2 layers is definitely high when compared to the values of 2.15/2.2+ obtained for the all other superconducting cuprates. Fig. 1 represents a three-dimensional structural model of the s = 2 member. Fig.1: A three-dimensional structural model of the s = 2 member: (Ce,Y)2O2Sr2(Cu2.75Mo0.25)O6+δ,.The oxygen on the basal plane have been omitted. The distance between 2 CuO2 layers is 5.998 Å. It increases to 8.746 and to 11.441 Å for the members with s = 3 and 4 respectively, while Tc remains constant at 56 K for both members. The higher-member structures are obtained by inserting additional 2-layer blocks (O2)(Ce,Y) in between (Ce,Y) and (CuO2). For example, for the member with s = 3 the sequence between two successive CuO2 layers is: (CuO2)(Ce,Y)(O2)(Ce,Y)(O2)(Ce,Y)(CuO2). The new samples of the Mo-cuprates allowed reliable structural refinements at room temperature. These refinements were performed at the Argonne National Laboratory 7 . The results confirmed the structural arrangement previously determined by x-ray, and electron diffraction techniques and HRED imaging. Moreover, they revealed important structural features concerning the basal plane occupied by 75% of Cu cations, 25% of Mo cations, and δ oxygen atoms. The results are summarized in Table 2. Table 2: Oxygen stoichiometry of the basal plane δ and the total oxygen content for the first four members of the homologous series (Ce,Y)sO2s-2Sr2(Cu2.75Mo0.25)O6+δ from neutron diffraction. s = 1 AS s = 1 HPO s = 2 AS s = 2 HPO s = 3 AS s = 3 HPO s = 4 AS δ 1.36 1.56 1.01 1.41 1.16 1.72 0.97 Total oxygen 7.36(16) 7.56(4) 9.01(8) 9.41(17) 11.16(9) 11.72(15) 12.97(7) The observation that after the HPO treatment the first member shows a Tc of 88 K while the members with s = 2, 3, 4 and 5 exhibit a constant Tc of ~56 K is a very strong indication that the charge reservoir layers must play an essential role for the superconducting properties. The first member has actually the YBa2Cu3O7 structure in which the Ba cations are substituted by Sr and 25% of the planar squarely-coordinated Cu cations are substituted by Mo 6+ cations. These substitutions make a substantial difference in the structure and consequently in the properties because the Mo 6+ cations require an octahedral coordination and therefore the basal z = 0 plane can incorporate a larger amount of oxygen than either YSr2Cu3O7 or YBa2Cu3O7. 8 In Table 3 the valence of the Cu cations belonging to the CuO2 layers, determined by the bond valence sum (BVS) from the interatomic distances obtained by neutron diffraction data are reported. They are compared to the corresponding values obtained by XANES experiments. The same trend was confirmed. Note that the total oxygen intake is reported in Table 2 whereas in Table 3 only that of the CuO2 planes is given. The high-valence value of the same group was able to produce very pure samples for the first 4 members, which allowed them to carry out XANES measurements leading to an estimate of the Cu valence and the holes’ concentration for these unusual cuprates. These results are summarized in Table 1. 11th European Conference on Applied Superconductivity (EUCAS2013) IOP Publishing Journal of Physics: Conference Series 507 (2014) 012031 doi:10.1088/1742-6596/507/1/012031