Quaternary palaeoecology and nature conservation: a general review with examples from the neotropics

A low-temperature topotactic route is used to assemble metal−anion arrays within a perovskite host. Ion exchange between RbLaNb2O7 and CuX2 (X = Cl, Br) results in a new set of layered copper−oxyhalide perovskites, (CuX)LaNb2O7. Rietveld structural analysis of X-ray powder diffraction data confirms the formation of a two-dimensional copper−halide network in the double-layered perovskite interlayer. This new structure type contains unusual CuO2X4 octahedra that corner-share with NbO6 octahedra from the perovskite slab and edge-share with each other along all four equatorial edges. Magnetic susceptibility measurements show that both products exhibit antiferromagnetic transitions below 40 K. Additionally, these materials are found to be low-temperature phases, decomposing completely by 700 °C. The synthetic approach described in this work is significant in that it demonstrates how host structures can be used as templates in the directed low-temperature assembly of extended metal−anion arrays.

Assembly of Metal−Anion Arrays within a Perovskite Host. Low-Temperature Synthesis of New Layered Copper−Oxyhalides, (CuX)LaNb2O7, X = Cl, Br

A systematic view stemmed for the most important parts from the authors’ own research results is presented for tailoring high-Tc superconductive copper oxides from the standpoint of materials chemistry. The present article first elucidates that, in the multi-layered superconductive copper-oxide structures presently known to exist, the distribution of charge/charge carriers is inhomogeneous over several different spatial dimensions throughout the crystal. At the same time useful guiding rules are derived based on the concepts of tolerance parameter and bond-valence for controlling the charge distribution. Then experimental and semi-experimental evidences are presented to establish the relationships between the inhomogeneous distribution of holes and superconducting properties. To reach this goal, phenomenological aspects of the high-Tc superconductive copper oxides are described systematically, and selected important techniques for estimating the local carrier concentration and distribution are discussed. That is, the following points are included in the present article: (i) general crystallographic categorization and naming scheme of multi-layered copper oxides based on the concept of homologous series, (ii) crystallographic and chemical factors to control the oxygen non-stoichiometry and charge distribution, (iii) techniques for probing the charge/carrier distribution in the layered copper-oxide crystal, and (iv) empirical relationships between the carrier distribution and superconducting properties including the superconductivity transition temperature, Tc, the magnetic irreversibility field, Hirr, and the so-called peak effect/fishtail phenomenon. Finally, the unified view is summarized and some indespensable questions to be still answered in future work are mentioned.

Control of the charge inhomogeneity and high-Tc superconducting properties in homologous series of multi-layered copper oxides

The mechanism of superconductivity in cuprates remains one of the big challenges of condensed matter physics. High-Tc cuprates crystallize into a layered perovskite structure featuring copper oxygen octahedral coordination. Due to the Jahn Teller effect in combination with the strong static Coulomb interaction, the octahedra in high-Tc cuprates are elongated along the c axis, leading to a 3dx2-y2 orbital at the top of the band structure wherein the doped holes reside. This scenario gives rise to 2D characteristics in high-Tc cuprates that favor d-wave pairing symmetry. Here, we report superconductivity in a cuprate Ba2CuO4-y, wherein the local octahedron is in a very exceptional compressed version. The Ba2CuO4-y compound was synthesized at high pressure at high temperatures and shows bulk superconductivity with critical temperature (Tc) above 70 K at ambient conditions. This superconducting transition temperature is more than 30 K higher than the Tc for the isostructural counterparts based on classical La2CuO4. X-ray absorption measurements indicate the heavily doped nature of the Ba2CuO4-y superconductor. In compressed octahedron, the 3d3z2-r2 orbital will be lifted above the 3dx2-y2 orbital, leading to significant 3D nature in addition to the conventional 3dx2-y2 orbital. This work sheds important light on advancing our comprehensive understanding of the superconducting mechanism of high Tc in cuprate materials.

https://www.pnas.org/content/pnas/116/25/12156.full.pdf

Superconductivity in a unique type of copper oxide

Syntheses and characterizations of complex perovskite oxynitrides LaMg1/3Ta2/3O2N, LaMg1/2Ta1/2O5/2N1/2, and BaSc0.05Ta0.95O2.1N0.9

RECENT work1,2has shown that superconducting copper oxyhalides can be synthesized, with the La2CuO4structure but with no oxygen in the ‘apical’ positions outside the CuO2 planes. For these materials to be rendered superconducting, charge carriers must be introduced into the CuO2 planes; in previous work, positive carriers (holes) have been introduced by the incorporation of interstitial fluorine in Sr2CuO2F2+δ (transition temperature Tc = 46 K)1, or by the substitution of sodium for calcium in (Ca,Na)2CuO2Cl2 (Tc = 26 K)2. Here we present an alternative doping approach for this copper oxyhalide family: holes are introduced by partially replacing the (monovalent) halogen that occupies the apical sites with (divalent) oxygen. We have obtained bulk superconductivity with Tc above 77 K in a new double-layer compound (Sr,Ca)3Cu2O4+δCl2-y, which is isostructural with (La,Sr)2-CaCu2O6 (ref. 3), with (Sr,Ca) at the (La,Sr) or Ca sites, and Cl at the apical sites outside the CuO2 planes.

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Superconductivity at 80 K in (Sr,Ca) 3 Cu 2 O 4 +δCl 2-y induced by apical oxygen doping

A new superconducting cuprate containing chlorine, (Sr,Ca)3Cu2O4+δCl2-y, has been synthesized using a high-pressure technique. Bulk superconductivity below 80 K has been observed. To render superconductivity in this compound, a new p-type charge carrier doping mechanism has been established by a partial substitution of oxygen for apical chlorine, namely by the “apical oxygen doping”. This demonstrates, for the first time, that a p-type doping can be achieved by anion substitution in high-Tc cuprates.

A New Approach to Hole-doping into Halo-oxocuprates —“Apical Oxygen Doping”—

Nearly single-phase Pb-1223 and a new Pb-based cuprate superconductor, Pb-1234, have been successfully synthesized in oxidizing atmosphere using a high-pressure technique. These phases belong to a homologous cuprate series which consists of “Pb-12(n-1)n” phases where n represents the number of CuO2 sheets sandwiched between two rock-salt sheets in which Pb is contained. It is found that more strict control of oxidizing atmosphere is necessary to synthesize the n=3 and n=4 phases than that required for the n=2 phase. As-prepared Pb-1234 sample exhibits a T c 107 K, which is slightly lower than that of Pb-1223.

N. Koshizuka

Papers

Superconductivity at 80 K in (Sr,Ca) 3 Cu 2 O 4 +δCl 2-y induced by apical oxygen doping

A New Approach to Hole-doping into Halo-oxocuprates —“Apical Oxygen Doping”—

Pb-based Cuprate Superconductor with the 1223 Structure