The mechanism of high-transition-temperature (high-T(c)) superconductivity in doped copper oxides is an enduring problem. Antiferromagnetism is established as the competing order, but the relationship between the two states in the intervening 'pseudogap' regime has become a central puzzle. The role of the crystal lattice, which is important in conventional superconductors, also remains unclear. Here we report an anomalous increase of the distance between copper oxide planes on cooling, which results in negative thermal volume expansion, for layered ruthenium copper oxides that have been doped to the boundary of antiferromagnetism and superconductivity. We propose that a crossover between these states is driven by spin ordering in the ruthenium oxide layers, revealing a novel mechanism for negative lattice expansion in solids. The differences in volume and lattice strain between the distinct superconducting and antiferromagnetic states can account for the phase segregation phenomena found extensively in low-doped copper oxides, and show that Cooper pair formation is coupled to the lattice. Unusually large variations of resistivity with magnetic field are found in these ruthenium copper oxides at low temperatures through coupling between the ordered Ru and Cu spins.
Cation ordering in Ba3VWO8.5 disrupts long-range oxygen diffusivity parallel to the c-axis resulting in reduced ionic conductivity.
As a pure and sustainable source of power, hydrogen (H2) is the desired chemical candidate for the future energy mix. Water electrolysis has been regarded as an effective method for producing clean and ultrapure hydrogen gas. However, its large-scale applications are hampered by its slow kinetics, particularly due to its slow anodic half-reaction i.e., the oxygen evolution reaction (OER). Another strategy based on chemical-assisted electrocatalytic energy-saving hydrogen production has recently been developed with great potential to address barriers associated with OER. In this case, OER is replaced by organic oxidation reactions that are thermodynamically more favorable, which substantially reduces the voltage required for H2 evolution and also facilitates the co-production of organic value-added products. Oxidation of biomass derivatives, such as alcohols, is the most suitable strategy for producing value-added chemicals with energy-saving hydrogen production. This Review focuses on the characteristics of making electrolytic hydrogen production more cost-efficient by using different alcohols. We have reviewed the fundamentals and key parameters for alcohol-assisted electrochemical hydrogen production and discussed several anodic alcohol oxidation reactions with value-added products. The choice of electrocatalysts, strategies to increase the reaction selectivity, and the possible cell architectures are elaborated in detail.
Soft" reactions, which alter local chemistry and structure while retaining the basic lattice arrangement, are important for synthesizing solids with novel properties. [1,2] For example, new layered oxides LaNiO 2 and SrFeO 2 were prepared by reduction of the corresponding perovskites at temperatures as low as 190 8C. [3,4] Precursors for "soft" chemistry are usually prepared at ambient pressure and sometimes under moderate gas pressures, for example, 10-30 MPa of oxygen gas was used to produce fully oxidized LaNiO 3 and SrFeO 3 perovskites for the latter reactions. "Hard" high-pressure (1-20 GPa) conditions can lock instabilities, such as unusual oxidation states or coordination environments, into dense phases that are metastable when recovered to ambient conditions, thus leading to unusual properties, such as intermetallic charge transfer and negative thermal expansion in LaCu 3 Fe 4 O 12 and BiNiO 3 . [5,6] Hence, it is attractive to explore combined "hardsoft" routes [7] to novel materials by partially relieving the instability of a high-pressure precursor through post-synthesis modification.We have explored "hard-soft" oxide chemistry by investigating the reduction of SrCrO 3 . Cr 4+ usually has tetrahedral coordination in oxides prepared at ambient pressure, and high pressures are needed to generate octahedral environments, for example, when Sr 2 CrO 4 is compressed from the ambient K 2 SO 4 -type to the denser K 2 NiF 4 -type polymorph. [8] Pressures higher than 4 GPa are needed to synthesize SrCrO 3 , which adopts the cubic perovskite structure with Cr 4+ in octahedral coordination. [9,10,11] SrCrO 3 thus offers the possibility for transforming Cr 4+ O 6 octahedra to Cr 4+ O 4 tetrahedra at low temperatures, while preserving the underlying perovskite arrangement, thus generating new structures through a "hard-soft" route.Polycrystalline SrCrO 3 samples were synthesized by a high-pressure technique and reduced by heating in flowing hydrogen gas or in sealed evacuated tubes with calcium hydride (experimental details and additional Tables S1 and S2 and Figures S1-S4 are shown in Supporting Information).Powder X-ray diffraction showed that two crystalline SrCrO 3Àd products were formed between 400 and 500 8C (Figure 1), both with complex diffraction patterns related to that of the cubic precursor. It proved very difficult to isolate single-phase samples of these two new compounds because of their similar compositions and facile re-oxidation to SrCrO 3 . However, both crystal structures were determined from powder diffraction patterns of mixed d = 0/0.2/0.25 phase samples, as described below. The d = 0.25 composition of the final phase was determined by thermogravimetry ( Figure S1) and confirmed by the crystal structure. No thermogravimetric plateau was observed for the intermediate phase and the d = 0.2 composition was deduced from the structural analysis. Attempts to prepare the SrCrO 3Àd phases without use of the high-pressure SrCrO 3 precursor (such as by reduction of the ambient-pressure compound SrCrO 4 ...
RuSr 2 ͑Nd,Y,Ce͒ 2 Cu 2 O 10−␦ ruthenocuprates have been studied by neutron diffraction, and magnetotransport and magnetization measurements, and the electronic phase diagram is reported. Separate Ru and Cu spin ordering transitions are observed, with spontaneous Cu antiferromagnetic order for low hole-doping levels p, and a distinct, induced-antiferromagnetic Cu spin phase in the 0.02Ͻ p Ͻ 0.06 pseudogap region. This ordering gives rise to large negative magnetoresistances which vary systematically with p in the RuSr 2 Nd 1.8−x Y 0.2 Ce x Cu 2 O 10−␦ series. A collapse of the magnetoresistance and magnetization in the presuperconducting region may signify the onset of superconducting fluctuations.
High critical-temperature superconductivity and large ('colossal') magnetoresistances are two important electronic conducting phenomena found in transition metal oxides. High-T c materials have applications such as superconducting magnets for MRI and NMR, and magnetoresistive materials may find use in magnetic sensors and spintronic devices. Here we report chemical doping studies of RuSr 2 (R 2-x Ce x )Cu 2 O 10-δ ruthenocuprates which show that a single oxide system can be tuned between superconductivity at high hole dopings and varied magnetoresistive properties at low doping levels. A robust variation of negative magnetoresistance with hole concentration is found in the RuSr 2 R 1.8-x Y 0.2 Ce x Cu 2 O 10-δ series, while RuSr 2 R 1.1 Ce 0.9 Cu 2 O 10-δ materials show an unprecedented crossover from negative to positive magnetoresistance with rare earth (R) ion radius.Although the mechanism for superconductivity in layered cuprates remains controversial, 1 the chemical tuning of their properties is well-established. Oxidation of the CuO 2 planes suppresses antiferromagnetic order of Cu 2+ S= ½ spins, and induces superconductivity in the doping range p= 0.06-0.25 (the equivalent Cu oxidation states are 2+p). Ruthenocuprates contain distinct RuO 2 and CuO 2 planes, and display coexisting ferromagnetism and superconductivity in both 1212-type (RuSr 2 RCu 2 O 8 ) 2,3 and 1222-type (RuSr 2 (R,Ce) 2 Cu 2 O 10-δ ) 4,5 structures, where R= Sm, Eu, or Gd. Large negative magnetoresistances (change of electrical resistivity ρ in an applied magnetic field H, defined as MR= (ρ(H) -ρ(0))/ρ(0)) have
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