Thermal Behaviour of Some Polyhydrocarboxylic Coordination Compounds with Neodium

New Nd-Co based polynuclear coordination compounds containing as ligand polyhydrocarboxylic acid as tartaric, malic and gluconic acids were prepared, namely: [NdCo(tart)₃]·4H₂O, (NH₄)[NdCo_0.5Cu_0.5(tart)₃]·4H₂O, (NH₄)[NdCo(malic)₃]·4H₂O, (NH₄)[NdCo_0.5Cu_0.5(malic)₃]·4H₂O, [NdCo(gluc)₄]·4H₂O and [Nd₂CoCu(gluc)₇]·5H₂O. A comparison between the thermal behaviour of the studied polynuclear coordination compounds concerning thermal stability and thermal decomposition stoichiometry was inferred. Oxalic and malonic intermediates were identified at about 300°C in the thermal decomposition of tartaric and malic compounds. In all the decomposition processes at about 400°C the presence of oxocarbonate is shown. The residual products are mixed oxides of perovskite type. This revised version was published online in August 2006 with corrections to the Cover Date.     

<Superscript>63</Superscript>Cu and <Superscript>139</Superscript>La NMR and chemical bonding in anion-deficient perovskite LACu<Subscript>0.81</Subscript>Ni<Subscript>0.19</Subscript>O<Subscript>2.5+δ</Subscript>

The ⁶³Cu and ¹³⁹La NMR spectra of the low-(δ = 0.25) and high-temperature (δ = 0) phases of lanthanum cuprate LACu_0.81Ni_0.19O_2.5+δ have been obtained. Quantum chemical calculations of the electronic structure of the high-temperature phase have been performed. It is found that the observed metal-semiconductor phase transition is attended by changes in the degree of atomic ordering and in the character of electronic conductivity over certain types of “copper” centers.     

Similar Structural Dynamics for the Degradation of CH3NH3PbI3 in Air and in Vacuum

We investigate the degradation path of MAPbI₃ (MA=methylammonium) films over flat TiO₂ substrates at room temperature by means of X‐ray diffraction, spectroscopic ellipsometry, X‐ray photoelectron spectroscopy, and high‐resolution transmission electron microscopy. The degradation dynamics is found to be similar in air and under vacuum conditions, which leads to the conclusion that the occurrence of intrinsic thermodynamic mechanisms is not necessarily linked to humidity. The process has an early stage, which drives the starting tetragonal lattice in the direction of a cubic atomic arrangement. This early stage is followed by a phase change towards PbI₂. We describe how this degradation product is structurally coupled with the original MAPbI₃ lattice through the orientation of its constituent PbI₆ octahedra. Our results suggest a slight octahedral rearrangement after volatilization of HI+CH₃NH₂ or MAI, with a relatively low energy cost. Our experiments also clarify why reducing the interfaces and internal defects in the perovskite lattice enhances the stability of the material.     

Conductivity‐Dependent Completion of Oxygen Reduction on Oxide Catalysts

The electric conductivity‐dependence of the number of electrons transferred during the oxygen reduction reaction is presented. Intensive properties, such as the number of electrons transferred, are difficult to be considered conductivity‐dependent. Four different perovskite oxide catalysts of different conductivities were investigated with varying carbon contents. More conductive environments surrounding active sites, achieved by more conductive catalysts (providing internal electric pathways) or higher carbon content (providing external electric pathways), resulted in higher number of electrons transferred toward more complete 4e reduction of oxygen, and also changed the rate‐determining steps from two‐step 2e process to a single‐step 1e process. Experimental evidence of the conductivity dependency was described by a microscopic ohmic polarization model based on effective potential localized nearby the active sites.     

Molten Ag2SO4‐based Ion‐Exchange Preparation of Ag0.5La0.5TiO3 for Photocatalytic O2 Evolution

Ag_0.5La_0.5TiO₃ with an ABO₃ perovskite structure was synthesized by a newly developed ion‐exchange method. Molten Ag₂SO₄ instead of traditional molten AgNO₃ was used as Ag⁺ source in view of its high decomposition temperature (1052 °C), thereby guaranteeing the complete substitution of Ag⁺ for Na⁺ in Na_0.5La_0.5TiO₃ with a stable ABO₃ perovskite structure at a high ion‐exchange temperature (700 °C). Under full‐arc irradiation, the O₂‐evolution activity of Ag_0.5La_0.5TiO₃ was about 1.6 times that of Na_0.5La_0.5TiO₃ due to the optimized electronic band structures and local lattice structures. On the one hand, the substitution of Ag⁺ for Na⁺ elevated the VBM and thus narrowed the band gap from 3.19 to 2.83 eV, thereby extending the light‐response range and, accordingly, enhancing the photoexcitation to generate more charge carriers. On the other hand, the substitution of Ag⁺ for Na⁺ induced a lattice distortion of the ABO₃ perovskite structure, thereby promoting the separation and migration of charge carriers. Moreover, under visible‐light irradiation, Ag_0.5La_0.5TiO₃ displayed notable O₂ evolution whereas Na_0.5La_0.5TiO₃ showed little O₂ evolution, thus demonstrating that the substitution of Ag⁺ for Na⁺ enabled the use of visible light to evolve O₂ photocatalytically. This work presents an effective route to explore novel Ag‐based photocatalysts.     

A Multiple Structure‐Design Strategy towards Ultrathin Niobate Perovskite Nanosheets with Thickness‐Dependent Photocatalytic Hydrogen‐Evolution Performance

Hydrogen production by catalytic water splitting using sunlight holds great promise for clean and sustainable energy source. Despite the efforts made in the past decades, challenges still exist in pursuing solid catalysts with light‐harvesting capacity, large surface areas and efficient utilities of the photogenerated carrier, at the same time. Here, a multiple structure design strategy leading to highly enhanced photocatalytic performance on hydrogen production from water splitting in Dion–Jacobson perovskites KCa₂Na_{n ‐3}Nb_n O_{3n +1} is described. Specifically, chemical doping (N/Nb⁴⁺) of the parent oxides via ammoniation improved the ability of sunlight harvesting efficiently; subsequent liquid exfoliation of the doped perovskites yielded ultrathin [Ca₂Na_{n ‐3}Nb_n O_{3n +1}]⁻ nanosheets with greatly increased surface areas. Significantly, the maximum hydrogen evolution appears in the n =4 nanosheets, which suggests the most favorable thickness for charge separation in such perovskite‐type catalysts. The optimized black N/Nb⁴⁺‐[Ca₂NaNb₄O₁₃]⁻ nanosheets show greatly enhanced photocatalytic performance, as high as 973 μmol h⁻¹ with Pt loading, on hydrogen evolution from water splitting. As a proof‐of‐concept, this work highlights the feasibility of combining various chemical strategies towards better catalysts and precise thickness control of two‐dimensional materials.     

有机无机杂化固态太阳能电池的研究进展

近年来, 由于钙钛矿材料优良的光学吸收和电荷传导特性, 有机无机杂化固态太阳能电池取得了突破性的进展. 自2009年首次报道了光电转换效率为3.8%的钙钛矿太阳能电池以来, 该类电池的效率不断突破. 基于介孔薄膜的电池已取得了超过16.7%的认证光电转换效率, 基于平板异质结结构电池光电转换效率达到19.3%, 已接近传统硅基太阳能电池的光电转换效率. 本文将介绍有机无机杂化钙钛矿作为光电材料的光学物理结构特性, 以及在固态太阳能电池中的应用. 基于固态钙钛矿太阳能电池结构上的差异, 分别介绍其在多孔结构、平板异质结结构、柔性结构以及无空穴传导材料结构电池工作特性和各自优势, 以及影响电池特性的主要影响因素, 特别是钙钛矿成膜控制等. 并阐述对钙钛矿电池的理解和进一步提高固态钙钛矿电池光电转换效率需要关注的重点以及展望.     

First-principles investigation of A-B intersite charge transfer and correlated electrical and magnetic properties in BiCu3Fe4O12

First-principles calculations using the augmented plane wave plus local orbitals method, as implemented in the WIEN2K code, have been carried out to study the A-B intersite charge transfer and the correlated electrical and magnetic properties of the perovskite BiCu(3)Fe(4)O(12), especially as regards the charge transfer. The results indicate that the charge transfer between A-site Cu and B-site Fe is by way of O 2p orbitals, and during this process orbital hybridization plays an important role. More importantly, the charge transfer is of 3d(9) + 4d(5)L(0.75) →3d(9)L + 4d(5) type (here L denotes an oxygen hole or a ligand hole). During this process, the magnetic interaction experiences a transition from Cu-Fe ferrimagnetic coupling to G-type antiferromagnetic coupling within B-site Fe with paramagnetic Cu(3+). As to electrical property, it undergoes a metal to insulator transition. All our calculated results are consistent with the available experimental results.     

Structural phase transition and magnetic properties of layered organic-inorganic hybrid compounds: p-Haloanilinium tetrachlorocuparate(II)

Compounds of the general formula A₂CuCl₄, (where A = 4-fluoroanilinium (1) and 4-chloroanilinium (2)) were prepared, structurally characterized and their thermal and magnetic properties studied. These compounds have a layered structure, distorted perovskite, where layers of CuCl₄²⁻ are sandwiched between a 4-haloanilinium cation bilayer. A single crystal X-ray diffraction study on (4-fluoroanilinium)₂CuCl₄, (1), shows that it crystallizes in the monoclinic P2₁/c space group with cell dimensions a = 15.5113(5) Å, b = 7.3788(2) Å, c = 7.0929(2) Å, β = 99.004(2)°, volume 801.81(4) Å³ at 150 K. Compound 2, (4-chloroanilinium)₂CuCl₄, crystallizes isostructurally to 1 at RT, but at 150 K it adopts the Pccn space group. This structural transition for 2 is reversible, and has been observed using Differential Scanning Calorimetric (DSC) measurements. The dc-magnetic studies using a SQUID magnetometer suggest that both compounds are soft ferromagnets and show an onset of long range magnetic ordering below 9 K. The ac-susceptibility measurements confirm the presence of this ferromagnetic ordering in both the compounds.     

The ferroelectric phase of CdTiO3: A powder neutron diffraction study

The synthesis of bulk samples of polycrystalline CdTiO₃ in both the rhombohedral ilmenite and orthorhombic perovskite forms is described and the structures of these have been refined using powder neutron diffraction data. This involved the preparation of samples enriched in ¹¹⁴Cd. Cooling perovskite-type CdTiO₃ to 4 K induces a ferroelectric phase transition, with the neutron data suggesting the low temperature structure is in Pna2₁. Mode analysis shows the polar mode to be dominant at low temperatures. The ilmenite-structure of CdTiO₃ is compared with that of ZnTiO₃. The refined scattering length of the ¹¹⁴Cd is estimated to be 5.56 fm. Attempts to dope CdTiO₃ with Ca and Sr are described.