Structural and Electronic Effect Driven Distortions in Visible Light Absorbing Polar Materials ATa2V2O11 (A = Sr, Pb).

Complex vanadates of tantalum(V), such as ATa2V2O11 (A = Sr, Pb), are rare and underrated materials, which have potential application domains that could be substantially expanded, mitigating the existing controversy on their atomic and electronic organization. Herein, we present a thorough structural examination combining synchrotron powder X-ray diffraction-aided distortion mode analysis with computational methods to study hettotypes of SrTa2V2O11 (STVO) and PbTa2V2O11 (PTVO). Being distinct from the perovskite family due to the presence of [VO4] groups, both compounds are polar dielectric materials with certain similarities to SBT and PBT Aurivillius phases. Applying the model of anions of metallic matrices to the analysis of electron localization functions calculated on top of as-established equilibrium structures helps retrace the effects in the Sr and Pb surroundings on the respective crystal packings of STVO and PTVO.

Na4Ni3P4O15-Ni(OH)2 core-shell nanoparticles as hybrid electrocatalysts for the oxygen evolution reaction in alkaline electrolytes.

There is wide interest in developing efficient, robust and low-cost electrode materials for the electrolysis of water to produce clean hydrogen fuel. It is especially important to improve the performance and durability of electrocatalysts for the OER. Here we have shown that the transformation of nanoparticle (n-NNP) and crystalline (c-NNP) forms of mixed phosphate Na4Ni3(PO4)2P2O7 in highly alkaline solutions occurs along various routes and provokes the generation of 2D Ni(OH)2 nanosheets or stable core(phosphate)-shell(Ni(OH)2) particles, respectively. In both cases, in the carbon matrix (through chemical and electrochemical conversion of phosphate in situ during electrolysis in a 6 M KOH or NaOH solution) stable OER electrocatalysts with low overpotentials of 250-290 mV at a current density of 10 mA cm-2 were obtained. The best candidate for the OER process is core-shell particles, which maintain overpotentials of around 250 mV in 6 M KOH for more than 3 days. The activity enhancement can be attributed to the formation of abundant NiOOH nanoparticles on the shell surface due to improved lattice matching. This report discusses future prospects for the creation of core-shell particles to reduce the overpotential of durable electrocatalysts for the OER.

CoOx(OH)y/C nanocomposites in situ derived from Na4Co3(PO4)2P2O7 as sustainable electrocatalysts for water splitting.

The great interest in developing efficient and stable bifunctional electrodes for electrocatalytic water splitting is due to the rapid growth in demand for sustainable and renewable energy sources. Here, we present an original electrode design strategy which can be used for the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). This strategy is based on the direct formation of active catalytic material on the electrode surface during electrolysis. In our case, cobalt hydroxides (ß-Co(OH)2 and CoO(OH) for HER and OER, respectively) in a carbon matrix are produced by the chemical and electrochemical transformation of various forms (glass, glass-ceramic and polycrystalline) of Na4Co3(PO4)2P2O7 during electrolysis in 1 M NaOH solution. The CoOx(OH)y/C composites demonstrate remarkable stability over time (more than 50 h), and bifunctional catalytic activity with an overall water splitting potential close to 1.8 V at a current density of 10 mA cm-2. The range of overpotentials for OER and HER is 346-365 and 326-369 mV, respectively. These electrodes can invert the OER and HER processes with a total overpotential of 0.6 V in the cell. This remarkable CoO(OH) ↔ Co(OH)2 inversion in the carbon matrix its further utility for creating bifunctional electrodes for long-life water splitting cells.

New complex phosphates Cs3MIIBi(P2O7)2 (MII - Ca, Sr and Pb): synthesis, characterization, crystal and electronic structure.

Herein, the peculiarities of complex phosphate formation in self-fluxes of Cs-MII-Bi-P-O (MII = Ca, Sr, Ba and Pb) systems with Cs/P = 0.7-1.3 at fixed ratios of Bi/P = 0.2 and Bi/MII = 1.0 were studied and discussed. Three novel isostructural diphosphates with the general composition Cs3MIIBi(P2O7)2 (MII = Ca, Sr and Pb) and the original framework topology were synthesized and characterized via single-crystal and powder X-ray diffraction, SEM, DTA, and FTIR- and UV-VIS-spectroscopy. In addition, electronic structure (DFT) and Voronoi-Dirichlet polyhedra (VDP) characteristics calculations and crystallochemical analysis were performed. In the structure of the new compounds, the MIIO7 and BiO6 polyhedra are connected via common oxygen vertices forming infinite helical-like chains, which are linked by P2O7 groups into a 3D-framework with pentagonal tunnels, where the Cs+ cations are located. The structural peculiarities are discussed considering perspectives for the creation of new luminescent materials. The dielectric bandgaps, Eg, of the Cs3MIIBi(P2O7)2 crystals reveal an ∼0.2 eV low-energy shift in the Ca-Sr-Pb sequence of MII cations, which reveals the possibility to tune the optical absorbance spectra of the crystals via the synthesis of solid solutions with various contents of MII cations. The glass-ceramic synthetic approach is also proposed as a convenient method for the creation of new diphosphates, and the applicability of this method is verified for Cs3CaBi(P2O7)2.