Interplay between Oxo and Fluoro in Vanadium Oxyfluorides for Centrosymmetric and Non-Centrosymmetric Structure Formation

Herein, we report the syntheses of two lithium-vanadium oxide-fluoride compounds crystallized from the same reaction mixture through a time variation experiment. A low temperature hydrothermal route employing a viscous paste of V2O5, oxalic acid, LiF, and HF allowed the crystallization of one metastable phase initially, Li2VO0.55 (H2O)0.45F5·2H2O (I), which on prolonged heating transforms to a chemically similar yet structurally different phase, Li3VOF5 (II). Compound I crystallizes in centrosymmetric space group, I2/a with a = 6.052(3), b = 7.928(4), c = 12.461(6) Å, and β = 103.99(2)⁰ , while compound II crystallizes in a non-centrosymmetric (NCS) space group, Pna21 with a = 5.1173(2), b = 8.612(3), c = 9.346(3) Å. Synthesis of NCS crystals are highly sought after in solid-state chemistry for their second-harmonic-generation (SHG) response and compound II exhibits SHG activity albeit non-phase-matchable. In this article, we also describe their magnetic properties which helped in unambiguous assignment of mixed valency of V (+4/+5) for Li2VO0.55 (H2O)0.45F5·2H2O (I) and +4 valency of V for Li3VOF5 (II)

NdBaScO4: aristotype of a new family of geometric ferroelectrics?

The authors acknowledge the University of St Andrews and the EPSRC (DTG: EP/K503162/1) for a studentship (to CALD). WZ and PSH thank the Welch Foundation (Grant E-1457), NSF DMR-1503573, and the Texas Center for Superconductivity for support. EB thanks the FRS-FNRS for supports and the Consortium des Equipements de Calcul Intensif (CECI), funded by the FRS-FNRS (Grants No. 2.5020.11 and No. 1175545). The research data supporting this publication can be accessed atat http://dx.doi.org/10.17630/197519dd-5b67-49f8-bd50-7f1fb98bec73NdBaScO4 represents the aristotype structure of a new series of -cut layered perovskites; it is suggested that compositional fine-tuning is likely to produce a family of new geometric ferroelectrics, driven primarily by octahedral tilting.PostprintPostprintPeer reviewe

The crystal and defect structures of polar KBiNb2O7

Funding: Experiments at the Diamond Light Source were performed as part of the Block Allocation Group award “Oxford/Warwick Solid State Chemistry BAG to probe composition-structure– property relationships in solids” (CY25166). Experiments at the ISIS pulsed neutron facility were supported by a beam time allocation from the STFC (RB 2000148). SM thanks Somerville College for an Oxford Ryniker Lloyd scholarship. ‘PSH and WZ thank the National Science Foundation (DMR-2002319) and Welch Foundation (Grant E-1457) for support.KBiNb2O7 was prepared from RbBiNb2O7 by a sequence of cation exchange reactions which first convert RbBiNb2O7 to LiBiNb2O7, before KBiNb2O7 is formed by a further K-for-Li cation exchange. A combination of neutron, synchrotron X-ray and electron diffraction data reveal that KBiNb2O7 adopts a polar, layered, perovskite structure (space group A11m) in which the BiNb2O7 layers are stacked in a (0, ½, z) arrangement, with the K+ cations located in half of the available 10-coordinate interlayer cation sites. The inversion symmetry of the phase is broken by a large displacement of the Bi3+ cations parallel to the y-axis. HAADF-STEM images reveal that KBiNb2O7 exhibits frequent stacking faults which convert the (0, ½, z) layer stacking to (½, 0, z) stacking and vice versa, essentially switching the x- and y-axes of the material. By fitting the complex diffraction peak shape of the SXRD data collected from KBiNb2O7 it is estimated that each layer has approximately a 9% chance of being defective-a high level which is attributed to the lack of cooperative NbO6 tilting in the material, which limits the lattice strain associated with each fault.Publisher PDFPeer reviewe

RbFe2+Fe3+F6: Synthesis, Structure, and Characterization of a New Charge-Ordered Magnetically Frustrated Pyrochlore-Related Mixed-Metal Fluoride

A new charge-ordered magnetically frustrated mixed-metal fluoride with a pyrochlore-related structure has been synthesized and characterized. The material, RbFe2F6 (RbFe2+Fe3+F6) was synthesized through mild hydrothermal conditions. The material exhibits a three-dimensional pyrochlore-related structure consisting of corner-shared Fe2+F6 and Fe3+F6 octahedra. In addition to single crystal diffraction data, neutron powder diffraction and magnetometry measurements were carried out. Magnetic data clearly reveal strong antiferromagnetic interactions (a Curie-Weiss temperature of -270 K) but sufficient frustration to prevent ordering until 16 K. No structural phase transformation is detected from the variable temperature neutron diffraction data. Infrared, UV -vis, thermogravimetric, and differential thermal analysis measurements were also performed. First-principles density functional theory (DFT) electronic structure calculations were also done. Crystal data: RbFe2F6, orthorhombic, space group Pnma (No. 62), a = 7.0177(6) {\AA}, b = 7.4499(6) {\AA}, c = 10.1765(8) {\AA}, V = 532.04(8) {\AA}3, Z = 4

Bibliografía

Reaction of the anion-deficient, cation-ordered perovskite phase Ba₂YFeO₅ with 80 atm of oxygen pressure at 410 °C results in the formation of the Fe⁴⁺ phase Ba₂YFeO_5.5. The topochemical insertion of oxide ions lifts the inversion symmetry of the centrosymmetric host phase, Ba₂YFeO₅ (space group <i>P</i>2₁/<i>n</i>), to yield a noncentrosymmetric (NCS) phase Ba₂YFeO_5.5 (space group <i>Pb</i>2₁<i>m</i> (No. 26), <i>a</i> = 12.1320(2) Å, <i>b</i> = 6.0606(1) Å, <i>c</i> = 8.0956(1) Å, <i>V</i> = 595.257(2) ų) confirmed by the observation of second-harmonic generation. Dielectric and PUND ferroelectric measurements, however, show no evidence for a switchable ferroelectric polarization, limiting the material to pyroelectric behavior. Magnetization and low-temperature neutron diffraction data indicate that Ba₂YFeO_5.5 undergoes a magnetic transition at 20 K to adopt a state which exhibits a combination of ferromagnetic and antiferromagnetic order. The symmetry breaking from centrosymmetric to polar noncentrosymmetric, which occurs during the topochemical oxidation process is discussed on the basis of induced lattice strain and an electronic instability and represents a new strategy for the preparation of NCS materials that readily incorporate paramagnetic transition metal centers

Polarity and ferromagnetism in two-dimensional hybrid copper perovskites with chlorinated aromatic spacers

We would like to thank the China Scholarships Council for a studentship to CH. PSH and WZ thank the Welch Foundation (Grant E-1457) and the National Science Foundation (DMR-2002319) for support.Two-dimensional (2D) organic−inorganichybrid copper perovskites have drawn tremendous attention as promisingmultifunctional materials. Herein, by incorporating ortho, metaand para-chlorine substitution in the benzylamine structure, we firstlyreport theinfluence of positional isomerism on the crystal structures of chlorobenzylammonium copper (II) chloride perovskites A2CuCl4. 2Dpolar ferromagnets (3-ClbaH)2CuCl4 and (4-ClbaH)2CuCl4(ClbaH+ = chlorobenzylammonium) are successfully obtained. They bothadopt a polar monoclinic space group Cc at room temperature, displayingsignificant differences in crystal structures. In contrast, (2-ClbaH)2CuCl4adopts a centrosymmetric space group P21/c at roomtemperature. This associated structural evolution successfullyenhances the physical properties of the two polar compounds with high thermalstability, discernible second harmonic generation (SHG) signals, ferromagnetism,and narrowoptical band gaps.These findings demonstrate that theintroduction of chlorine atoms into the inter-layer organicspecies is a powerful tool to tune crystalsymmetries and physical properties, and this inspires further exploration of designinghigh-performance multifunctional copper-based materials.Publisher PDFPeer reviewe

Polar and Magnetic Mn2FeMO6 (M = Nb, Ta) with LiNbO3-type Structure - High Pressure Synthesis

Polar oxides are of much interest in materials science and engineering. Their symmetry-dependent properties such as ferroelectricity/multiferroics, piezoelectricity, pyroelectricity, and second-order harmonic generation (SHG) effect are important for technological applications. However, polar crystal design and synthesis is challenging, because multiple effects, such as steric or dipole-dipole interactions, typically combine to form non-polar structures; so the number of known polar materials, especially polar magnetoelectric materials, is still severely restricted. Therefore, it is necessary for the material science community to develop new strategies to create these materials

Polarizable Anionic Sublattices Can Screen Molecular Dipoles in Noncentrosymmetric Inorganic-Organic Hybrids

We report the growth and photophysical characterization of two polar hybrid lead halide phases, methylenedianiline lead iodide and bromide, (MDA)Pb2I6 and (MDA)Pb2Br6, respectively. The phases crystallize in noncentrosymmetric space group Fdd2, which produces a highly oriented molecular dipole moment that gives rise to second harmonic generation (SHG) upon excitation at 1064 nm. While both compositions are isostructural, the size dependence of the SHG signal suggests that the bromide exhibits a stronger phase-matching response whereas the iodide exhibits a significantly weaker non-phase-matching signal. Similarly, fluorescence from (MDA)Pb2Br6 is observed around 630 nm below 75 K whereas only very weak luminescence from (MDA)Pb2I6 can be seen. We attribute the contrasting optical properties to differences in the character of the halide sublattice and postulate that the increased polarizability of the iodide ions acts to screen the local dipole moment, effectively reducing the local electric field in the crystals

The role of Zn-OR and Zn-OH nucleophiles and the influence of para-substituents in the reactions of binuclear phosphatase mimetics

Analogues of the ligand 2,2'-(2-hydroxy-5-methyl-1,3-phenylene)bis(methylene)bis((pyridin-2-ylmethyl)azanediyl)diethanol (CH(3)H(3)L1) are described. Complexation of these analogues, 2,6-bis(((2-methoxyethyl)(pyridin-2-ylmethyl)amino)methyl)-4-methylphenol (CH(3)HL2), 4-bromo-2,6-bis(((2-methoxyethyl)(pyridin-2-ylmethyl)amino)methyl)phenol (BrHL2), 2,6-bis(((2-methoxyethyl)(pyridin-2-ylmethyl)amino)methyl)-4-nitrophenol (NO(2)HL2) and 4-methyl-2,6-bis(((2-phenoxyethyl)(pyridin-2-ylmethyl)amino)methyl)phenol (CH(3)HL3) with zinc(II) acetate afforded [Zn-2(CH(3)L2)(CH3COO)(2)](PF6), [Zn-2(NO(2)L2)(CH3COO)(2)](PF6), [Zn-2(BrL2)(CH3COO)(2)](PF6) and [Zn-2(CH(3)L3)(CH3COO)(2)](PF6), in addition to [Zn-4(CH(3)L2)(2)(NO2C6H5OPO3)(2)(H2O)(2)](PF6)(2) and [Zn-4(BrL2)(2)(PO3F)(2)(H2O)(2)](PF6)(2). The complexes were characterized using H-1 and C-13 NMR spectroscopy, mass spectrometry, microanalysis, and X-ray crystallography. The complexes contain either a coordinated methyl-(L2 ligands) or phenyl-(L3 ligand) ether, replacing the potentially nucleophilic coordinated alcohol in the previously reported complex [Zn-2(CH(3)HL1)(CH3COO)(H2O)](PF6). Functional studies of the zinc complexes with the substrate bis(2,4-dinitrophenyl) phosphate (BDNPP) showed them to be competent catalysts with, for example, [Zn-2(CH(3)L2)](+), k(cat) = 5.70 +/- 0.04 x 10(-3) s(-1) (K-m = 20.8 +/- 5.0 mM) and [Zn-2(CH(3)L3)](+), kcat = 3.60 +/- 0.04 x 10(-3) s(-1) (K-m = 18.9 +/- 3.5 mM). Catalytically relevant pK(a)s of 6.7 and 7.7 were observed for the zinc(II) complexes of CH(3)L2(-) and CH(3)L3(-), respectively. Electron donating para-substituents enhance the rate of hydrolysis of BDNPP such that k(cat) p-CH3 > p-Br > p-NO2. Use of a solvent mixture containing H2O18/H2O16 in the reaction with BDNPP showed that for [Zn-2(CH(3)L2)(CH3COO)(2)](PF6) and [Zn-2(NO(2)L2)(CH3COO)(2)](PF6), as well as [Zn-2(CH(3)HL1)(CH3COO)(H2O)](PF6), the O-18 label was incorporated in the product of the hydrolysis suggesting that the nucleophile involved in the hydrolysis reaction was a Zn-OH moiety. The results are discussed with respect to the potential nucleophilic species (coordinated deprotonated alcohol versus coordinated hydroxide)