Christophe Payen

DFT-NMR Investigation and 51V 3QMAS Experiments for Probing Surface OH Ligands and the Hydrogen-Bond Network in a Polyoxovanadate Cluster: The Case of Cs4[H2V10O28]·4H2O

This work shows that the combination of first-principles calculations and (51)V NMR experiments is a powerful tool to elucidate the location of surface hydroxyl groups and to precisely describe the hydrogen bond network in the complex decavanadate cluster Cs(4)[H(2)V(10)O(28)].4H(2)O, enhancing the strength of NMR crystallography. The detailed characterization of H-bond networks for these kinds of inorganic compounds is of primary importance and should benefit from the DFT-NMR predictions by considering explicitly the periodic boundary conditions. The determination of the Cs(4)[H(2)V(10)O(28)].4H(2)O structure by single-crystal X-ray diffraction was not sufficiently accurate to provide the location of protons. From available diffraction data, five different protonated model structures have been built and optimized using DFT-based methods. The possible interconversion of two decavanadate isomers through a proton exchange is evaluated by calculating the energy barrier and recording variable-temperature (1)H MAS NMR spectra. First-principles calculations of (51)V NMR parameters clearly indicate that these parameters are very sensitive to the local intermolecular hydrogen-bonding interactions. Considering the DFT error limits, the fairly good agreement between calculated and experimental NMR parameters arising from the statistical modeling of the data allows the unambiguous assignment of the five (51)V NMR signals and, thus, the location of OH surface ligands in the decavanadate cluster. In particular, first-principles calculations accurately reproduce the (51)V quadrupolar parameters. These results are fully consistent with (51)V 3QMAS NMR spectra recorded with and without (1)H decoupling. Finally, correlations are established between local octahedral VO(6) deformations and (51)V NMR parameters (C(q) and Deltadelta), which will be useful for the characterization of a wide range of chemical species containing vanadium(V).

Effect of Nonmagnetic Substituents Mg and Zn on the Phase Competition in the Multiferroic Antiferromagnet MnWO4

The effects of substituting nonmagnetic Mg2+ and Zn2+ ions for the Mn2+ (S = 5/2) ions on the structural, magnetic, and dielectric properties of the multiferroic frustrated antiferromagnet MnWO4 were investigated. Polycrystalline samples of Mn1−xMgxWO4 and Mn1−xZnxWO4 (0 ≤ x ≤ 0.3) solid solutions were prepared via a solid-state route and characterized via X-ray and neutron diffraction, magnetization, and dielectric permittivity measurements. Mg and Zn substitutions result in very similar effects. The Neel temperature (TN), the AF3-to-AF2 magnetic phase-transition temperature (T2), and the critical ferroelectric temperature (Tc = T2) of MnWO4 are reduced upon nonmagnetic doping. At the lowest temperature (T = 1.5 K), the incommensurate magnetic structure for x(Mg) = 0.15 and x(Zn) = 0.15 corresponds to either a sinusoidal spin arrangement or an elliptical spin-spiral phase similar to the polar AF2 structure observed in MnWO4. These findings were discussed by considering the effects of the Mg and Zn substi...

Frustrated magnetism in the S = 1 kagomé lattice BaNi3(OH)2(VO4)2.

Frustrated magnets with integer spin are predicted to have exotic physical properties including spin nematicity, yet few are known to exist. We report a new, frustrated S = 1 magnet, BaNi(3)(OH)(2)(VO(4))(2), which is the structural analogue of the mineral vesignieite. Magnetic frustration arises from a competition between ferromagnetic and antiferromagnetic ordering leading to a glassy transition at 19 K.

A new family of 2D antiferromagnets: the layered phosphonates MII (RPO3) · H2O; M Mn, Fe, Co, Ni; R = alkyl, phenyl

Abstract Magnetic susceptibility data are reported for the divalent metal phosphonates II (RPO 3 ) · H 2 O (M  Mn, Fe, Co, Ni; R = alkyl, phenyl). The quasi 2D square lattice formed by the M II ions within these compounds leads to 2D antiferromagnetic properties.

New manganese pyrophosphates: The syntheses, crystallographic characterizations and magnetic properties of BaMnP2O7 and CaMnP2O7

Abstract The new compound BaMnP2O7 was obtained in two crystallographic modifications, (monoclinic-1) and (triclinic-2), by heating mixtures of BaCO3, P2O5 and MnO2 to 1100°C and to 1000°C for 72 h, respectively. CaMnP2O7 (3) was obtained by heating a mixture of CaCO3, P2O5 and MnO2 to 1050°C for 48 h. Both crystallographic forms of BaMnP2O7 and CaMnP2O7 (3) were investigated by single-crystal X-ray diffraction analysis. The high-temperature monoclinic form of BaMnP2O7 could not be obtained free of the low-temperature triclinic form in the bulk form. In monoclinic-1 the manganese ions exist in a distorted MnO6 octahedron surrounded by five closely and one remotely positioned oxygen atoms. In triclinic-2 and -3 forms the manganese ions are associated in pairs by the formation of Mn2O10 units that share one edge of two adjacent octahedra. The magnetic properties of the triclinic-2 and -3 forms were also investigated. The effective magnetic moments, μeff, are 5.7 B.M. and 5.8 B.M./Mn atom for triclinic-2 and -3, respectively, and are consistent with a high-spin Mn2+ ion in an octahedral environment with five unpaired electrons. The temperature-dependent magnetic measurements of 2 and 3 have revealed a combination of short-range antiferromagnetic coupling, J, between the two Mn ions within the Mn2O10 units and a longer range weaker antiferromagnetic coupling, J′, between the neighbouring Mn2O10 units, |J′/J| = 0.18 and 0.074 for 2 and 3, respectively.

A new mixed metal titanate: The synthesis and characterization of Ba2Fe2Ti4O13

Abstract The new mixed metal titanate Ba 2 Fe 2 Ti 4 O 13 ( 1 ) was obtained in crystalline form by heating a mixture of BaCO 3 , Fe 3 O 4 and TiO 2 at 1350°C for 48 h. Compound 1 was characterized by single-crystal X-ray diffraction methods. It is isomorphous to the related known compounds Na 2 Ti 6 O 13 and Ba 2 NiTi 5 O 13 . The X-ray diffraction and Mossbauer measurements of 1 obtained at room temperature indicate that the iron ions are unequally distributed between the three independent sites occupied by the transition metal ions.

BaCo2Si2O7 : a new one-dimensional antiferromagnet based on chains of oxide bridged CoO4 tetrahedra

Abstract The new compound BaCo 2 Si 2 O 7 was obtained by heating the mixture BaCO 3 , CoCO 3 and SiO 2 to 1300°C for 48 h. The material was characterized by single-crystal X-ray diffraction and magnetic susceptibility measurements. Each cobalt ion is surrounded by four oxygen atoms arranged in a distorted tetrahedral shape. These tetrahedral are linked into chains through the sharing of one of the oxygen atoms. The CoO 4 chains are cross-linked by bridging Si 2 O 7 groups. The barium ions occupy channels that run parallel to the crystallographic c -axis. The effective magnetic moment, μ eff = 4.70 B.M., at 25°C is consistent with Co 2+ in a tetrahedral environment having three unpaired electrons. Anti-ferromagnetic coupling of the electrons occurs between the nearest neighbour cobalt ions, as indicated by the sign of the Weiss constant, θ p = −46 K, and by the presence of a broad maximum in the susceptibility curve. At 26 K it undergoes a three-dimensional magnetic transition into a field-induced weak ferromagnet that is characteristic of a one-dimensional metamagnet.

Magnetic Structure and Ferroelectric Polarization of MnWO4 Investigated by Density Functional Calculations and Classical Spin Analysis

The ordered magnetic states of MnWO4 at low temperatures were examined by evaluating the spin exchange interactions between the Mn2+ ions of MnWO4 on the basis of first principles density functional calculations and by performing classical spin analysis with the resulting spin exchange parameters. Our work shows that the spin exchange interactions are frustrated within each zigzag chain of Mn2+ ions along the c direction and between such chains of Mn2+ ions along the a direction. This explains the occurrence of a spiral-spin order along the c and a directions in the incommensurate magnetic state AF2, and that of a up arrow up arrow down arrow down arrow spin order along the c and a directions in the commensurate magnetic state AF1. The ferroelectric polarization of MnWO4 in the spiral-spin state AF2 was examined by performing Berry phase calculations for a model superstructure to find that the ferroelectric polarization occurs along the b direction, in agreement with experiment.

Novel structural arrangement for divalent metal phosphonates: synthesis of tert-butylphosphonates and structure of Co[(CH3)3CPO3].H2O

The synthesis and characterization of new divalent metal tert-butylphosphonates are described: MII[(CH3)3CPO3]·xH2O [MII= Co, Mn (x= 1) MII= Zn (x= 2/3)] and Cu1.75O0.75[(CH3)3CPO3]·H2O. Co[(CH3)3CPO3]·H2O crystallizes in the monoclinic space group P21/c with a= 12.256(1)A, b= 17.939(1)A, c= 10.769(1)A, β= 93.57(1)°, V = 2363.1 (4)A3, Z= 12, with R= 0.064 and Rw= 0.075 for 1805 observed reflections, according to the criterion I > 3σ(I). The layer arrangement of this new phase is different from that observed in the case of the n-alkyl MII( RPO3)·H2O analogues owing to the steric effect of the bulky tert-butyl group. The structural relationship between the cobalt, manganese and zinc compounds is discussed. The observed magnetic moments are consistent with the presence of high-spin MnII or CoII and with the existence of both octahedral and tetrahedral environments about these metals.

Incorporation of Jahn-Teller Cu(2+) Ions into Magnetoelectric Multiferroic MnWO4: Structural, Magnetic, and Dielectric Permittivity Properties of Mn1-xCuxWO4 (x ≤ 0.25).

Polycrystalline samples of Mn1-xCuxWO4 (x ≤ 0.5) have been prepared by a solid-state synthesis as well as from a citrate synthesis at moderate temperature (850 °C). The goal is to study changes in the structural, magnetic, and dielectric properties of magnetoelectric type-II multiferroic MnWO4 caused by replacing Jahn-Teller-inactive Mn(2+) (d(5), S = 5/2) ions with Jahn-Teller-active Cu(2+) (d(9), S = 1/2) ions. Combination of techniques including scanning electron microscopy, powder X-ray and neutron diffraction, and Raman spectroscopy demonstrates that the polycrystalline samples with low copper content 0 ≤ x ≤ 0.25 are solid solution that forms in the monoclinic P2/c space group. Rietveld analyses indicate that Cu atoms substitutes for Mn atoms at the Mn crystallographic site of the MnWO4 structure and suggest random distributions of Jahn-Teller-distorted CuO6 octahedra in the solid solution. Magnetic susceptibility reveals that only 5% of Cu substitution suppresses the nonpolar collinear AF1 antiferromagnetic structure observed in pure MnWO4. Type-II multiferroicity survives a weak Cu substitution rate (x < 0.15). Multiferroic transition temperature and Néel temperature increase as the amount of Cu increases. New trends in some of the magnetic properties and in dielectric behaviors are observed for x = 0.20 and 0.25. Careful analysis of the magnetic susceptibility reveals that the incorporation of Cu into MnWO4 strengthens the overall antiferromagnetic interaction and reduces the magnetic frustration.