Ulises Amador

Lattice Dynamics of β-V2O5: Raman Spectroscopic Insight into the Atomistic Structure of a High-Pressure Vanadium Pentoxide Polymorph

We report here the Raman spectrum and lattice dynamics study of a well-crystallized β-V(2)O(5) material prepared via a high-temperature/high-pressure (HT/HP) route, using α-V(2)O(5) as the precursor. Periodic quantum-chemical density functional theory calculations show good agreement with the experimental results and allow one to assign the observed spectral features to specific vibrational modes in the β-V(2)O(5) polymorph. Key structure-spectrum relationships are extracted from comparative analysis of the vibrational states of the β-V(2)O(5) and α-V(2)O(5) structures, and spectral patterns specific to the basic units of the two V(2)O(5) phases are proposed for the first time. Such results open the way for the use of Raman spectroscopy for the structural characterization of vanadium oxide-based host lattices of interest in the field of lithium batteries and help us to greatly understand the atomistic mechanism involved in the α-to-β phase transition of vanadium pentoxide.

Microwave-assisted synthesis, microstructure, and physical properties of rare-earth chromites.

The full rare-earth (RE) chromites series (RE)CrO(3) with an orthorhombic distorted (Pnma) perovskite structure and the isostructural compound YCrO(3) can be synthesized through a simple microwave-assisted technique, yielding high-quality materials. Magnetization measurements evidence that the Néel temperature for antiferromagnetic Cr(3+)-Cr(3+) ordering strongly depends on the RE(3+) ionic radius (IOR), and a rich variety of different magnetic spin interactions exists. Dielectric spectroscopy on sintered pellets indicates electronic inhomogeneity in all samples as manifested by the presence of at least two dielectric relaxation processes associated with grain boundary and grain interior bulk contributions. X-ray diffraction, Raman spectroscopy, and temperature-dependent dielectric permittivity data do not indicate potential noncentrosymmetry in the crystal or concomitant ferroelectricity. Strong correlations between the magnetic and dielectric properties were not encountered, and microwave-synthesized (RE)CrO(3) may not be classified as magnetoelectric or multiferroic materials.

Coordination compounds of 4,2′-6′,4″-terpyridine, [MCl2(4,2′-6′,4″-terpyridine)], M = Mn(II), Co(II), Ni(II), Cu(II) or Zn(II). Crystal structure of catena-poly [(dichlorozinc)-μ-(4,2′-6′,4″-terpyridine)]

Abstract MCl2(4,2′-6′,4″-terpyridine) compounds, where M = Mn(II), Co(II), Ni(II), Cu(II) or Zn(II), have been synthesized and characterized by IR and electronic spectroscopy and susceptibility magnetic measurements. Crystalline structure of the Zn(II) complex has been determine by X-ray diffraction. The Zn(II) atom shows to be in a highly distorted tetrahedral environment, defined by two Cl atoms and two N atoms of two ligand molecules, that act as bridges bound through the N atoms of the external pyridine rings. The structure is made of chains that do not interact among them.

High-pressure investigation of Li2MnSiO4 and Li2CoSiO4 electrode materials for lithium-ion batteries.

In this work, the high-pressure behavior of Pmn2(1)-Li(2)MnSiO(4) and Pbn2(1)-Li(2)CoSiO(4) is followed by in situ X-ray diffraction at room temperature. Bulk moduli are 81 and 95 GPa for Pmn2(1)-Li(2)MnSiO(4) and Pbn2(1)-Li(2)CoSiO(4), respectively. Regardless of the moderate values of the bulk moduli, there is no evidence of any phase transformation up to a pressure of 15 GPa. Pmn2(1)-Li(2)MnSiO(4) shows an unusual expansion of the a lattice parameter upon compression. A density functional theory investigation yields lattice parameter variations and bulk moduli in good agreement with experiments. The calculated data indicate that expansion of the a lattice parameter is inherent to the crystal structure and independent of the nature of the transition-metal atom (M). The absence of pressure-driven phase transformation is likely associated with the incapability of the Li(2)MSiO(4) composition to adopt denser structures while avoiding large electrostatic repulsions.

Electrochemical Study of Li3Fe ( MoO4 ) 3 as Positive Electrode in Lithium Cells

Li 3 fe(MoO 4 ) 3 undergoes a complex electrochemical reaction with lithium in which the reduction of Fe 3 + and Mo 6 + takes place along the first discharge of the cell at about 2.4 and 1.8 V, respectively. The intercalation process involved is fully reversible for low lithium contents, Li 3 + x Fe(MoO 4 ) 3 with 0 < x < 1, the inserted compound Li 3 + 1 Fe 2 + (MoO 4 ) 3 retaining the Li 3 Fe 3 + (MoO 4 ) 3 parent framework with only a slight increase of the cell volume (0.85%). In contrast, the electrochemical reaction of Li 3 Fe(MoO 4 ) 3 with five lithium ions originates an irreversible decomposition of this material into a mixture of a FeO-type compound and an amorphous lithium-molybdate phase. This in situ formed blend is electrochemically active, being able to intercalate and deintercalate three lithium ions at an average voltage of 2 V (reversible specific capacity of 150 Ah/kg). The full discharge of the cells (down to the vicinity of 0 V) proceeds through the complete and irreversible reduction of Li 3 fe(MoO 4 ) 3 with 25 lithium ions, resulting in the breakdown of any existing crystalline framework.

The rare-earth H.T.S.C. family Ba2(RE)Cu3O7; structural, electrical and magnetic studies (RE=Y,Nd,Sm,Eu,Gd,Dy,Ho,Er,Tm)

Abstract A systematic study of the title family has been performed in the light of structural, electric and magnetic properties. All samples are orthorhombic and the unit cell parameters show an interesting linear relationship with the lanthanide ion dimension.

Ba2PrCu3O7: Crystal growth, structure and magnetic properties

Abstract A novel procedure for growing crystals of the title phase has been found by dissolving Pr 2 CuO 4 in a mixture of CuO and BaO 2 . The crystal structure determination shows this solid to be isostructural with, but of higher symmetry than “Ybacuo”, being tetragonal with unit cell parameters a =3.8918(2) A , c =11.6484(8) A . Magnetic susceptibility measurements together with crystallochemical considerations allow us to estimate the oxidation states of the cations in this material which can be formulated as Ba 2 Pr 4+ 3 Pr 3+ .7 Cu 3+ .7 Cu 2+ 2.3 O 7 . In spite of having the same oxygen stoichiometry than the best superconducting “Ybacuo”, this material does not show any superconducting transition down to 4.2 K.

Influence of carboxylic acids on the reactions with chlorotetraacetatodiruthenium(II,III): X-ray crystal structure of [Ru2(μ-O2CC4H3S)4(OPPh3)2]BF4·2H2O

Abstract The reaction of Ru 2 Cl(μ-O 2 CCH 3 ) 4 with indole-2-carboxylic, N -methyl-pyrrole-2-carboxylic, furane-2-carboxylic, thiophene-2-carboxylic and benzofurane-2-carboxylic acids, which contain nitrogen, oxygen or sulphur atoms in the α position with respect to the carboxylate group, leads to compounds of the type Ru 2 Cl(μ-O 2 CR) 4 . In these compounds O,O-coordination of the carboxylate ligands has been found. The analogous reaction with quinoline-2-carboxylic acid (Hquin) gives a disproportionation process with formation of Ru(quin) 3 and Ru 2 (quin) 4 ; in this case, N,O-coordination of the ligand has been observed. The indole-2-carboxylato derivative has a non-polymeric structure, whereas with the other ligands polymeric structures with chlorine atoms bridging Ru 2 5+ units are formed. Treatment of Ru 2 Cl(μ-O 2 CR) 4 in thf or acetone with AgBF 4 gave [Ru 2 (μ-O 2 CR) 4 L 2 ]BF 4 (L = thf, acetone; R = N -methyl-2-pyrrolyl, 2-furyl, 2-thienyl) or [Ru 2 (μ-O 2 CR) 4 ]BF 4 (R = 2-benzofuryl). The axial positions of these compounds can be occupied by OPPh 3 to give the corresponding [Ru 2 (μ-O 2 CR) 4 (OPPh 3 ) 2 ]BF 4 The compounds have been characterized by analytical, spectroscopic and magnetic data. The structure of [Ru 2 (μ-O 2 CC 4 H 3 S) 4 (OPPh 3 ) 2 BF 4 ·2H 2 O has been determined by X-ray crystallography. The dinuclear cation has two ruthenium atoms linked by four bridging thiophene-2-carboxylate ligands and two OPPh 3 ligands coordinated to axial positions, with an Ru—Ru distance of 2.2747(11) A.

CuO2-plane vibrational modes in single crystals of PrBa2Cu3O7−δ

Abstract We report Raman spectra of tetragonal, non superconducting PrBa 2 Cu 3 O 7−δ (δ ≈ O) single crystals. Phonons which correspond to those observed in the homologous superconducting compounds are identified. The Raman phonon of B 1g tetragonal symmetry, which corresponds to the silent modes of perovskite is observed at 292 cm −1 . Contrary to the case of the superconductors this mode shows neither Fano lineshape nor a softening when cooling from rt to 10 K. We thus confirm that these phenomena are associated with the superconducting transition. The stretching vibration of the Cu1-0IV bands occurs at rather high frequencies (554 cm −1 ). From these mode frequencies we estimate that there is a significant amount of Pr +4 in the material.

Insight into Ramsdellite Li2Ti3O7 and Its Proton-Exchange Derivative

Despite being proven to be a good lithium-ion conductor 30 years ago, the crystal structure of the ramsdellite-like Li(2)Ti(3)O(7) has remained uncertain, with two potential models for locating the lithium ions in the structure. Although the model presently accepted states that both lithium and titanium occupy the octahedral sites in the framework, evidence against this model are provided by (6)Li and (7)Li MAS NMR spectroscopy. Thus, about 14% of these octahedral positions are empty since no lithium in octahedral coordination is present in the material. When Li(2)Ti(3)O(7)-ramsdellite is treated with nitric acid a complete exchange of lithium by protons is produced to yield H(2)Ti(3)O(7). The crystal structure of this proton-exchanged ramsdellite has been re-examined combining X-ray diffraction (XRD), neutron powder diffraction (NPD), and spectroscopic ((1)H and (7)Li MAS NMR) techniques. Two kinds of protons are present in this material with different acidity because of the local environments of oxygen atoms to which protons are bonded, namely, low acidic protons strongly bonded to highly charged oxygen atoms (coordinated to two Ti(4+) and a vacancy); and protons linked to low charged oxygen atoms (bonded to three Ti(4+) ions) which will display a more acidic behavior. H(2)Ti(3)O(7) absorbs water; proton mobility is enhanced by the presence of absorbed water, giving rise to a large improvement of its electrical conductivity in wet atmospheres. Thus, it seems that water molecules enter the tunnels in the structure providing a vehicle mechanism for proton diffusion.