Search

An ammonium vanadyl oxalatophosphite has been synthesized by hydrothermal treatment. The formula is (NH 4 ) 2 (VOHPO 3 ) 2 C 2 O 4 2·9H 2 O (1) . The crystal structure of the compound has been determined by single crystal X-ray diffraction and solid state Nuclear Magnetic Resonance spectroscopy (NMR). Compound 1 crystallizes in triclinic symmetry with space group P −1, a = 6.3844(4) Å, b = 7.2278(4) Å, c = 9.2965(5) Å, α = 67.260(4)°, β = 72.927(4)°, γ = 85.848(3)°. The vanadium phosphite framework consists of infinite chains of corner-sharing vanadium octahedra and hydrogenophosphite tetrahedra. The oxalate groups ensure the connection between the chains. The ammonium ions and the water molecules are located between the anionic [(VO) 2 (HPO 3 ) 2 C 2 O 4 ] 2 − layers. The thermal behaviour of 1 was carefully studied by combining thermogravimetric analyses, in situ X-ray diffraction and Infrared spectroscopy. The formation of given vanadyl phosphate catalysts was shown to be atmosphere-dependent of (NH 4 ) 2 (VOHPO 3 ) 2 C 2 O 4 2·9H 2 O thermal decomposition. [ABSTRACT FROM AUTHOR]

Two new lithium vanadyl oxalatophosphites have been synthesized by hydrothermal treatment. The respective formula are Li2(VOHPO3)2C2O4 6H2O (1) and Li2(VOHPO3)2C2O4 4H2O (2). The structures of the compounds have been determined by single crystal X-ray diffraction. Compound 1 crystallizes in triclinic symmetry in space group P-1, a=6.3592(2)Å, b=8.0789(3)Å, c=9.1692(3)Å, α=64.390(2), β=87.277(2)°, γ=67.624(2) and, compound 2 in monoclinic symmetry, space group P21/a, a=6.3555(2)Å b=12.6368(7)Å c=9.0242(4)Å β=105.167(3)°. The vanadium phosphite framework consists of infinite chains of corner-sharing vanadium octahedra and hydrogenophosphite tetrahedra. The oxalate groups ensure the connection between the chains. The lithium ions and the water molecules are located between the anionic [(VO)2(HPO3)2C2O4]2− layers. Thermal behavior of both compounds was carefully studied by combining thermogravimetric analyses and thermal dependant X-ray diffraction in order to study the thermal stability of the layered oxalatophosphites and to see the influence of the decomposition of the carbon-based anions into the final lithium vanadyl phosphate. Various intermediate phases were evidenced and for both compounds the final product was LiVOPO4. [ABSTRACT FROM AUTHOR]

Abstract: Following our previous work on the tavorite-like LiFePO4·OH and FePO4·H2O phases, we report here the magnetic and NMR characterizations of analogous LiMnPO4·OH, MnPO4·H2O and VPO4·H2O phases together with the DFT calculations of the NMR shifts. The first two compounds exhibit Curie–Weiss type magnetic behavior with Curie constants close to the theoretical ones for HS Mn3+, while the vanadium compound is very close to a pure Curie-type behavior. 7Li, 31P and 1H MAS NMR spectra are reported for the three compounds, and show strong Fermi-contact shifts for the first two nuclei, while the sign and magnitude of the 1H shifts are very different for the three phases. DFT calculations (FLAPW in GGA+U approximation) using the WIEN2k code and the experimental susceptibilities are shown to reproduce closely the experimental data. This situation is compared to the case of the homologous and isostructural Fe compounds, which exhibit much more complex magnetic behaviors. [Copyright &y& Elsevier]

Abstract: 77Se (I=1/2) solid-state NMR is a very sensitive probe of the local structure of selenide glasses, which themselves are promising for optical applications. In this work, we show that although 77Se has a low natural abundance (7.58%) and a wide spectral range, the sensitivity can be dramatically increased using Carr–Purcell–Meiboom–Gill (CPMG) trains of rotor-synchronized π pulses during the detection of 77Se magnetization but may be affected by chemical shift anisotropy when the Magic Angle Spinning rate is not fast enough and by offset effects. The indirect dimension of the T2 CPMG-resolved spectrum shows a strong influence of the J-couplings between naturally occurring 77Se pairs. The resulting spectra show that the structural model known as “chains crossing model” is not entirely suitable to describe the glassy network of the Se-rich compositions. [Copyright &y& Elsevier]

We carried out a detailed investigation of the local ordering and dynamics of the lithium intercalation in paramagnetic Li x VOPO 4.2H 2 O (with 0 < x ≤ 1) materials. This question was addressed using a combination of X-ray diffraction, 31P and 7Li MAS NMR experiments. We first studied the structure of the fully ordered end-member of the series, Li 1 VOPO 4.2H 2 O, revisiting the X-ray single crystal diffraction data on the basis of the information provided by 31P MAS NMR. We then carried out 7Li MAS and exchange NMR experiments and 31P MAS experiments on the polycrystalline powders obtained after partial lithium insertion in VOPO 4.2H 2 O phases. These experiments evidenced an unexpected ageing of the material related with lithium dynamics between the VOPO 4 layers and a V4+/V5+ charge ordering mechanism within the layers. Image 1 • Combination of single crystal X-Ray diffraction and 31P MAS NMR allowed us to revisit the symmetry of Li 1 VOPO 4.2H 2 O. • 7Li EXSY NMR combined to XRD on Li 0.5 VOPO 4.2H 2 O evidenced an ordering mechanism of Li cations (timescale of several days). • 31P NMR are showing a similar rearrangement taking place within the layers with an ordering of the V4+ versus V5+ locations. [ABSTRACT FROM AUTHOR]