We present the synthesis and a detailed investigation of structural and magnetic properties of polycrystalline NH4[(V2O3)2(4,4 -bpy)2(H2PO4)(PO4)2]·0.5H2O by means of x-ray diffraction, magnetic susceptibility, electron spin resonance, and 31 P nuclear magnetic resonance measurements. Temperature dependent magnetic susceptibility could be described well using a weakly coupled spin-1/2 dimer model with an excitation gap ∆/kB 26.1 K between the singlet ground state and triplet excited states and a weak inter-dimer exchange coupling J /kB 4.6 K. A gapped chain model also describes the data well with a gap of about 20 K. The electron spin resonance intensity as a function of temperature traces the bulk susceptibility nicely. The isotropic Landé g-factor is estimated to be about g 1.97, at room temperature. We are able to resolve the 31 P NMR signal as coming from two inequivalent P-sites in the crystal structure. The hyperfine coupling constant between 31 P nucleus and V 4+ spins is calculated to be A hf (1) 2963 Oe/µB and A hf (2) 1466 Oe/µB for the P(1) and P(2) sites, respectively. Our NMR shift and spin-lattice relaxation rate for both the 31 P sites show an activated behaviour at low temperatures, further confirming the singlet ground state. The estimated value of the spin gap from the NMR data measured in an applied field of H = 9.394 T is consistent with the gap obtained from the magnetic susceptibility analysis using the dimer model. Because of a relatively small spin gap, NH4[(V2O3)2(4,4 -bpy)2(H2PO4)(PO4)2]·0.5H2O is a promising compound for further experimental studies under high magnetic fields.
We present the synthesis and a detail investigation of structural and magnetic properties of polycrystalline [VO(HCOO)2·(H2O)] by means of x-ray diffraction, magnetic susceptibility, highfield magnetization, heat capacity, and electron spin resonance measurements. It crystallizes in a orthorhombic structure with space group P cca. It features distorted VO6 octahedra connected via HCOO linker (formate anions) forming a two-dimensional square lattice network with a bilayered structure. Analysis of magnetic susceptibility, high field magnetization, and heat capacity data in terms of the frustrated square lattice model unambiguously establish quasi-two-dimensional nature of the compound with nearest neighbour interaction J1/kB 11.7 K and next-nearest-neighbour interaction J2/kB 0.02 K. It undergoes a Néel antiferromagnetic ordering at TN 1.1 K. The ratio θCW/TN 10.9 reflects excellent two-dimensionality of the spin-lattice in the compound. A strong in-plane anisotropy is inferred from the linear increase of TN with magnetic field, consistent with the structural data.
A vanadium formate (VF) coordination polymer and its composite with partially reduced graphene oxide (prGO), namely VF‐prGO, can be applied as anode materials for Li‐ion based electrochemical energy storage (EcES) systems in the potential range of 0–3 V (vs Li+/Li). This study shows that a reversible capacity of 329 mAh g−1 at a current density of 50 mA g−1 after 50 cycles can be realized for VF along with a high rate capability. The composite exhibits even a higher capacity of 504 mAh g−1 at 50 mA g−1. A good capacity retention is observed even after 140 cycles for both VF and the composite. An ex‐situ X‐ray photoelectron spectroscopy study indicates the involvement of V3+/V4+ redox couple in the charge storage mechanism. A significant contribution of this reversible capacity is attributed to the pseudocapacitive behavior of the system.
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