Magnetic Properties of a New One-Dimensional Vanadium Oxide with the Hollandite Structure.
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Magnetic Properties of a New One-Dimensional Vanadium Oxide with the Hollandite Structure. Natasha A. Chernova, J. Katana Ngala, Peter Y. Zavalij, and M. Stanley Whittingham Institute for Materials Research, State University of New York at Binghamton, Binghamton, NY 13902-6000, U.S.A. ABSTRACT The magnetic properties of the first hollandite-type vanadium oxide containing anions in the 2×2 channels, V7.22O8(OH)8(Cl)0.77(H3O)2.34, are studied using static (DC) and dynamic (AC) magnetic susceptibilities. From the high-temperature Curie-Weiss behavior the effective magnetic moment is found consistent with the 3+ vanadium oxidation state; the negative CurieWeiss temperature Θ ≈ -500 K indicates strong antiferromagnetic exchange. The DC magnetic susceptibility shows a rapid increase and the AC susceptibility shows a maximum at about 20 K, indicating magnetic phase transition. The field-cooled and zero-field-cooled susceptibilities diverge below the transition temperature. The real and imaginary components of the AC susceptibility show frequency dependence and shift of maximum toward lower temperatures with decreasing frequency. Analysis of the frequency dependences reveals at least three different relaxation processes existing around and below the transition temperature. The temperature dependences of their relaxation times were obtained using Cole-Cole analysis. We show that the magnetic behavior observed is well explained by the random-field Ising model, with randomness brought on by vacancies in vanadium sites. INTRODUCTION Open framework compounds are being considered as cathode materials for rechargeable lithium batteries. The hollandite-type structure contains tunnels built of double chains of edgesharing MO6 octahedra (here M is usually a transition metal, for example, Mn in original hollandite MnO2; V, Fe, Ni, Mo, etc). The channels are most often occupied by alkaline, alkaline earth or other metal cations (Figure 1). These channels provide easy pathways for lithium intercalation and deintercalation, which are the essential processes of lithium battery performance. It has been shown that the hollandite compounds, AxMnO2·nH2O, where A is Na+, K+, Rb+ or NH4+, reversibly intercalate lithium and have discharge capacities of about 200 mAh/g [1]. Hollandite-type compounds possess interesting Figure 1. Hollandite structure viewed magnetic and electrical properties due to one or more of the following factors: quasi one-dimensional (1D) along c axis. Spheres within the 2x2 character of the tunnel walls, mixed valence of the tunnels represent chloride and transition metal, partial occupancy of tunnels or MO6 hydroxonium ions.
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octahedra. BixV8O16 (1.72
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