51 V and 93 Nb high resolution NMR study of NbVO 5
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V and
93
Nb high resolution NMR study of NbVO5
J. Davis, D. Tinet,a) and J. J. Fripiat Department of Chemistry and Laboratory for Surface Studies, University of P.O. Box 413, Milwaukee, Wisconsin 53201
Wisconsin-Milwaukee,
J. M. Amarilla, B. Casal, and E. Ruiz-Hitzky Instituto de Ciencia de Materiales, C.S.I.C, Serrano 115 bis, 28006, Madrid, Spain (Received 29 May 1990; accepted 5 November 1990)
NbVO5 is characterized by 51V and 93Nb NMR resonance frequencies strongly upfield shifted when compared to those in model compounds V2O5 and LiNbO3. The chemical shift anisotropy dominates the 51V observed spectrum in a magnetic field of 11.7 T. The asymmetry parameter rjc is 0.2 and the quadrupole coupling constant is relatively small (1 MHz). The quadrupolar Hamiltonian overwhelmingly dominates the 93Nb spectrum (T)Q = 0.9) and the quadrupole coupling constant is huge (16.5 MHz). In agreement with the structure obtained from the x-ray powder diagram the isotropic chemical shift of 51V suggests that NbVO5 is indeed an orthovanadate. Interestingly, in NbVO5 the isotropic chemical shift of 93Nb reveals a better shielding of the 93Nb nucleus and a lower electric field gradient than in LiNbO3. Nb octahedra in NbVO5 are sharing corners whereas they share edges in LiNbO3.
I. INTRODUCTION
The synthesis and characterization of NbVO5 has recently been described by Amarilla, Casal, and Ruiz-Hitzky.1 This orthorhombic niobo-orthovanadate is isostructural with TaVO5 described earlier by Chahboun, Groult, and Raveau.2 After Rietveld refinement3 the unit cell parameters of NbVO5 are a: 11.866(8) A; b: 5.5288(4) A, and c: 6.9340(5) A. Their structure can be visualized as built from slabs of two Nb octahedra connected through isolated VO4 tetrahedra. The presence of pentagonal channels running along the b axis and the possibility for V and Nb to change their oxidation states make these structures good candidates for intercalation of alkali metals or hydrogen. In fact, Li has been intercalated within NbVO53 while H can be intercalated within NbVO5, forming a hydrogen bronze with approximate composition Hi.3NbVO5.4 Because of our interest in intercalated materials and of the uncertainty in assigning structures from x-ray powder diffraction, we decided to study NbVO5 by high resolution solid state NMR of the 51V and 93Nb nuclei in the hope of shedding an additional light on the structural features, the ultimate goal being to have a base of comparison for the future study of the intercalated materials. As far as the 51V resonance is concerned, V2O5 and NH4VO3 have been used as model materials, while for the 93Nb resonance, the LiNbO3 perovskite has been used for that purpose.
a)
On leave of absence from CSOCI-CNRS, Orleans, France. J. Mater. Res., Vol. 6, No. 2, Feb 1991
http://journals.cambridge.org
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In V2O5, V5+ is in a square pyramid. However, it can be considered as in a pseudo-octahedral environment with one short " V = O " and one long V — O bond, the oxygen of the latter belonging to the next plane.
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