title: Multinuclear Solid-State NMR Investigation of Hexaniobate and Hexatantalate Compounds.
abstract: This work determines the potential of solid-state NMR techniques to probe proton, alkali, and niobium environments in Lindqvist salts. Na7HNb6O19·15H2O (1), K8Nb6O19·16H2O (2), and Na8Ta6O19·24.5H2O (3) have been studied by solid-state static and magic angle spinning (MAS) NMR at high and ultrahigh magnetic field (16.4 and 19.9 T). (1)H MAS NMR was found to be a convenient and straightforward tool to discriminate between protonated and nonprotonated clusters AxH8-xM6O19·nH2O (A = alkali ion; M = Nb, Ta). (93)Nb MAS NMR studies at different fields and MAS rotation frequencies have been performed on 1. For the first time, the contributions of NbO5Oμ2H sites were clearly distinguished from those assigned to NbO6 sites in the hexaniobate cluster. The strong broadening of the resonances obtained under MAS was interpreted by combining chemical shift anisotropy (CSA) with quadrupolar effects and by using extensive fitting of the line shapes. In order to obtain the highest accuracy for all NMR parameters (CSA and quadrupolar), (93)Nb WURST QCPMG spectra in the static mode were recorded at 16.4 T for sample 1. The (93)Nb NMR spectra were interpreted in connection with the XRD data available in the literature (i.e., fractional occupancies of the NbO5Oμ2H sites). 1D (23)Na MAS and 2D (23)Na 3QMAS NMR studies of 1 revealed several distinct sodium sites. The multiplicity of the sites was again compared to structural details previously obtained by single-crystal X-ray diffraction (XRD) studies. The (23)Na MAS NMR study of 3 confirmed the presence of a much larger distribution of sodium sites in accordance with the 10 sodium sites predicted by XRD. Finally, the effect of Nb/Ta substitutions in 1 was also probed by multinuclear MAS NMR ((1)H, (23)Na, and (93)Nb).
authors.affiliation: PSL Research University , Chimie ParisTech-CNRS, Institut de Recherche de Chimie Paris, 11 rue Pierre et Marie Curie, 75005 Paris, France.|ERAMET Research , Hydrometallurgy department, 1 avenue Albert Einstein, F-78193 Trappes, France.
authors.forename: Gauthier J-P
authors.initials: GJ
authors.lastname: Deblonde
authors.affiliation: Institut des Matériaux de Paris Centre (IMPC-UPMC-FR2482) , site collège de France, 75005 Paris, France.
authors.forename: Cristina
authors.initials: C
authors.lastname: Coelho-Diogo
authors.affiliation: PSL Research University , Chimie ParisTech-CNRS, Institut de Recherche de Chimie Paris, 11 rue Pierre et Marie Curie, 75005 Paris, France.
authors.forename: Alexandre
authors.initials: A
authors.lastname: Chagnes
authors.affiliation: PSL Research University , Chimie ParisTech-CNRS, Institut de Recherche de Chimie Paris, 11 rue Pierre et Marie Curie, 75005 Paris, France.
authors.forename: Gérard
authors.initials: G
authors.lastname: Cote
authors.affiliation: Vice-Chancellor's Office, University House, Lancaster University , Lancaster LA1 4YW, U.K.
authors.forename: Mark E
authors.initials: ME
authors.lastname: Smith
authors.affiliation: Department of Physics, University of Warwick , Coventry CV4 7AL, U.K.
authors.forename: John V
authors.initials: JV
authors.lastname: Hanna
authors.affiliation: Department of Physics, University of Warwick , Coventry CV4 7AL, U.K.
authors.forename: Dinu
authors.initials: D
authors.lastname: Iuga
authors.affiliation: Sorbonne Universités , UPMC Univ Paris 06, UMR CNRS 7574, Laboratoire de Chimie de la Matière Condensée de Paris, Collège de France, 11 place Marcelin Berthelot, 75005, Paris, France.
authors.forename: Christian
authors.initials: C
authors.lastname: Bonhomme