Insight into the factors influencing NMR parameters in crystalline materials from the KF–YF3 binary system

Abstract

Solid state NMR signals are very sensitive to the local environment of the observed nucleus; however, their interpretation is not straightforward. On the other hand, first-principles DFT calculations of NMR parameters can now be applied to periodic compounds to predict NMR parameters. Thus, ab initio calculations can help to interpret the NMR spectra exhibited by complex materials, to assign NMR lines to structural environments, and even to enlighten the environmental factors influencing the NMR parameters for a given nucleus. Both techniques have been applied to crystalline compounds of the KF–YF₃ binary system, γ-K₃YF₆, K₂YF₅, KYF₄, β-KY₂F₇ and α-KY₃F₁₀, which present a variety of YF_n and KF_m polyhedra. First, the structure of K₂YF₅ was refined in the Pnma space group and, for all compounds, atomic positions were optimized by DFT. The ¹⁹F, ⁸⁹Y and ³⁹K NMR spectra have been recorded and the measured NMR parameters are compared to those calculated from the first-principles DFT method, allowing unambiguous assignments of NMR lines to crystallographic sites. Linear correlations between the experimental δ_iso and calculated σ_iso values for the three nuclei are used to predict the theoretical ¹⁹F spectra of KYF₄ (24 F sites) and β-KY₂F₇ (19 F sites) as well as the ³⁹K spectrum of KYF₄ (6 K sites). For ⁸⁹Y and ³⁹K, both computational and experimental results show a decrease of the isotropic chemical shift values when the cation coordination number increases. Above all, ⁸⁹Y isotropic chemical shift values correlate with the number of K atoms present in the Y second coordination sphere. For ¹⁹F, the combination of isotropic chemical shift and chemical shift anisotropy allows for distinguishing four kinds of F environments.