Characterizations of F-superconductors and selected F-compounds, amorphous carbon and (VO)2P2O7 by 19F, 13C, 1H, 31P NMR and a new probe for multiple pulse, MAS and DAS NMR

Pan, Hongjun
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[superscript]19F nuclear magnetic resonance (NMR) studies were performed on the polycrystalline inorganic conductor Ag[subscript]2F and the polycrystalline insulators AgF, YOF, EuOF, YF[subscript]3, CuF[subscript]2, BaF[subscript]2 and KF. The Ag[subscript]2F spectrum showed a [superscript]19F pea clearly shifted downfield with respect to AgF. The two types of nonequivalent F's in YF[subscript]3 have the same isotropic chemical shift. Comparing the signals from the insulators and Ag[subscript]2F with the signals found in so-called 1-2-3 type superconductors of the formula RBa[subscript]2Cu[subscript]3O[subscript] 7-xF[subscript] x (R = Y and Eu) type, no detectable evidence for a Knight shifted built-in fluorine signal was found in the 1-2-3 type superconductors. The only signal found was from the BaF[subscript]2 which is an impurity in the superconductors;Amorphous carbon has been studied by [superscript]13C and [superscript]1H NMR with techniques of static single excitation, magic angle spinning and cross-polarization/magic angle spinning (CP/MAS). A spin-lattice relaxation time 0.7s was obtained by the progressive saturation method. Two different components are clearly seen in the spectrum of the static sample. The downfield component corresponds to sp[superscript]2-like carbons which comprise 93.6% of the total signal, and has an axially symmetric shift tensor with [sigma][subscript] ǁ = -28 ppm, [sigma][subscript]⊥ = 209 ppm and [macron][sigma] = 130 ppm. The upfield component corresponds to sp[superscript]3-like carbons which comprise 6.4% of the total signal and has a symmetric shift tensor with [macron][sigma] = 62 ppm. Two thirds of carbons are not seen in the static measurement because of high concentration of unpaired electrons leading to severe inhomogeneous line broadening. Those carbons are detected by magic angle spinning method in which sidebands spread over a range of 2000 ppm with the first moment located at 130 (±5) ppm. The upper limit of the fraction of hydrogenated carbons is 1.5%;The temperature dependence of the chemical shift of [superscript]31P in (VO)[subscript]2P[subscript]2O[subscript]7 has been studied. A Curie-Law-type temperature dependence of the isotropic chemical shift was observed. This makes (VO)[subscript]2P[subscript]2O[subscript]7 a viable internal temperature standard in high temperature NMR;A stable rotor-stator system is designed that is suitable for NMR experiments of homonuclear decoupling, magic angle spinning, and dynamic angle spinning experiments. A maximum speed of 16 KHz is achieved. The angle of rotation can be changed by 45° in 9 ms with no loss in rotation stability when spinning at 10 KHz.

Chemistry, Physical chemistry