Origin of NMR shielding in fluorides
In this work, we analyze in detail the relation between electronic structure and fluorine nuclear magnetic resonance (NMR) shielding in a series of solid state alkali fluorides (LiF, NaF, KF, RbF, and CsF). For that purpose, we use solid-state NMR calculations implemented in the density functional theory full potential WIEN2K code (APW + lo). Both measurements and calculations show that the NMR shielding varies across the series by approximately 200 ppm. We focus our discussion on an explanation of the origin of the observed trend, and we show that the variation is mainly determined by contributions from “semicore” metal-p and valence F-p bands. More specifically, the trend in fluorine shielding can be related to the small but significant change in the hybridization of these states. A second important ingredient determining the value of the shielding is the presence and position of metal-d states in the unoccupied part of the Kohn-Sham bands. Although the present analysis has been demonstrated for the 19F nucleus in alkali fluorides, the main results are more general and can explain similar trends observed in other solids and for other nuclei.
Accurate Determination of NMR Chemical Shifts in Alkali Halides and Their Correlation with Structural Factors
High-resolution solid-state NMR spectra of alkali halides have been measured using the magic angle spinning (MAS) of the sample, and chemical shifts of 7Li, 23Na, 87Rb, 133Cs, 35Cl, 79Br, and 127I have been determined accurately. The 39K chemical shifts have been determined by NMR without MAS. As the ratio of the cation radius to the anion radius (Rm) increases, the chemical shifts of halogen nuclei show an upfield shift when Rm 0.6. The chemical shifts of the halogen nuclei are explained in terms of the first and the second nearest neighbor interactions, both of which cause a downfield shift. On the other hand, the chemical shifts of alkali nuclei show a downfield shift for the ratio of the anion radius to the cation radius (Rx) less than 1.6, while they show an upfield shift for Rx>1.6. The chemical shifts of the alkali nuclei are expressed by the sum of the nearest neighbor interaction in the absence of the halogen-halogen interaction and the “isolation ef...
Alkali feldspars: ordering rates, phase transformations and behaviour diagrams for igneous rocks
Abstract Homogeneous and heterogeneous phase relationships in the alkali feldspars are reviewed, and behaviour diagrams developed. Al,Si ordering is almost certainly continuous and higher order in both albite and potassium feldspar and has been established reversibly or nearly so down to below 500°C in albite and possibly to ∼ 200°C in potassium feldspar. The degree of order in intermediate albite changes strongly over a range of ∼ 75–150°C depending on pressure, low albite being stable up to about 620–650°C and high albite above about 725°C at low pressure. Symmetry is broken at ∼ 980°C mainly by a cooperative shearing of the whole framework and not by Al,Si ordering alone; there is a thermal crossover near 700°C shearing being dominant above (high albite) and ordering dominant below (intermediate albite). In potassium feldspar symmetry is broken by Al,Si ordering at a temperature of about 500°C The change in degree of order with respect to temperature has been followed easily and reversibly in sanidine from ∼ 1075 to ∼ 550°C and to a lesser extent in microcline from 450 to 200°C. Ordering rates in sanidine down to 500°C and ordering rates in microcline between 450 and 200°C are almost as fast as in albite. Ordering in sanidine at 500°C and below slows and then stops with the development of the tweed orthoclase domain texture. The tweed texture acts as a barrier to further order because the strain energy associated with the (incipient) twin domain texture balances or nearly balances the free energy decrease resulting from ordering. Ordering stops not because of the kinetics of Al,Si diffusion, but because the total driving force is very small or nil. Ordering can readily proceed to completion, with the formation of low microcline, only if the domain-texture barrier is overcome by processes involving fluids or strong external stresses. There is no barrier in albite. The symmetry-breaking process in alkali feldspar changes with composition from mainly shearing in albite to ordering in potassium feldspar. Symmetry is broken equally at a compositional crossover (metastable with respect to exsolution) near Ab80-75 at low pressure and progressively displaced towards Or at higher pressures. Ordering in pure albite occurs by a (nearly) one-step path which progressively becomes two-step with substitution of Or. Diagrams showing the near-equilibrium variation of the order parameters at low pressure with composition and T are given, as well as two extreme phase and behaviour diagrams for complete coherent and complete incoherent (strain-free) relationships. These diagrams can be used to understand feldspar relationships and microtextures in hypersolvus and subsolvus rocks, the occurrence of orthoclase, and of intermediate and low microcline.