Computer simulations of magnetic resonance spectra observed in polycrystalline and glassy samples
TL;DR: In this paper, the authors developed a general computer program which simulates the NMR or ESR spectra observed in powdered and glassy samples to facilitate calculation of the relevant Hamiltonian or spin-Hamiltonian parameters in a variety of cases.
About: This article is published in Journal of Magnetic Resonance.The article was published on 1970-05-01. It has received 99 citations till now.
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TL;DR: In this paper, the relative fractions of boron atoms in Na2O-B2O3SiO2 glasses of high soda content were determined using NMR techniques.
Abstract: NMR techniques are employed to determine the relative fractions of boron atoms in Na2OB2O3SiO2 glasses of high soda content. The data show that if enough Na2O is added, four-coordinated borons are destroyed and borons with one or two non-bridging oxygens are created, but that both the beginning point and the rate of these processes depend strongly on the amount of silica present. These findings are shown to be quantitatively inconsistent with structural models previously suggested in the literature. Utilizing the concept of proportionate atomic sharing of the additional Na2O, a new structural model is proposed for K
440 citations
TL;DR: In this paper, two distinct oxygen-associated trapped-hole centers (OHCs) are identified in samples of room-temperature γ-irradiated, high-purity fused silica.
Abstract: Two distinct oxygen-associated trapped-hole centers (OHCs) are identified in samples of room-temperature γ-irradiated, high-purity fused silica. One, which we label the “wet” OHC, predominates in the high-OH-content (wet) silicas while the other, the “dry” OHC, is more prevalent in low-OH (dry) silicas. Excellent computer simulations of the low-temperature electron-spin-resonance spectra are obtained for both wet and dry silicas using only the relative abundance of the “wet” and “dry” OHCs as an adjustable parameter. Analysis of the 17 O-hyperfine structure which occrs in samples of wet silica enriched in 17 O provides direct confirmation that the “wet” OHC is a hole trapped in a single nonbonding 2p-orbital of an oxygen (presumed nonbridging). Correlation of optical absorption and electron spin resonance via isochronal pulse anneals indicates that the “dry” OHC has an optical transition ay 7.6 eV. In addition, it is reported that the “dry” OHC can be induced in the dry silicas by the fiber drawing process. From the present results, an O 2 − molecular ion model appears most attractive for the “dry” OHC.
365 citations
TL;DR: In this article, the structure of glasses in the system Na2OB2O3SiO2 was analyzed using the 11B NMR spectra.
Abstract: The 11B NMR spectra have been used to study the structure of glasses in the system Na2OB2O3SiO2. The fraction of BO4 units, and the fraction of BO3 units with one or two nonbridging oxygens, are measured and analyzed according to a structural model. The results indicate that: (1) for a sodium oxide to boron oxide ratio of 0.5 or less, the Na+1 ions are attracted primarily by the borate network; therefore, the ternary glasses can be viewed as binary sodium borate glasses diluted by SiO2; (2) when the sodium oxide to boron oxide ratio exceeds 0.5, the additional Na2O results in the formation of [BSi4O10]−1 units at the expense of diborate and SiO4 units. In this process, Na+1 ions are still taken up only by the borate network. After all the available SiO4 units are consumed to form [BSi4O10]−1 units, additional Na+1 ions are proportionally shared between the borate and silicate networks.
358 citations
TL;DR: In this article, the phosphorusoxygen-hole center (POHC) was shown to occur in two variants comprising holes trapped on one or two nonbridging oxygens, and a singlet resonance S due to E′ type defects was observed to grow in with annealing above ∼800 K, regardless of whether or not the sample was irradiated.
Abstract: Defect centers induced by ionizing radiation (50–100‐keV x rays, 60Co γ rays) in high purity P‐doped silica glass have been observed and elucidated by ESR spectroscopy. Four generic species are well characterized on the basis of the observed 31P hyperfine splittings and g values as defects analogous to PO2−3 (phosphoryl), PO4−4 (phosphoranyl), PO2−2 (phosphinyl), and PO2−4 radicals. The latter species, also termed the phosphorus‐oxygen‐hole center (POHC), is shown to occur in two variants comprising holes trapped on one or two nonbridging oxygens. Radiation‐induced Si E′ centers with and without P next‐nearest‐neighbors were also identified, and a singlet resonance S due to E′ type defects such as (OSi2)Si⋅ and/or (O2Si)Si⋅ was observed to grow in with annealing above ∼800 K, regardless of whether or not the sample was irradiated. The structures, formation mechanisms, and precursors of these defects have been determined or inferred for all centers. Radiation‐induced optical absorption spectra over the ran...
346 citations
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TL;DR: In this article, the second moment of the frequency deviation of the absorption line of the electron was calculated and the fourth moment was also computed to examine how good an approximation is the conventional assumption of a Gaussian shape.
Abstract: In regular crystals, the width of the absorption lines arising from the magnetic moment of the electron or nucleus is caused primarily by the interaction between the magnetic dipoles. It is prohibitively difficult to determine the precise shape of the absorption line theoretically, but the invariance of the diagonal sum in quantum mechanics permits the calculation of the second moment of the frequency deviation, and hence the r.m.s. line breadth. The latter agrees excellently with the observations of Pake and Purcell on the magnetic absorption of the F nucleus in Ca${\mathrm{F}}_{2}$, both in absolute magnitude, and in the dependence on the direction between the magnetic field and the principal cubic axes. The fourth moment was also computed to examine how good an approximation is the conventional assumption of a Gaussian shape. As long as no exchange is present (the nuclear case) the Gaussian model is moderately good. For the 100 direction in a cubic crystal, the theoretical ratio of root mean fourth to root mean square breadth is 1.25. Pake and Purcell's measurements yield 1.24. A Gaussian model would require 1.32. The theory is extended to include crystals with two kinds of spin moments (two types of nuclei, or simultaneous nuclear and electronic spin). Coupling between unlike moments is less effective (by a factor ⅔ in the r.m.s. width) than that between like in broadening the lines.In the paramagnetic absorption caused by electronic spin, it is imperative to include the effect of exchange coupling. This interaction does not contribute to the second moment, but greatly increases the fourth. As a result, the lines are peaked much more sharply than one would compute from the second moment with the Gaussian model for line shape. This "exchange narrowing" explains why microwave paramagnetic absorption lines are much narrower than one first conjectures from the amount of dipolar coupling.The theoretical calculations are given in Sections II-IV. The final sections V-VI give the comparison with the experiments of Pake and Purcell, and with the model of Bloembergen, Purcell, and Pound, for r-f absorption in liquids.
2,139 citations
TL;DR: In this article, the general formalism of Kramers indicating the existence of superexchange interaction has been reduced, under simplifying assumptions, to the point where actual formulas for the interaction can be written down directly in terms of spin operators, with certain exchange and transition integrals as parameters.
Abstract: In this paper the general formalism of Kramers indicating the existence of superexchange interaction has been reduced, under simplifying assumptions, to the point where actual formulas for the interaction can be written down directly in terms of spin operators, with certain exchange and transition integrals as parameters. Two results of physical interest are the following: (a) superexchange must be expected to show the directional properties (as far as directional relations of interacting magnetic ions are concerned) of the orbitals in the outer shell of the non-magnetic connecting ions; and (b) the sign of the effective exchange integral depends upon the sign of the internal exchange coupling of an added electron on the magnetic ion.
1,304 citations
TL;DR: The line shapes arising from randomly oriented particles in a fixed position have been calculated in this article and it has been found that it is possible to determine the three principal g factors with relatively high accuracy and without investigating single crystals.
Abstract: The line shapes arising from randomly oriented particles in a fixed position have been calculated. It has been found that it is possible to determine the three principal g factors with relatively high accuracy and without investigating single crystals. Measurements on powders of CuCl2·2H2O and CuSO4·5H2O are used as examples. In addition, the influence of Brownian rotation in viscous liquids on line shapes has been studied. Equations have been derived for the case of high viscosities, supplementing the well‐known ones given by McConnell [J. Chem. Phys. 25, 709 (1956)]. To a first approximation, the effect of decreasing viscosity can be described by an over‐all diminishing line width and an unchanged line shape function. The hyperfine splittings are neglected, but can be taken into account.
481 citations
TL;DR: In this article, paramagnetic resonance data are collected together for crystalline solids containing ions of the transition groups, and those parts of the theory necessary for an understanding of the results are presented in a fairly simple way.
Abstract: In this report, which is complementary to that of Bleaney and Stevens in Reports on Progress in Physics, Volume XVI, paramagnetic resonance data are collected together for crystalline solids containing ions of the transition groups, and those parts of the theory necessary for an understanding of the results are presented in a fairly simple way. After a brief introduction, the magnetic properties of various types of compounds are considered, and a simple discussion is given of the spin Hamiltonian which is used to describe the lowest energy levels. Some details are also given of the structures of the ten crystal types which are most commonly used, and of the relation between the structure and the resonance spectrum of a single crystal. Finally, the resonance data which are available for ions of the 3d, 4d, 5d, 4f and 5f transition groups are tabulated. These data are expressed as values of the parameters in the appropriate spin Hamiltonian, and, where applicable, formulae are given for (1) energy levels, (2) susceptibility, (3) specific heat tail CT2/R, in terms of these parameters.
481 citations