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Nuclear magnetic resonance in kernite Waterman, Harold Hughton

Abstract

This thesis describes the experimental investigation of the complete resonance absorption spectrum of B¹¹ nuclei in a single crystal of kernite (Na₂B₄O₇.4H₂O) placed in a uniform external magnetic field, and its interpretation in terms of the perturbation of the nuclear Zeeman energy, levels by the interaction of the nuclear electric quadrupole moment with the crystalline electrostatic field gradient. Some observations on the Na²³ spectrum in kernite are also included. The present experiment is one of a series carried out in this laboratory on the study of such nuclear electric quadrupole interactions. In the two previous cases studied, those of Li⁷ and Al²⁷ in LiAl(SiO₃)₂ (spodumene), the atomic positions and symmetry of the crystal are such that there is only one kind of site for each of the two nuclear species studied as far as the electric field gradient is concerned. For both these cases the simplest possible spectrum of 2I lines was observed for any orientation of the crystal with respect .to the erternal magnetic field. In the Li⁷ case first order perturbation theory was adequate to explain the observed spectrum while second order theory was required for the Al²⁷ spectrum. In order to study in detail one case of a complex spectrum due to nuclei of the same species in different non-equivalent positions in the unit cell, the methods of radio frequency spectroscopy used for the previous experiments were applied to the study of the complete B¹¹ [formula omitted] spectrum in kernite (Na₂B₄O₇.4H₂O) in a magnetic field of 7060 gauss. The B¹¹ resonance absorption spectrum was observed in detail as the crystal was rotated about three mutually perpendicular axes. Eleven lines due to B¹¹ were observed when the symmetry axis of the crystal was perpendicular or parallel to the magnetic field, and twenty-one lines were observed in all other orientations of the crystal. In addition to some of these lines were slightly split in some orientations of the crystal. This observed number of lines was interpreted by assuming four essentially non-equivalent boron sites in the crystal with four others obtained from them by a 180° rotation about the b axis of the monoclinic kernite crystal. This interpretation drawn directly from the experimental data is in agreement with the previously proposed symmetry of the crystal on the basis of X-ray data. The complete set of properties of the quadrupole coupling tensor [formula omitted] was determined at each of the non-equivalent B¹¹ sites. The values of the quadrupole coupling constants [formula omitted] at the four essentially non-equivalent sites are .645±.003, .588±003, 2.563±007, 2.567±.010 Mc/s, and the corresponding asymmetry parameters are .54±.01, .60±.02, .163±.010, .116±.010. The orientation of the electric field gradient tensors at the four nuclear sites are given in the thesis. The preliminary investigation of the Na²³ spectrum in kernite leads to a direct contradiction of the positions for these atoms proposed on the basis of earlier X-ray data. Whereas the previously proposed structure postulates three non-equivalent sodium positions, two of which are on axes of symmetry, the Na²³resonance spectrum shows definitely that there are essentially two non-equivalent sodium sites neither of which is on an axis of symmetry.

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