Dependence upon Volume of Nuclear Quadrupole Interactions in Crystals

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Abstract
The electric quadrupole splitting of the Na23 nuclear magnetic resonance has been measured in single crystals as a function of pressure and temperature for NaNO3, and as a function of temperature for NaBrO3. It is found that the electric field gradient at the sodium nuclei varies as V—3.8 in NaNO3 and as V—2.0 in NaBrO3. These results, together with the previously reported V—1.9 dependence for Na23 in NaClO3, are compared with theoretical values computed from point charge models for the crystal lattices. It is concluded that the large volume dependence in NaNO3 arises from the anisotropic thermal expansion and compressibility of the hexagonal crystal plus the retention of size and configuration of the NO3— group. The experimental results for NaClO3 can be explained in the same manner, assuming a rigid ClO3— group; however, this model does not account for the V—2 dependence of the Na23 quadrupole coupling observed in NaBrO3. The assumed rigidity of the ClO3— group is supported by the small volume dependence, V—0.10, found for the Cl35 quadrupole coupling in NaClO3, and also by calculations of the interionic contribution to the field gradient at the chlorine nuclei in NaClO3 and KClO3.