Jump diffusion and rotational tunnelling of ammonium in beta -alumina by NMR

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Abstract
The motion of NH4+ ions in non stoichiometric beta -alumina single crystals has been observed at liquid helium temperatures and between 77 and 360K, and the effects on the proton NMR lineshape are reported. The authors show that at 4.2K, the ions in BR positions exhibit rapid quantum tunnelling and have a C2 axis parallel to the c axis of the crystal. The ions in mO positions are motionless. Spin isomerism theory allows a quantitative interpretation of the spectra. Between 77 and 240K there is a single line corresponding to rapid 90 degrees reorientation around a C2 axis perpendicular to c. Above 240K jump diffusion begins to occur and there is a corresponding line narrowing. The activation energies found from NMR and conductivity are the same, showing that the elementary motion of the ion is a translation plus a rotation jump. Good quantitative agreement is found between electrical conductivity and NMR data.