| Proton relaxation times (T(,1) and T(,2)) and linewidth studies have been performed on NH(,4)('+) beta (single crystal and polycrystalline forms) and NH(,4)('+) - H(,3)O('+) beta'' (single crystal) alumina using NMR pulse (10.3 and 18.0 MHz) and cw techniques (27.7 MHz), respectively, over the temperature range of 77 to 475 K. Both compounds are fast ion (protonic) conductors. The relaxation data suggests that protonic diffusion takes place in these compounds. The first known NMR report of an angular dependence of the form 1/T(,2) (or (DELTA)H) (alpha) (3 cos('2) (theta) - 1)('2) + const is made for nuclei in these types of compounds where (theta) is the angle between the crystal's c-axis and the dc magnetic field direction. This angular dependence in the layered two dimensional structure of these compounds is discussed in terms of a diffusion theory of A. Avogadro and M. Villa in a two dimensional system. The NH(,4)('+) beta alumina T(,1) data displays three minima over the temperature range of 77 to 475 K. Two T(,1) minima at higher temperature are discussed in terms of the NH(,4)('+) ion rotational motion proposed by others who have observed similar T(,1) behavior of NH(,4)('+) in other compounds. Motional narrowing takes place at about 255 K in NH(,4)('+) beta and two T(,2) values exist at T > 255 K while only one T(,2) value is seen for T < 255 K.;Based on the T(,1) data, the activation energy for proton motion in NH(,4)('+) beta and NH(,4)('+) - H(,3)O('+) beta'' alumina is 0.21 ev and 0.15 ev, respectively. The values determined for (tau)(,o) at 10.3 MHz are 3.71 x 10('-13) s for beta and 1.51 x 10('-11) sec for beta'' and at 18.0 MHz (tau)(,o) increases to 4.64 x 10('-13) s for beta and 2.17 x 10('-11) s for beta''. It is suggested that a controlled doping of paramagnetic impurities into these compounds could provide information on the translational motion of the protonic species.;A significant difference exists in the T(,1) values for NH(,4)('+) - H(,3)O('+) beta'' alumina in that the two minima are not observed at the higher temperatures and the T(,1) values are generally lower when compared to that for NH(,4)('+) beta. At 77 K the T(,1) values of both compounds agree. EPR measurements show that the beta'' compound contains about 2.0 x 10('20) Mn('+2) ions/cm('3) which is considerably more than the concentration of 6.0 x 10('17) Mn('+2) ions/cm('3) found in beta. It is proposed that Mn('+2) paramagnetic impurities govern the relaxation behavior of protons in the beta'' compound when translation occurs. A single T(,2) relaxation time was found in NH(,4)('+) - H(,3)O('+) beta'' alumina over the temperature range studied and motional narrowing appears to take place at about 115(DEGREES)K. Both compounds exhibit a classic V shape for the T(,1) minima at higher temperatures and this is interpreted to indicate a single correlation time. The T(,1) and T(,2) relaxation for both compounds appear to be exponential at all temperatures. Estimates of the proton spacing in the NH(,4)('+) ion for T(,1) data are found to be resonable for a NH(,4)('+) ion. |
