High field 207Pb spin-lattice relaxation in solid lead nitrate and lead molybdate

Spin-lattice relaxation rates of lead have been measured at 17.6 T (156.9 MHz) as a function of temperature in polycrystalline lead nitrate and lead molybdate. Comparing the results with relaxation rates measured at lower fields, it is found that at high fields and low temperature, chemical shift anisotropy (CSA) makes small but observable contributions to lead relaxation in both materials. At 17.6 T and 200 K, CSA accounts for about 15% of the observed relaxation rate. Above 300 K, the dominant relaxation mechanism even at 17.6 T is an indirect Raman process involving modulation of the (207)Pb spin-rotation tensor, as first proposed by Grutzner et al. [J. Am. Chem. Soc. 123, 7094 (2001)] and later treated theoretically in more detail by Vega et al. [Phys. Rev. B 74, 214420 (2006)]. The improved signal to noise ratio at high fields makes it possible to quantify relaxation time anisotropy by analyzing saturation-recovery functions for individual frequencies on the powder pattern line shape. No orientation dependence is found for the spin-lattice relaxation rate of either material. It is argued from examination of the appropriate theoretical expressions, derived here for the first time, that the lack of observable relaxation time anisotropy is probably a general feature of this indirect Raman mechanism.