Phase transition by Li-7 NMR in a LiNH4SO4 single crystal

The temperature dependence of the Li-7 nuclear magnetic resonance in a LiNH 4SO4 single crystal grown by the slow evaporation method has been investiga ted by employing a FT NMR spectrometer. The rotation patterns of the Li-7 N MR spectra were obtained in three mutually perpendicular planes under an ap plied magnetic field of 4.7 T. The Li-7 NMR spectra demonstrates the occurr ence of the first-order phase transition at T-C (= 285 K). Two different gr oups of Li resonances were recorded below T-C. The two groups represent the transition of Li-7 NMR Lines due to Li(I) and Li(II), respectively; the tw o signals are associated with two physically inequivalent positions of lith ium atoms in the unit cell. The principal axes of the EFG tensor below T-C are not the same as those above T-C. The temperature dependence of e(2)qQ/h and eta in the temperature range of 170-400 K is explained with a single t orsional frequency of the Li-O ion by means of the simple Bayer theory. (C) 2000 Elsevier Science Ltd. All rights reserved.