Sodium NMR and the phase transition in NaN3

摘要

The sodium nuclear quadrupole interaction has been used as a microscopic probe of the static and dynamic behavior of the crystalline lattice near the phase transition in NaN3at 293 K. The phase transition from a rhombohedral to monoclinic structure is very evident in the experimental measurements of the quadrupole coupling constante2qQ/hand asymmetry parameter eegr;. These measurements were facilitated by the use of the nuclear quadrupole echo and pulsed Fourier transform techniques. In the rhombohedral phasee2qQ/his nearly independent of temperature and equal to 130 kHz while eegr; is zero, which is consistent with the crystal structure. BelowTc,e2qQ/his larger and eegr; is nonzero. As the temperature approachesTcfrom below, both parameters become smaller and the rate increases rapidly nearTc. A simple point charge model calculation indicates that eegr; is a measure of the order parameter squared for the phase transition. The sodium spinndash;lattice relaxation rate 1/T1is proportional toT2throughout the temperature region investigated and shows no effects due to the phase transition. The relaxation is a result of the quadrupolar spinndash;phonon interaction. Because the relaxation rate shows no Arrheniushyphen;type temperature dependence, molecular reorientation of the N3ion is thought not to be a factor in the dynamics of the phase transition.