PRESSURE-DEPENDENCE OF NUCLEAR-QUADRUPOLE SPIN-LATTICE RELAXATION

A theory has been developed for the pressure dependence of the time of nuclear quadrupole spin-lattice relaxation for the principal mechanism of relaxation: libration, reorientation, and modulation. A thermodynamic approach has been used in separating the contributions to the observed temperature dependence of the time T1 from factors that are dependent and independent of the volume change. It has been shown that librational motions of the molecules in the crystal may lead to a deviation from the law of a quadratic temperature dependence of the relaxation rate. Deviations from the power law are related to the magnitude and sign of the contribution from the volume effect. With reorientational motion of the molecules or their fragments, it is necessary, in determining the activation energy of such motion, to study both the temperature and pressure dependences of T1; and it is also necessary to know the ratio of the coefficients of thermal expansion and isothermal compressibility of the crystal. The conclusions from the theory are compared with the available literature data on NQR in molecular crystals.