Quadrupolar Echoes in Solids

Abstract

In a classic paper, Solomon showed that the presence of inhomogeneous, first-order quadrupolar interactions leads to the formation of extra "allowed" spin echoes in nuclei for which the spin I-%. We demonstrate (both theoretically and experimentally) that shifting the rf phase of the second pulse by 90° (in a spin-f system) enhances the extra allowed echoes by a factor of almost 5 in amplitude over the unshifted case. Using the density-matrix formulation (and assuming no magnetic inhomogeneities), we have derived, for a 90° phase shift, the amplitude and shape dependence on the second-pulse turning angle of the |r, 2r, and 3T echoes. Experimental echo amplitudes and shapes (for both 0° and 90° phase shifts) were obtained, at room temperature, on 127 I in a fused sample of KI, and these show good agreement with the calculations. Because of the enhancement, this technique affords a much easier separation of the respective distributions arising from the f <-> f and from the J <-> | satellite transitions than is possible in the unshifted case. Another feature of the phase-shifted case is that, in favorable circumstances, the 3r echo may be observed although the 2T echo is obscured by the receiver recovery time. Preliminary data on quadrupole distributions at Al sites in NiAl and AuAl 2 intermetallic compounds are presented. Spin-spin relaxation of the satellite transitions is noted. Results expected for systems which have spin values other than 7=f (both integral and half-integral) are mentioned.

Solid-state NMR spectroscopy of the quadrupolar halogens: chlorine-35/37, bromine-79/81, and iodine-127

“…I-127 SSNMR studies of diamagnetic solids are, to date, predominantly limited to periodate (IO 4 ) salts 146,159,188,198,199,201 -204,208,209,213 in addition to cubic alkali metal iodides. 79,85,110,117,185,196,200,207,211,212,214,215 The first solid-state 127 I NMR study (of a single crystal of KI) seems to be that of Pound in 1950. 117 Some early studies reported on trends in the solid-state iodine chemical shifts of some of the alkali metal halides.…”

The application of frequency swept pulses for the acquisition of nuclear quadrupole resonance spectra

“…However, pulsed Fourier transform techniques, such as the standard Hahn-echo sequence [23] and quadrupolar echo sequences [24][25][26] (Fig. 1a), are now the preferred methods for acquiring NQR spectra.…”

29Si and 17O (Q)CPMG-MAS solid-state NMR experiments as an optimum approach for half-integer nuclei having long T1 relaxation times

“…Another advantage of this method of signal accumulation is for in situ kinetic experiments, which have been almost impossible for 29 Si in natural abundance because of long acquisition times. Static QCPMG experiments on 2 H in DMS-d6 have been shown to provide sensitivity gains of about an order of magnitude compared to quadrupole echo (QE) [4,5] experiments as well as introducing the spinecho sidebands whose lineshape is very sensitive to dynamics [6]. Therefore it is conceivable that pulse train techniques could now provide a method for examining in situ the recovery of radiation damage in ceramics or glasses at repository temperatures (200-300°C).…”