NQR is observed in solid state (mono- and polycrystalline) and in frozen liquids. The existence of EFG allows one to observe NQR in the absence of external fields. Typical nuclei on which NQR is observed are 35Cl, 37Cl, 79Br, 81Br, 127I, 121Sb, 123Sb, 75As, 63Сu, 65Сu,14Ν, 10Β, 11Β etc. NQR is often used for the identification of certain chemicals (explosives and poisons), quality control of pharmaceuticals, in the investigations of ferroelectricity, solar-cell semiconductors, optical memory units, superconductors and other materials, as well as in fundamental research of molecular and ionic crystals.
NQR allows one to study
• symmetry and structure of crystals, phase transitions;
• distributon of the electron density around certain nucleus;
• non-equivalence of nuclei in the crystal lattice;
• structural defects and ordering effects;
• nature of chemical bonds;
• mobility of groups of atoms;
• quadrupolar coupling constant and asymmetry parameters of EFG.
NQR frequencies can lie in the broad range, from 100 kHz to 10 GHz. Depending on the chemical structure and surroundings particular nucleus can resonate at various frequencies. For example, the resonance frequency of 35Cl lie in the range 30-40 MHz (see the table below). In inorganic compounds the NQR frequency range of chlorine spans from 2.4 MHz (FeCl2) to 70.7 MHz (FCl).
| Bond type | NQR frequency, MHz |
|---|---|
|
N-Cl |
44-56 |
|
S-Cl |
30-40 |
|
C-Cl |
32-35 |
|
Si-Cl |
15-20 |
At CMR, NQR measurements can be performed in the range 27-120 MHz at temperatures 4-400 K. Samples are placed in a glass tube (35 mm high, 5 mm in diameter). Sample preparation can be done at CMR (if the sample has to be protected from oxygen or water vapor the sample tube should be hermetically sealed by the user).
1. T1 relaxation time measurements
Several methods can be used for T1 measurements:
- pulse sequence 90°—τ—90° (if T1≈ T2 ≈ T2*).
- pulse sequence 180°—τ—90° (“inversion-recovery”) or stimulated echo (if case of short T2*).
- saturation method.
Example:
The measurement of spin-lattice relaxation time of 35Cl in KCl was done on Tecmag Redstone spectrometer at the frequency 28.1 MHz by the inversion-recovery method at room temperature. In the Figure the signal intensity is given as a function of the delay between pulses. The T1 time was estimated to be about 20 ms.
2. T2 relaxation time measurements
Several methods can be used for T2 measurements:
- pulse sequence 90°—τ—180°;
- Carr Purcell method;
- Carr Purcell Meiboom Gill method (CPMG).
Multi-pulse sequences Carr Purcell and CPMG represent a way for express-analysis of T2 relaxation and can be used if the T2* ˂ T2 condition is fulfilled.
