Proton spin–lattice relaxation times (T1) and second moments (M2), at 14.7 MHz, have been investigated for tetramethylammonium cadmium chloride. Discontinuities in T1 at 104 and 119 K indicate the occurrence of crystallographic transformations. It is proposed that, in the low temperature phase below 104 K, the correlation time for hindered motion of one of the methyl groups is somewhat different from that of the other three. Using this model, the calculated T1 and M2 values are in satisfactory agreement with the experimental data. For the low temperature phase, the activation energies of the tetramethylammonium ion, 2.0±0.3 and 1.6±0.3 kcal/mole (8.4±1.3 and 6.7±1.3 kJ/mole), are considerably lower than the values 5.5–11 kcal/mole observed for the tetramethylammonium halides. In the high temperature phase, the activation energy decreases even further to 0.7±0.2 kcal/mole (2.9±0.8 kJ/mole). This decrease is in accordance with other order–disorder phase transitions involving tetrahedral ions.

The subject of temperature measurement below 0.3 K is reviewed, with particular attention paid to developments reported in the period [1970][1971][1972][1973][1974]. Sensors, measurement techniques, primary and secondary thermometers, and fixed points are discussed and attempts are made to assess the accuracy and relative merits of the various devices and methods.

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We have made measurements of the low-temperature heat capacity, magnetic susceptibility, and chlorine nuclear quadrupole resonance (NQR) of CeCU, PrCU, NdCl 3 , SmCl 3 , and CeBr 3 . Some measurements have been made on PrBr 3 and NdBr 3 . The experimental results can be closely correlated with antiferromagnetic linear chain models. The NQR shows that long-range order occurs in PrCl 3 at 0.428 K. In NdCl 3 , the NQR line disappeared at 0.50 K and the resonances produced by an internal field appeared at 0.17 K. In CeCl 3 , the NQR resonance disappeared at 0.110 K and no lines were found above 0.054 K. In SmCl 3 , the NQR line disappeared at 0.21 K and no resonance was observed above 0.1 K. We have measured the internal field, which is proportional to the sublattice magnetization, at the chlorine nucleus in PrCl 3 and NdCl 3 below their Neel temperatures, and have determined the magnitude and the direction of the internal field at the chlorine nucleus relative to the principal axes of the electric field gradient.

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A precise measurement of the temperature dependence of the NQR frequency of 35Cl in KClO3 has been made at constant pressure in the range 12°—90°K. Analysis using the Bayer—Kushida theory predicts lattice modes at 60 and 126 cm−1. A comparison is made with the Raman spectrum.