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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