Proton spin–lattice relaxation times, T1, have been measured as a function of temperature for KBH4, NaBH4, and LiBH4. For NaBH4 and KBH4, 23Na and 11B relaxation measurements were also made. In all cases, the magnetization recovery is approximately exponential. Correlation times, τc, derived from the T1 data were used to calculate activation energies, V, for BH4− ion reorientations. For the cubic phase of KBH4, V = 14.8 ± 0.4 kJ/mole (3.55 ± 0.1 kcal/mole) (± always refers to rms error) from measurements on proton and 11B. For NaBH4, V was found to be 11.2 ± 0.5 and 14.8 ± 0.7 kJ/mole (2.7 ± 0.1 and 3.5 ± 0.2 kcal/mole) for the high- (cubic) and low-temperature (tetragonal) phases; an anomaly in τc was observed at temperatures slightly below the phase transition, and may be interpreted as a relatively sudden change in V associated with the phase transition. In LiBH4, a rather broad minimum was observed for the proton T1 vs temperature; this has been interpreted as due to two inequivalent BH4− tetrahedra with activation energies of 20 ± 1 and 16 ± 1 kJ/mole (4.7 ± 0.3 and 3.8 ± 0.3 kcal/mole). The proton and 11B nuclei are relaxed by magnetic dipolar interactions, but quadrupolar fluctuations are the dominating relaxation mechanism for 23Na in the cubic phase of NaBH4.

The deuteron nuclear magnetic resonance of single crystals of (ND4)2SO4 has been studied from 77° to 300°K and particularly in the region of the first-order ferroelectric phase transition at 223°K. At high temperatures four closely spaced pairs of lines (10-G max separation) are observed, which shift abruptly as the crystal is cooled through the transition temperature (Tc). These lines are assigned to the two inequivalent ND4+ ions (I and II) in a unit cell, which are slightly distorted from the tetrahedral configuration and rapidly reorienting so as to average the quadrupolar splitting to a small value. At 130°K only two pairs of closely spaced lines are observed due to ND4+ (II); at 77°K eight pairs of widely spaced (400-G max separation) lines are observed corresponding to ND4+ (I). In the transition to the ferroelectric phase the principal coordinate system of the electric-field gradient (EFG) tensor rotates by about 30° for each ion with little change in the coupling constants or asymmetry parameters. The spontaneous polarization produced by the distorted ions has been calculated from the observed EFG tensors and is in reasonable agreement with the experimental value. Proton spin—lattice relaxation times (T1) and proton relaxation times along the rf field (T1ρ) have been measured for polycrystalline (NH4)2SO4. T1 versus temperature (T) has two minima, and may be interpreted as due to the two inequivalent NH4+ tetrahedra reorienting at different frequencies and coupled to each other via dipole—dipole interaction. Correlation times τc at various temperatures have been obtained for the two inequivalent NH4 ions from the T1 and T1ρ measurements; above Tc and below 170°K, logτc is a linear function of T−1, with normal values of pre-exponential factors. The activation energies change only slightly at the transition, in agreement with recent neutron inelastic scattering data. τc is, however, anomalously short immediately below Tc due to the volume contraction of the crystal. Correlation times have also been estimated from deuteron resonance linewidths and proton second moments. The results are consistent with those derived from the proton T1 and T1ρ data. An order—disorder mechanism is proposed for the phase transition in which NH4+ tetrahedra are disordered with respect to the crystal ab plane. A first-order transition is described by the molecular field approximation for a sufficiently strong dependence of the interaction parameter on the spontaneous polarization.

The low-temperature {1. 3 -20.0 K) high-magnetic-field (0 -10 T) heat capacity and the magnetization and magnetic susceptibility (1.7 -300 K) of the strongly Pauli paramagnetic RCo2 (R =Sc, Y, or Lu) compounds with the MgCu2-type structure were measured. The heat-capacity results for ScCo2, YCo~, and LuCo2 show that the electronic specific-heat constant decreases with increasing magnetic fields (by 7%, 4%%uo, and 10', respectively, at 10 T). For YCo2 the coefficient of the T term (P) in the heat capacity is found to increase by 18% at 10 T, but for ScCo2 and LuCo2 P remains constant within experimental error. Analyses based on several theoretical models of the quenching of spin fluctuations by high magnetic fields suggest that the characteristic spinfluctuation temperature is -20 K for ScCo2, -35 K for YCo2, and -16 K for LuCo2. The magnetization and the field dependence of the magnetic susceptibility of the same samples as used in the heat-capacity measurements indicate the presence of ferromagnetic impurities in the samples, but the estimated concentrations are sufficiently low that they probably have no effect on the observed heat capacities. Maxwell's thermodynamics relationship between the field dependence of the heat capacity and the temperature dependence of the magnetic susceptibility has been examined.

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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.

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