Ion Motion in Tetrafluoroborate Salts. II

Fluorine nuclear relaxation and linewidths have been studied as a function of temperature in a series of tetrafluoroborate salts containing the following cations: ND4+, K+, Rb+, Cs+ NO+, NO2+ and Na+. Anion tumbling correlation times have been extracted from the nonexponential decays of longitudinal magnetization in the temperature region near the orthorhombic-cubic phase transition and exhibit a close relationship to the temperature T0 of the phase transition: τF(T0)∼10−10 sec. Activation energies decrease smoothly with increasing cation radius. Previously unreported phase transitions have been observed in RbBF4, CsBF4, NOBF4 and NO2BF4 at 523°, 449°, 433°, and 335°K, respectively. Room-temperature quadrupole coupling constants are reported for 11B in the alkali metal salts.