Ion Motion in Tetrafluoroborate Salts. I. NH4BF4 and ND4BF4

Proton and fluorine NMR data in NH4BF4 and its totally deuterated analog reflect the presence of isotropic and rapid BF4− tumbling as well as onset of isotropic NH4+ ion motion in the orthorhombic phase. A linewidth transition in the 19F spin system from 19±1 G2 at 133°K to 2.5±0.5 G2 at 198°K and 19F thermal relaxation times measured in ND4BF4 from 210° to 478°K yield a span of 19F correlation times from 3×10−4 sec (168°K) to 6.4×10−11 sec (472°K). The transition to the cubic modification at 472°K decreases the 19F correlation time to 2.6×10−12 sec (478°K). Above 318°K, fluorine relaxation in NH4BF4 is dominated by intraionic dipolar interactions, while below this temperature cross relaxation with protons located on neighboring ammonium ions becomes evident. Deuteration removes this contributor to the 19F relaxation. The apparent activation energy for BF4− motion is slightly dependent upon temperature, and increases somewhat with increasing temperature. The pre-exponential factor associated with 19F correlation times above 318°K is abnormally small, possibly reflecting a decrease in the effective moment of inertia by mechanical coupling between BF4− motion and NH4+ motion. Crystallographic measurement of the orthorhombic unit cell parameters from 190° to 296°K shows rapid linear expansion of a and c axes in the range 190° to 260°K, with a marked decrease in the rate of variation of these axis lengths with temperature above 260°K. In the same temperature region a shallow proton T1 minimum is observed, which may be associated with the modification of NH4+ ion motion by lattice expansion or simply due to cross relaxation with the 19F spin system. Proton T1 data also show a shallow plateau at higher temperatures due to interionic dipolar coupling and the BF4− motion. The x-ray data suggest the possibility of a subtle crystallographic change at low temperatures. A small linewidth transition is observed for the proton spin system in the range 173°K (4.0±0.1 G2) to 223°K (2.9±0.1 G2) apparently associated with interionic dipolar coupling.