Nuclear Magnetic Resonance and Nonexponential Spin-Lattice Relaxation in Ferroelectric Ammonium Fluoroberyllate

Nuclear spin-lattice relaxation times

T_{1}

and relaxation times along the rf field (

T_{1 ρ}

) have been measured for

H^{1}

and

F^{19}

{(N H_{4})}_{2}

F_{4}

. Below 250°K the relaxation is due to reorientation of N

H_{4}^{+}

ions; proton

T_{1}

versus

T

has two minima due to two inequivalent N

H_{4}^{+}

ions in the unit cell. Above the transition temperature

T_{c}

(176°K) and below 130°K, the logarithm of the correlation time

τ_{c}

is a linear function of

T^{- 1}

with normal values of pre-exponential factors. Immediately below

T_{c}

, the time

τ_{c}

is anomalously short, as found previously in

{(N H_{4})}_{2}

O_{4}

. Above 250°K, both proton and

F^{19}

relaxation become nonexponential and may be characterized by the same pair of relaxation times

{T_{1}}^{'}

and

{T_{1}}^{''}

. The shorter component (

{T_{1}}^{'}

) is ascribed to H-F dipolar interactions, and the longer component (

{T_{1}}^{''}

) to H-F, H-H, and Be-F dipolar interactions. On deuteration, only the longer component remains; also,

{T_{1}}^{'}

and

{T_{1}}^{''}

depend only on the rate of reorientation of Be

F_{4}

tetrahedra. Deuteron-resonance results indicate that the dipole moment per N

H_{4}^{+}

ion in

{(N H_{4})}_{2}

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