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

Nuclear spin-lattice relaxation times T₁ and relaxation times along the rf field (T_1ρ) have been measured for H¹ and F¹⁹ in (NH₄)₂BeF₄. Below 250^∘{}K the relaxation is due to reorientation of NH₄⁺ ions; proton T₁ versus T has two minima due to two inequivalent NH₄⁺ ions in the unit cell. Above the transition temperature Tc (176^∘{}K) and below 130^∘{}K, the logarithm of the correlation time τc is a linear function of T^-1 with normal values of pre-exponential factors. Immediately below Tc, the time τc is anomalously short, as found previously in (NH₄)₂SO₄. Above 250^∘{}K, both proton and F¹⁹ relaxation become nonexponential and may be characterized by the same pair of relaxation times T₁^' and T₁^''. The shorter component (T₁^') is ascribed to H-F dipolar interactions, and the longer component (T₁^'') to H-F, H-H, and Be-F dipolar interactions. On deuteration, only the longer component remains; also, T₁^' and T₁^'' depend only on the rate of reorientation of BeF₄ tetrahedra. Deuteron-resonance results indicate that the dipole moment per NH₄⁺ ion in (NH₄)₂BeF₄ is about half that in (NH₄)₂SO₄, in agreement with spontaneous-polarization measurements. The phase transition in (NH₄)₂BeF₄ is not as abrupt as in (NH₄)₂SO₄, and may be described by a weaker dependence of the interaction parameter on the spontaneous polarization.