Proton dynamics at low and high temperatures in a novel ferroelectric diammonium hypodiphosphate (NH4)2H2P2O6 (ADhP) as studied by 1H spin-lattice relaxation time and second moment of NMR line.

Proton spin-lattice relaxation times T1 at 24.7 MHz and 15 MHz and second moment of NMR line have been applied to study molecular dynamics of a novel ferroelectric (NH4)2H2P2O6 (T(c)=178 K) in the temperature range 10-290 K. Low-temperature T1 behaviour below Tc is interpreted in terms of Haupt's theory and Schrödinger correlation time of tunnelling jumps. A shallow T1 minimum observed around 39 K is attributed to the C3 classical motion of "intra" proton-proton vectors of NH3 (ammonium groups NH4(+) may perform stochastic jumps about any of the four C3 symmetry axes). The tunnelling splitting of the ground state vibrational level, (νT)v0, of the same frequency for both ammonium groups was estimated as high as 900 MHz ((ħωT)v0=3.7 μeV). This tunnelling splitting exists only in the ferroelectric phase. Magnetisation recovery is found to be non-exponential in the temperature regime 63-48 K. The temperature of 63 K is the discovered T(tun) above which the probability of stochastic tunnelling jumps equals zero. The T1 relaxation time is temperature independent below 25 K, which is related to a constant value of the correlation time characterising tunnelling jumps according to Schrödinger. The T1 minima observed in the paraelectric phase (204 K at 15 MHz and 213 K at 24.7 MHz) as well as second moment reduction at about 130K are attributed to isotropic motion of all protons.