Wydział Chemii

Wojciech Medycki

L. Latanowicz

Przemysław Szklarz

Ryszard Jakubas

2013

Journal of Magnetic Resonance

231

54-60

10.1016/j.jmr.2013.03.005

Naukowa

Angielski

Artykuł

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 T_c 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 may perform stochastic jumps about any of the four C₃ symmetry axes). The tunnelling splitting of the ground state vibrational level, (ν_T)_v0, of the same frequency for both ammonium groups was estimated as high as 900 MHz . 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 130 K are attributed to isotropic motion of all protons.

Solit state, Paraelectric and ferroelectric phases, Internal dynamics, Proton spin–lattice relaxation time, Proton second moment of NMR line, Complex motion, Schrödinger equation, Tunnelling splitting, Tunnelling jump

http://dx.doi.org/10.1016/j.jmr.2013.03.005