Two-Dimensional Exchange NMR and Relaxation Study of the Takagi Group Dynamics in Deuteron Glasses

Evidence for Ferroelectric Nucleation Centres in the Pseudo-spin Glass System Rb1−x (ND4) x D2PO4: A 87Rb NMR Study

“…13 is E a = 80 meV. However, as we have reported in [12], this is only an apparent value, due to the fact that the biased random walk of unpaired Takagi groups (T1,T3) is responsible for the dominant contribution to the spin-lattice relaxation, and the apparent value is in fact the sum of the hindering potential for symmetric bonds and the ice bias, i.e. E a + 2E b = 80 meV.…”

“…In contrast to the pure end members RDP and KDP, the macroscopic strain cannot develop properly in the mixed crystals. In the glass phase regions the system remains pseudo-tetragonal down to very low temperatures, though the local symmetry is "apparently" broken (the system is only pseudo-frozen and restores the symmetry in the long time scale [11,12]). The strain can only develop when the FE clusters, and with them the coherence length of the soft mode, become wide enough.…”

“…Another interesting nucleus is the 31 P that is located in the centre of the PO 4 tetrahedra. Via the chemical shift interaction, 31 P NMR can, for example, distinguish between the six Slater states of its PO 4 group and thus observe changes of these configurations [11,12]. Furthermore, the characteristic time scale of NMR for time averaging is in the order of milliseconds, and can be extended to the order of 100 s by means of 2D exchange NMR measurements.…”

“…There is, however, a non-vanishing probability that the Takagi pair separates into two unpaired Takagi groups that are stable. The existence of unpaired Takagi groups in a KDP-type lattice was proved by means of 31 P two-dimensional (2D) exchange NMR [11,12]. Recently the separation of a Takagi pair in the FE bulk of KDP was beautifully demonstrated by means of ab initio electronic structure calculations [14].…”

“…In the temperature dependence of 87 Rb spin-lattice relaxation we observe essentially two competing mechanisms: a soft mode component related to the displacive part of the FE phase transition and a thermally activated component that can be assigned to the biased random walk of unpaired Takagi groups T1 (HPO 4 ) and T3 (H 3 PO 4 ) [11,12]. They can move through the lattice without essential hindering, similar to the ion states HO -and H 3 O + in hexagonal ice.…”