RADP system

The reason why NMR spectroscopy is so successful in the study of D-RADP system is the existence of local and non-local NMR probes in this system. While, the covalently bonded deuteron sees only the local polarization, but not the polarization of the neighbouring deuteron, the ionically bonded Rb observes a range about 2 lattice constants and is thus capable of distinguishing between different spatial correlations of deuteron order. Furthermore, the characteristic time scale of NMR for time averaging is in the order of millisecond, and can be extended to the order of 100 s by means of 2D NMR exchange measurements. It can thus be considered as a quasi-static investigation method.24... 

Dolinsek et al.7 have reported the existence of quantum tunnelling in these systems. This has been observed from the temperature-independent behaviour of 8 Rb and 2H NMR SLR times at low temperatures (1.6Quantum tunnelling in these systems refers to the tunnelling of protons/deuterons in the double-well potential. In a 31P NMR relaxation time measurements by Chen et al.30 in D-RADP systems, have found an additional T1 minimum at low temperatures apart from the BPP T1 minimum at high temperatures. The low temperature minimum has been attributed to the extreme slowing down of the O-D—O intra-bond motion which is unique to the glassy phase. 

The request that no ND4 should be within the cube is represented by n = 0. The inter-centre distance increases here so fast with x that the behaviour of the system cannot be explained with ND4 clean cubes. Since the density of nucleation centres should not increase drastically on increasing x, we have either to reduce the clean volume with x, or to allow for some stray ND4 keeping the volume constant. The latter possibility is shown in Fig. 14. The open circles correspond to the values where n is increased by one when x is increased by 0.05, starting with n = 0 for x = 0.15. It should be noted that these points still correspond to Rb-rich areas. This can be seen in Fig. 14 where the minima of the curves indicate the average concentration x = n/32. The spoiling of the nucleation centres has, of course, to be paid with a lower local transition temperature, which is indeed observed in the real system. Going along with this procedure, we would obtain cluster sizes of about 100 formula units for D-RADP-25 and of 80 formula units for D-RADP-30, i.e. of about 2 nm in... 

NMR is a powerful technique for providing information about the distribution and dynamics of local RFs, characteristic of such systems. While the quadrupole-perturbed NMR line shape analysis gives details about the distribution of local RFs, spin-lattice relaxation (SLR) studies can give information on the dynamics in the frustrated state of these systems. From the literature, it can be seen that most of the NMR experiments have been carried out in RADP mixed systems and its deuterated analogues. ETFI group published a number of results21-23 on various mixed crystals. 

Salient experimental features exhibited by RADP/D-RADP type mixed systems are (1) diffuse scattered intensities at temperatures less than 100 K, (2) a broad rather than a sharp Cusp in the dielectric susceptibility experiment at the PG transition temperature (unlike in spin glasses), (3) deviation of the temperature... 

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