D. Michel

Dielectric and NMR Studies of Nanoporous Matrices Loaded with Sodium Nitrite

NMR and dielectric studies have been performed on NaNO2 loaded in mesoporous matrices of MCM-41 and SBA-15 with pore sizes of 20, 37, and 52 Å. The spin-lattice relaxation rate and 23Na NMR line shape, as well as the complex impedance, were measured within a broad temperature interval including the ferroelectric phase transition in bulk NaNO2. Two different phases of sodium nitrite, the crystalline and melt phases, are shown to coexist under conditions of a restricted geometry. The crystalline phase undergoes a ferroelectric phase transition. The melt fraction increases with temperature. The existence of two phases accounts for all experimental data on NaNO2 under conditions of a restricted geometry.

NMR studies of structure and ferroelectricity for Rochelle salt nanoparticles embedded in mesoporous sieves

NMR studies were carried out for Rochelle salt embedded in molecular sieves. 23 Na magic angle spinning (MAS) and multiple quantum (MQ) MAS NMR spectra revealed a complex structure of the confined crystalline material. The major part of particles within nanopores had a structure similar to that of bulk Rochelle salt. The 23 Na spin‐lattice relaxation times at various temperatures associated with this modification were also similar to those for bulk Rochelle salt and showed broad minima that corresponded to the ferroelectric and re-entrant phase transitions under nanoconfinement at temperatures just below the relevant transitions in bulk. This result suggests that the bulk-like modification within pores is ferroelectric in between. Fast spin relaxation in the rest of the confined material reflected high molecular mobility.

Effect of confined geometry on linear and nonlinear dielectric properties of triglycine sulfate near the phase transition

The temperature dependence of the linear permittivity and the third harmonic generation amplitude of nanocomposites representing nanoporous silica matrices (opal matrix and SBA-15) with triglycine sulfate embedded in pores has been studied in the region of the ferroelectric phase transition. A broadening of the phase transition and an increase its temperature in comparison with bulk triglycine sulfate have been revealed. The latter becomes more significant as the pore size decreases. It has been shown that the nonlinear dielectric properties of nanocomposites near the phase transition differ significantly from the properties of bulk triglycine sulfate.

Dielectric and calorimetric investigations of KNO3 in pores of nanoporous silica matrices MCM-41

The temperature dependences of the linear dielectric permittivity, the third harmonic amplitude, and the heat capacity of nanoporous silica matrices MCM-41 with cellular channels (3.7 and 2.6 nm in diameter) filled with KNO3 have been investigated in comparison with those obtained for bulk potassium nitrate. Measurements have been performed during heating and cooling in the range from room temperature to 463 K. Anomalies corresponding to structural phase transitions have been observed. A significant broadening of the temperature region of the existence of the ferroelectric phase III of potassium nitrate upon cooling has been revealed. This broadening increases with a decrease in the size of pores. It has been shown that, in the nanocomposites with potassium nitrate, the ferroelectric phase can also be formed during heating. The efficiency of observation of the third harmonic generation for studying nanocomposites with the ferroelectric phase has been demonstrated.

35Cl NMR studies of the domain structure of tetramethylammonium cadmium chloride (TMCC) at lower temperatures

Quadrupolar perturbed 35Cl NMR and 35Cl NQR investigations are performed on single crystals of tetramethylammonium cadmium chloride, (CH3)4NCdCl3 (TMCC), in order to study the formation of ferroelastic domains in the ferroelastic phase II below 118 K in which the hexagonal symmetry of the high temperature phase I is lost. The experimental results cannot simply be explained in terms of the well known monoclinic domain structure but additionally a twin domain structure is found to exist in phase II: six orientational domains are verified from the NMR rotational pattern at 113 K in contrast to the expected number of three for a classical 6/mF2/m transition. The rotational angle Θ between the epitaxially grown twins is calculated from the strain tensors of the three orientation states, applying an algebraic approach. The theoretical value Θ = 3.5° is in very good correspondence with the experimental result from the 35Cl NMR studies (~4°). Similar results were also derived in recent x-ray studies in phase II. In the low temperature phase III below 104 K the number of non-equivalent Cl positions is consistent with both the space group of phase III and the sixfold enlarged size of the unit cell in phase III with respect to that in phase I as observed in XRD studies.

Barium-137 nuclear magnetic resonance study in the various phases of

quadrupole perturbed NMR measurements on a multidomain crystal were performed in order to determine the quadrupole tensors (which are proportional to the electric field gradient (EFG) tensors) in the tetragonal, orthorhombic and rhombohedral phase of . Because of the relatively large quadrupole splitting, only the central transitions were analysed. By means of the well known Volkoff theory the principal values and the directions of the principal axes were evaluated from the angular dependent NMR spectra. Furthermore, the correct determination of the principal values of the quadrupole tensors has been controlled by means of measurements on polycrystalline samples of the same origin. The conclusions from the measurements on the multidomain crystals are in very good agreement with the results of NMR line shape measurements on powdered samples. Moreover, the NMR studies on the crystal sample allow a study of the domain structure in the different ferroelectric phases in complete agreement with the predictions from symmetry considerations.

Two-dimensional2H NMR exchange spectroscopy on conducting ionic crystals

The two-dimensional (2-D) deuterium nuclear magnetic resonance (NMR) exchange spectroscopy is applied to two types of conducting ionic crystals for the study of hydrogen mobility and conductivity, viz. partially deuterated ammonium hydrogen selenate, NH4HSO4 (AHSe), and partially deuterated mixed crystals of betaine phosphate (DBP) and betaine phosphite (DBPI), DBP1−xDBPIx. In both crystals chemical exchange processes of deuterons between different hydrogen bridges occur which are studied by the 2-D-2H-NMR technique over a wide temperature range in the slow-exchange regime. For AHSe exchange only occurs between two sites which are involved in hydrogen bonds. Two Arrhenius-like exchange processes were found the activation energies of which could be determined. For the case of DBP1−xDBPIx, with several deuteron sites taking part in the exchange, the analysis of the quantitative exchange behavior required a combination of time-domain analysis of our 2-D NMR data with mixing-time- and temperature-dependent measurements. Different exchange rates for each two-site exchange, all showing Arrhenius behavior, were obtained for DBP0.3, DBPI0.7. For crystals with different phosphite concentrationx the differences towards DBP0.3, DBPI0.3 were established. With the help of conductivity data from dielectric measurements quantitative relations between the exchange and conductivity processes are obtained for AHSe as well as for DBP1−xDBPIx. Finally, an estimation of effective charge carrier densities is discussed in view of possible conductivity models for both crystals.

Dielectric properties of crystalline binary KNO3—AgNO3 mixtures embedded in nanoporous silicate matrices

The results of dielectric studies of MCM-41 sili cate matrices with cellular pores (37.0 and 26.1 Å in diameter) filled by binary mixtures of K1−xAgxNO3 (x=0, 0.05, 0.10) are reported in comparison with those obtained for bulk salts of the same composition in the temperature range of structural phase transitions. It has been revealed that, upon heating, the nanocomposites undergo transitions from crystal modification II to phase I, as in bulk KNO3, whereas the bulk mixtures with x=0.05 and 0.10 have a complex structure in accordance with the phase diagram. It has been shown that embedding binary salts and pure potassium nitrate in pores with a diameter of 26.1Å results in the formation of an intermediate ferroelectric phase upon cooling. The permittivity and electrical conductivity are found to increase with increasing AgNO3 concentration in bulk samples, as well as with decreasing pore size in the nanocomposites for all values of x.

Chemical exchange in NH4HSeO4 single crystals studied by two-dimensional 2H nuclear magnetic resonance

Processes of chemical exchange of deuterons in partially deuterated ammonium hydrogen selenate, NH4HSeO4 (AHSe), crystals are investigated by means of 2H nuclear magnetic resonance (NMR) experiments over a wide temperature range. The temperature dependencies of the quadrupole line splittings in the one-dimensional spectra of AHSe above 350 K revealed line-shape changes which are characteristic for chemical exchange processes. A detailed study of these exchange processes in AHSe is achieved by means of two-dimensional 2H NMR experiments. In the temperature range investigated, a chemical exchange occurs only between those deuteron (proton) sites which are involved in hydrogen bonds (- and -positions). It was established that the rates of exchange between all types of hydrogen-bound deuteron are approximately the same. Exchange between these positions and the deuterons in the ND4 groups could not be detected. On the basis of our findings, we finally discuss a model for the microscopic mechanism of hydrogen transport in AHSe.

Micromechanism of proton conduction in a KHSeO4 crystal

The proton exchange in a potassium hydrogen selenate crystal is investigated in detail. Partial deuteration of the crystal (approximately 80% of the protons are replaced by deuterons) makes it possible to use the method of nuclear magnetic resonance (NMR) on quadrupole nuclei to perform research into proton (deuteron) transport. In addition to conventional Fourier-transform NMR spectroscopy, elementary processes of deuteron chemical exchange are studied by two-dimensional NMR spectroscopy, which provides unique information regarding these processes. Slow exchange between protons of hydrogen bond chains and dimers consisting of two SeO4 groups is revealed. It is established that this process is responsible for the proton conduction in the potassium hydrogen selenate. The NMR data are compared with the results of dielectric measurements carried out at a frequency of 1 kHz.