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The structural phase transitions between the intercalated and the non-intercalated phases of the layer structure compound n-decylammoniumchloride and n-hexylammoniumchloride have been studied via proton-14N and proton-35Cl nuclear quadrupole double resonance. The results have been related to an order-disorder model of the alkylammonium chains. The temperature dependence of the order parameters of the -NH3 head groups has been determined.

The14N NMR spectra and spin-lattice relaxation timeT1 of [N(CH3)4]2ZnI4 have been studied between room temperature and 200 K. Two phase transitions atTc1=255 K and atTc1=217 K are observed. The14N NMR lineshape andT1 data suggest that the intermediate phase is commensurate rather than incommensurate in spite of the presence of a Lifshitz invariant in the expansion of the free energy density in powers of the order parameter. We also discuss the phenomenological theory of structural phase transitions in [N(CH3)4]2ZnI4.

The structural phase transitions between the non-intercalated phases of the layer structure compound N-octylammoniumchloride have been studied via proton-14N and proton-35Cl nuclear quadrupole double resonance. The results have been related to an order-disorder model of the octylammonium chains. The temperature dependence of the order parameters of the -NH3 head groups has been determined.

Using a proton-nitrogen double resonance technique we have determined the quadrupole coupling of14N in the room temperature orthorhombic (Cmca), the low temperature tetragonal (P42/ncm), and the monoclinic low temperature (P21/c) phases of (CH3NH3)2CdCl4. In all these phases all nitrogens are chemically equivalent demonstrating that the disorder in the orientations and H-bonding arrangements of the CH3-NH3 groups in theC m c a andP42/ncm phases is indeed dynamic and not static. In the monoclinic phase the14N quadrupole coupling constant equalse2qQ/h=880 kHz and the asymmetry parameter is?=0.20, wherease2qQ/h=790 kHz,?=0.1 in the tetragonal low temperature phase ande2qQ/h=726 kHz,?=0.21 in the room temperature orthorhombic phase. The observed increase in the14N quadrupole coupling constant on going from the orthorhombic phase to the tetragonal low temperature phase which is coupled with a simultaneous decrease in the asymmetry parameter can be understood in terms of a partial freezing in of the dynamic disorder in the C-N bond directions whereas the14N quadrupole coupling tensor in the monoclinic phase is characteristic of a frozen in C-N bond in a deformed lattice, where the N-H — Cl bonds are of different length.

The investigation of thermal, optical and dielectric properties of Ag2H3IO6 and (NH4)2H3IO6 revealed phase transitions in two steps with unknown intermediate phases. Reflectivity measurements in the far infrared region confirm that the main step is caused by a freezing in of the proton tunnelling modes. From the results of NMR-NQR-spectroscopy the low temperature structures for both compounds are deduced. The complete group analysis of lattice vibrations and proton ordering modes of the room temperature phases is performed. In the case of Ag2H3IO6 the transitions can be discussed in the frame-work of the Landau theory, starting with a condensation of an antiphase vibration of the silver atoms in theF point of the trigonal Brillouin zone, whereas in the case of (NH4)2H3IO6 a simultaneous ordering of protons inZ, F, andL-points of the Brillouin zone is proposed for the first step. A new definition of antiferroelectric transitions is suggested which is based on symmetry arguments only. The proton-ordering transitions in both compounds can then be classified as antiferroelectric.

The orientations and asymmetry parameters of the EFG tensors at the different lattice sites of I127 in (NH4)2H3IO6 have been determined by proton-iodine crossover relaxation at room temperature and at 94°K. Based on these results it was possible to prove the existence of superstructure for the hexagonala- andc-directions and to reduce the number of allowed space groups of the low temperature phase to two: andR32. In both space groups half of the hydrogen bonds remain symmetric and only half of them become asymmetric (ordered) on lowering the temperature through the phase transition, but only inR32 there are antipolar sublattices. The transition entropies calculated for this model of the hydrogen arrangement agree fairly well with the measured value. With the evidence available it is not possible to rule out one of the two space groups.