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The 127 I NQR, IR absorption and Raman spectra of impurity-doped and mixed lithium iodate Li 1− x H x IO 3 crystals grown from water solutions with different LiIO 3 /HIO 3 ratios were investigated depending on the content of the impurity hydrogen x . The NQR results suggested that, at small concentration of doping iodic acid x x 1− x H x IO 3 are formed.

The geometric isotopic effect in hydrogen-containing crystals of the KDP-type is investigated. The mechanism of changes in the hydrogen bond lengths under deuteron substitution is supposed. It is shown that (in the framework of a three-dimensional model for hydrogen bonds based on the Einstein approximation for anharmonic oscillators) the correspondence between experimental and theoretical data can be achieved if one takes into account the interactions between the neighboring H-bonded chains in the crystal.

The theoretical aspects of the structural order–disorder phase transitions in systems with four-minimum potential were considered. A theoretical model of the phase transition is proposed summarizing the well-known Ising model. The proposed theory predicts the existence of two or three crystal phases.

The ferroelectric crystal of ammonium iodate NH4IO3 was investigated by the methods of Raman and IR spectroscopy. Powder IR and Raman temperature dependent spectra of the title crystal were measured in the temperature range 300–400 K near the Curie temperature (Tc=367 K). The 127I nuclear quadruple resonance (NQR) spectra of the NH4IO3 crystal, which were measured in the same temperature range, are discussed. The effect of hindered rotation of NH4+ and IO3− ions was revealed in the pretransition temperature range and is explained by anomalous crystal thermal expansion.

The study of temperature dependency of the electric field gradient (EFG) tensor asymmetry parameter η(T) from 127I nuclear quadrupole spectra (NQR) for NH4IO3 crystals in the temperature range of 100–400 K including the phase transition temperature are presented. It is shown that the phase transition in the NH4IO3 crystal has the properties of phase transitions of both first and second orders. It is shown that some decrease in the barrier energy for reorientation vibrations of IO3− ions (U=24 kcal/mol) can be explained by the anomalous expansion of the crystal in the pretransition temperature range of 335–367 K and by some weakness of the hydrogen bond subsystem.

The influence of geometric isotopic effects on the Curie temperature of hydrogen-containing crystals was considered. The experimental pressure and concentration dependencies of the Curie temperature have been explained in a framework of the pseudo-spin Ising model, by taking into account the changes in geometry of hydrogen bonds under pressure and due to isotopic exchange.

We propose a new approach to the band shape formation in crystals with strong symmetric hydrogen bonds. Our consideration is based on the assumption that the interaction between the incident light applied to the crystal and polarized optical photons is stronger than that between molecular excitations and the photons. In this case the formation of polaritons takes place and hence molecular excitations begin to interact with the polaritons. As a result, a new kind of correction to the absorption coefficient caused by the beam of the spectroscopic instrument appears. The complete band shape of the OH stretching band is computed. A comparison with the available experimental data is carried out.

FT-IR (4000-400 cm −1 ) and FIR (400-10 cm −1 ) spectra of α-modification of acid potassium iodate (α-KIO 3 ·HIO 3 ) were measured in the paraelectric (at 300-220 K) and ferroelectric (at 223-20 K) phases. FIR spectra of the ferroelectric phase were found to exhibit doublet structure of many bands while FT-IR spectra mainly remain invariable. The results are in good agreement with the order-disorder mechanism of ferroelectric phase transition connected with localization of the proton within a strong hydrogen bond.

An investigation of the one-dimensional model for the hydrogen bond was made using the double adiabatic approximation. The dependence of the frequency of proton and deuteron stretching vibrations on the distances between equilibrium states of the oxygen atoms was determined whilst taking into account the Ubbelohde effect.

The mutual effect of protons (deutrons) and heavy ions is examined within the limits of adiabatic approximation. The dependence of the length of the hydrogen bond on the proton state function is obtained. It is shown that the mutual effect of the proton and ion subsystems, taken into account by adiabatic corrections, should increase the phase transition temperature.

The pressure dependencies of the electric field gradient tensor asymmetry parameters for 127 I NQR spectra in hydrogen-bonded iodate crystals has been studied. The proton locations in the unit cells and their variation under hydrostatic pressure or due to structural phase transitions are discussed.

The lattice dynamics of pseudo-binary system LiIO 3 ·HIO 3 has been investigated by means of IR spectroscopy and NQR technique. It is shown that at compositions close to 0.2 ≤ x ≤ 0.34 mixed crystals Li 1-x H x IO 3 based on 2LiIO 3 ·HIO 3 are formed. The spectral changes observed in the 2600 and 400 cm −1 region indicate the formation of “intermolecular” hydrogen bondings between the components. The concentration phase transition to amorphous glass phase at x near 0.26 has been detected.

It is shown that the phase transition in the KIO 3 crystal at 120K is caused bv the ordering of an impurity in an asymmetrical two-minimum potential. The shape of this potential changes under hydrostatic pressure and the role of proton tunnel effect increases with increasing pressure. The transformation of intemoiecular hydrogen bond. length in the ∝ -HIO 3 crystal under pressure is studied. The value of the O-H....O bond length variation and the compressibility factor for the ∝-HIO 3 crystal is estimated.

The NQR Zeeman effect of 127 I (± 1 2 ←→ ± 3 2 transition) for the LiIO 3 crystal of hexagonal modification has been investigated, and the EFTG asymmetry parameter for nuclei of 127 I determined. Measurements of optical properties of the z -cut crystal in polarized light within the region of stretching vibrations and their overtones for IO − 3 revealed that transmission (reflection) depends on the direction of the incident light wave electric vector. The results obtained indicate that crystals of LiIO 3 grown from aqueous solutions show anomalous biaxiality.

The NQR Zeeman effect of 127I quadrupole resonance (± 1 2 ↓ ± 3 2 transition) and the polarized IR spectra of a single crystal of β-LiIO3 have been studied. The z-axes of the EFG orientation in the unit cell were determined; the EFG asymmetry parameter for the 127I nuclei was refined giving η = 0.027 ± 0.003 at 300 K for β-LiIO3. The dichroic ratio in the (100) crystal plane over the I-O stretching vibration region has been measured from polarized spectra and calculated on the basis of an “oriented gas” model by the use of refined structural data. Comparison of some spectral, structural and quantum-chemical characteristics of both β- and α-modifications of LiIO3 crystal has been made.

It is shown that calculation of the temperature dependence of the 127J NQR frequency (± 12 ↔ ± 32) on the basis of Bayer-Kushida theory allows one to interpret the IR and Raman spectra (ν < 350 cm−1) of hexagonal crystal LiJO3.