J.-M. Crettez

Electrical conductivity of α-LiIO3 acid type crystals at 1 kHz

Abstract The temperature dependence of the electrical conductivity of α-LiIO3 acid type crystals is studied. By applying a very low amplitude electric field at 1 kHz and performing a continuous sampling of measurements, differences, reproducible for all the investigated samples, appeared between the first and subsequent heatings The anomalies occurring during the first heating are attributed mainly to inclusions of mother liquor, HIO3 and Li1-xHxIO3. The ‘intrinsic’ conductivity is measured after a first annealing at about 470 K; the activation energies are then calculated.

Structure of lithium iodate - iodic acid solid solution Li1-xHxIO3 (x = 0.33)

Abstract The structure of the solud solution Li 1-x H x IO 3 ( x = 0.33) is determined at room temperature by X-ray single crystal and neutron powder diffraction. The space group is found to be P6 3 like α-LiIO 3 . Hydrogen atoms are in general positions with the occupation factor 0.33 and form weak bonds with neighbouring oxygens. The structure is briefly compared with that of α-LiIO 3 .

Lithium iodate: Phase transitions revisited

Abstract The sequence of phase transitions of lithium iodate powders has been investigated by neutron thermodiffractometry. Upon heating, powders obtained by grinding hexagonal crystals exhibit the well known α → γ → β sequence. The intermediate orthorhombic phase γ is never observed as a single phase but always coexists either with α or with β-LiIO3. The transition temperatures and the α → γ transformation rate are found to be strongly dependent on the crystallite dimension, especially for grain sizes smaller than 100 μm. The hysteresis of the reversible α ⇄ γ transition, the kinetics of both α → γ and γ → β transitions, and the influence of proton contamination are also investigated. Contrary to this, natural powders obtained by fast evaporation of an aqueous solution do not exhibit any intermediate phase and transform directly from α to the β tetragonal modification. Further DTA and X-ray investigations show that the occurrence of the γ phase is related to the mechanical treatment of the sample (grinding, polishing…), i.e. to the presence of defects. Previous literature data are reviewed and the apparent discrepancies concerning the transition processes are elucidated on the basis of the present results.

Dielectric response of α-LiIO3 acid type crystals

Abstract α-LiIO 3 is often used as non-linear optical material. In view of its preparation, low pH aqueous solutions are used as mother solutions, which give crystals with some hydrogen incorporated to the lattice. Thus, dielectric characterization appears as a very important tool. In this work, the dielectric properties of acid type α-LiIO 3 crystals (conductivity, permittivity, loss-angle tangent) are investigated as functions of temperature and frequency, through an original method allowing seemingly continuous measurements and avoiding space charge contributions to the measurements. The results show the dipolar and low frequency conductivity contributions to the dielectric response. The role of the protons in this ionic conductor is also discussed.

Raman study of crystals

In this work we present a polarized Raman study of single crystals for several values of the concentration made using different scattering geometries. The Raman spectra, composed of broad bands, have been fitted in accordance with a symmetry analysis which allowed us to assign the vibrational modes, and determine their frequencies and damping constants. The results are compatible with an average hexagonal symmetry for the solid solutions with x in the range . In each of the spectra we found two bands at about 590 and , probably associated with the existence of structures in the solid solutions.

Electrical conductivity and micro-Raman scattering studies of ionic conduction in Li1−xHxIO3 solid solutions

Abstract Li 1− x H x IO 3 solid solutions have been investigated by ac electrical conductivity and micro-Raman techniques, for x ≤0.32. The presence of protons leads to a continuous reduction of the anisotropic intrinsic conduction of the system. The in-plane conduction mechanism would be by Li + vacancy hopping, while a mixed process by interstitial Li + and H + would account for the conduction along the c -axis. The solid solutions undergo the same phase transition sequence as the pure crystal, but the presence of the protons shifts the transition temperatures to lower values. Micro-Raman spectroscopy has been successfully introduced to demonstrate that proton mobility occurs preferentially along the c -axis.

Comparative study of electrical behavior and phase transitions in pure and chromium doped α-LiIO3 single crystals

Abstract The polymorphic phase transitions of undoped and chromium doped α-LiIO3 crystals have been investigated by means of DTA experiments at slow heating rates and electrical measurements vs temperature and frequency. Upon heating the well known α ⇔ γ ⇒ β sequence appears to be influenced by the samples morphology, acidity of the growing solution and doping. Dielectric response at room temperature is close to the type “superposition of d.c. ionic conductivity and dipolar response”. We describe the thermal behavior of conductivity along the c polar axis at three frequencies. Electrical measurements also highlight a higher value of ionic conductivity and a less destructive transition for doped crystals.

Structural disorder and ionic conductivity in Li1−x H x IO3 solid solutions

Abstract Li1−x H x IO3 solid solutions have been obtained for several values of the concentration x. The crystalline samples have been analyzed by Raman spectroscopy and dielectric techniques. The polarized Raman spectra reveal an average hexagonal symmetry for the disordered solid solutions formed for x between 0.22 and 0.36. The dielectric constants and the AC resistivities show a low anisotropic character for the ionic conduction in the disordered sample, as compared with the pure α–LiIO3 crystal.

Temperature Dependence of the Structure of Li1-xHxIO3 Studied by High Resolution Neutron Powder Diffraction between 10 and 380 K

Thermal variations of structural parameters of Li1-xHxIO3 are reported between 10 and 380 K, and are compared to isotypic α-LiIO3 in the same temperature range. Beside the presence of a weakly bound, randomly distributed proton in general position of P63, differences appear concerning the thermal expansion coefficient along the polar axis and a significant shift of the Li position is observed from 210 K to the decomposition temperature (≈400 K). This predicts differences in the Li1-xHxIO3 pyroelectric and piezoelectric effects when compared to the α-LiIO3 crystal widely used for technological applications.

Thermal expansivity of α, λ and β-LiIO3 by neutron thermodiffractometry

Abstract We describe neutron powder diffraction experiments and show how thermodiffractograms obtained by means of a position-sensitive-detector diffractometer can be used to determine accurately the thermal variations of cell parameters and the principal linear expansion coefficients of materials. The case of the three phases α, λ and β of lithium iodate is investigated, results are subsequently compared to previously published ones.