François Gervais

Soft mode spectroscopy in barium titanate

Infrared reflectivity measurements are performed with a scanning interferometer on a single crystal over a wide range of temperature (300-1350K) especially in the cubic paraelectric phase of barium titanate (Tc=395K). The temperature dependence of TO and LO frequencies, dampings and TO oscillator strengths is reported. An overdamped soft mode, which follows a Curie-Weiss law over 800 degrees , is evidenced. A deviation from this law occurs over a hundred degrees above the tetragonal-cubic transition. The discrepancy of the value of dielectric constant obtained from optical mode frequencies and that obtained by direct dielectric measurements is discussed in terms of the possibility of a disorder which would appear progressively on cooling, in the cubic phase, and which complicates the displacive-type mechanism.

Infrared reflectivity spectroscopy of silicate glasses

Infrared reflection spectra of more than fifty silicate glasses have been obtained with a Fourier-transform infrared spectrometer and are reported in the range from 20 to 4000 cm −1 . Results have been systematically analysed with both Kramers-Kronig inversion and fit of the factorized form of the dielectric function to reflectivity spectra. Good agreement between both methods has been found. The detailed analysis of this set of data shows localized modes characteristic of the response of cations like Na, K or Ca, as opposed to the coupled-mode behavior of Al-O-Si vibrations. The degree of coherence of vibrations is also discussed. The specific potentialities of this spectroscopy and the kind of conclusions that can be drawn about the structure of silica glasses — when complete spectra are available — is illustrated by several examples in silica glasses containing Na, K, Ca, Al, Zr, F, B or Mg ions.

Optical conductivity of oxides

Optical conductivity spectra of conducting oxides, cuprates, including high-Tc superconductors, nickelates, manganites, doped titanates, barium–lead–bismuth oxides, barium–potassium–bismuth–lead oxides, oxides showing metal–insulator phase transitions, and conducting polymers for the sake of comparison, are reviewed in the spectral range from the milli-electron-volt up to a few electron-volts. The optical conductivity profiles evolve from Drude-like to incoherent scattering and even pseudo-gap-like, depending on doping and temperature. When Drude or Drude-like models do not apply, the screening of phonons by charge carrier is generally incomplete and the polaron concept is considered. Spectra are fairly well modeled within a general approach compatible with the Drude framework and extended by factorizing over all complex poles and zeroes of the dielectric response, allowing discrimination of trapped and mobile polarons. Other models are reviewed: extended Drude with either frequency-dependent relaxation time or non-persistent electron velocity, marginal Fermi liquid, Luttinger liquid, localization, Anderson disordered metal, and those implying opening of gaps or pseudo-gaps in the conductivity. The signatures of stripes resulting from ordering of charges and spins—commensurate or incommensurate with the period of the crystal lattice—observed in certain nickelates, cuprates and manganites are discussed.

Ab initio investigation of phonon modes in the MgAl2O4 spinel

Ab initio infrared and Raman phonon modes in the normal cubic MgAl2O4 spinel are calculated at the first Brillouin zone centre using density functional theory with plane-wave basis and norm-conserving pseudopotentials. Good agreement with available experimental infrared and Raman spectra is found for both natural and synthetic spinel. A new set of infrared measurements on synthetic spinels shows extra vibrational infrared modes which are not predicted by calculations on normal spinel. The ionic character of spinels is illustrated by comparison between calculated and experimental Born effective charges.

Oxide ion conduction in Ba, Ca and Sr doped apatite-type lanthanum silicates

The apatite-type compounds La9.33□0.67Si6O26, La10 − xBax(SiO4)6O2 + δ with x = 0.5 → 2, La9Ca1(SiO4)6O2 + δ and La9Sr1Si6(SiO4)6O2 + δ were prepared by high temperature solid state reaction with a high purity level. The La9Ba1Si6O26.5 composition is a good electrolyte for SOFCs at intermediate temperature, with a pure ionic conductivity of 1.16 × 10−2 S cm−1 at 700 °C and low activation energy of 0.74 eV. Moreover the conductivity is stable after one month of ageing at 900 °C under air or reducing atmosphere and is increased with the ceramic grain size. The comparison of undoped La9.33□0.67Si6O26 apatite and doped La9AE1(SiO4)6O26 apatites with AE= Ca, Sr, Ba shows an increase in conductivity with the doping concentration. For doped apatites, the conductivities and activation energy are related to the dopant size, the best properties being obtained for the barium doping. The existence of some extra oxide ions was demonstrated by NPD experiments for oxygen excess apatites but there was no evidence for a new crystallographic interstitial oxygen site in the hexagonal channel. The oxide ion O(5) in the hexagonal channel exhibits a large spread of the nuclear density along the c-axis in all samples, probably due to a static disorder. The cooperative displacements of the SiO4 tetrahedra and of the La(3) site towards the O(5) channel suggest a conduction mechanism involving mainly the extra oxide ions with a migration path between the O(5) channel and the SiO4 tetrahedra along the [001] direction.

Synthesis of Ca0.25Cu0.75TiO3 and infrared characterization of role played by copper

Abstract Ca0.25Cu0.75TiO3 compounds with a perovskite-type structure have been reported to display the double advantage for capacitance applications to show high dielectric constant and little dependence on temperature near room temperature. Powders have been prepared by using an organic gel-assisted citrate process. The infrared reflectivity spectra obtained on sintered ceramic discs are very different from those of simple cubic perovskites. Copper indeed is expected to form hybridized bonds with oxygen, contrary to calcium. As a result, instead of remaining equidistant from calcium or copper as in simple cubic perovskite, oxygen forms shorter bonds with copper inducing a strong distortion of oxygen octahedra. Spectra have been Kramers–Kronig transformed and fitted with the factorized form of the dielectric function. Results show that the origin of high dielectric constant is little phononic but rather due to a relaxational process with the characteristic frequency in the gigahertz range. Thin films grown by PLD on (100) SrTiO3 substrate show preferential (100) orientation.

Soft vibrational mode in LiNbO3 and LiTaO3

Abstract The temperature dependence of the far-infrared reflectivity as obtained with a scanning interferometer for the A1- and E-type modes of both LiNbO3 and LiTaO3 is reported in the ferroelectric phase. Results of a Kramers-Kronig analysis are compared with Raman and neutron scattering data which are controversial about the existence of a soft vibrational mode. For LiTaO3, spectra are obtained 300 K above the Curie temperature, in the paraelectric phase. The lowest-frequency A1 (TO) mode is unambiguously found as soft and becomes rapidly overdamped.

Displacive - order-disorder crossover IFI ferroelectrics

Abstract The temperature dependence of the soft mode components in BaTiO3 and KNbO3 and their contribution to the low-frequency dielectric constant are reexamined under the 1ight of recent hyper-Paman and infrared reflectivity data. Correlated fluctuations are less and less dominant with the decrease of critical dimensionality on cooling in the four successive phases and the soft mode triggers the last transition to the rhombohedra1 phase.

Aluminum Oxide (Al 2 O 3 )

Publisher Summary Aluminum oxide (Al203) generally crystallizes in the corundum (α-Al203) rhombohedral structure. It is also commonly known as “sapphire.” It is noted that the system is uniaxial and optical properties measured on a single crystal are therefore anisotropic. The structure of the oxygen sublattice is close-packed. As a result, the anisotropy of the refractive index in the visible region is mainly related to the anisotropy of the oxygen polarizability, which is small. In the millimeter-wave range, the anisotropy reaches 10%. A study including the original measurements and a compilation of previous data, extends over a wide energy range, from 6eV up to 1.6 x 103 eV, has been performed at the Deutsches Elektronen synchrotron. It is observed that results agree well with previous measurements at low energies, but extend over a much wider energy range. The indexes of refraction of a synthetic sapphire prism is measured over its entire range of transmission is also reported.Publisher Summary Aluminum oxide crystallizes in the corundum (α-Al2O3) rhombohedral structure. It is also known as sapphire. The system is uniaxial and optical properties measured on a single crystal are, therefore, anisotropic. The structure of the oxygen sublattice is close-packed. As a result, the anisotropy of the refractive index in the visible region—related to the anisotropy of the oxygen polarizability—is small. In the millimeter-wave range, the anisotropy reaches 10%. The optical constants selected for the same are presented in this chapter. In randomly oriented materials, a reflection band necessarily develops between the two values that cancel the real part of the dielectric response. There is necessarily a maximum between two minima of the dielectric response. In single crystals, the dielectric function can be modeled. As soon as a dielectric function model fits the infrared reflectivity, the optical constants can be deduced more accurately than with a Kramers–Kronig analysis. The absorption coefficient is extremely high. The penetration depth of the infrared radiation is smaller than 1 m at a peak corresponding to a normal vibrational mode. As a result, the infrared beam in these spectral regions probes the surface of the sample rather than the bulk. In the vicinity of vibrational-mode frequencies, the results reflect bulk properties on etched surfaces or annealed crystals. Disorder or alteration of the sample surface increases mode damping and, therefore, lowers the reflectivity level. The higher the reflectivity, the better is the sample quality.

“Ba2BiO4” surprisingly found as a cubic double perovskite Ba2(Ba23Bi13)BiO6−δ

Abstract Instead of being the hypothetical end member of the tetragonal K 2 NiF 4 -type structure Ba 2 Pb 1− x Bi x O 4 ( x = 1) solid solution, “Ba 2 BiO 4 ” is shown to be a cubic perovskite Ba 2 ( Ba 2 3 Bi 1 3 ) BiO 6−δ . This series was synthetized by comparison with the series BaPb 1− x Bi x O 3 which displays superconductivity around x = 0.25. The new compound “Ba 2 BiO 4 ” is described as the probable end member ( x = 0.67) of an oxygen-defect series the generic formula of which is Ba 2 (Bi 1- x 3+ Ba x )Bi 5+ O 6−δ . The structural determination performed on a powdered sample for x = 0.67 points to a new example of a 1/1 ordered A 2 (BB′)O 6 cubic double perovskite ( a = 8.7670(1) A ). The cubic perovskite structure is also checked from infrared reflectivity spectra. A full Bi 3+ -Bi 5+ separation occurs as a result of the simultaneous presence of large Ba 2+ cation both in dodecahedral and octahedral coordination. Barium in octahedral site breaks the BiOBi bonding and stabilizes insulating properties of the system.