We report on the lattice evolution of BiFeO 3 as function of temperature using far infrared emissivity, reflectivity, and X-ray absorption local structure.A power law fit to the lowest frequency soft phonon in the magnetic ordered phase yields an exponent β=0.25 as for a tricritical point. At about 200 K below T N ~640 K it ceases softening as consequence of BiFeO 3 metastability. We identified this temperature as corresponding to a crossover transition to an order-disorder regime. Above ~700 K strong band overlapping, merging, and smearing of modes are consequence of thermal fluctuations and chemical disorder. Vibrational modes show band splits in the ferroelectric phase as emerging from triple degenerated species as from a paraelectric cubic phase above T C ~1090 K. Temperature dependent X-ray absorption near edge structure (XANES) at the Fe K-edge shows that lower temperature Fe 3+ turns into Fe 2+ . While this matches the FeO wüstite XANES profile, the Bi L III -edge downshift suggests a high temperature very complex bond configuration at the distorted A perovskite site. Overall, our local structural measurements reveal high temperature defect-induced irreversible lattice changes, below, and above the ferroelectric transition, in an environment lacking of long-range coherence. We did not find an insulator to metal transition prior to melting.

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Two innovative experimental techniques for measuring the high temperature near infrared optical spectra of glass melts are compared. The critical experimental features of both techniques, one based on transmission and the other based on emittance measurements, are reviewed. Typical results of both techniques, including high temperature spectra and values for the Rosseland mean absorption coefficient and thermal radiation conductivity versus temperature for similar glass melts, are compared. The study is focused on sulfate fined soda lime silicate glass melts colored with iron oxide and chromium oxide and on the effect of the glass redox state on the thermal radiation conductivity. It is shown that essentially different measuring principles provide consistent results for similar glass melt types, that is, colors. Using the high temperature spectra of a large variety of (colored) glasses, a new semi-empirical model is developed for predicting the Rosseland radiation conductivity of arbitrary sulfate fined soda lime silicate glass melts, colored with iron oxide and chromium oxide. By separating the effects of (a) the temperature-dependent redox state, (b) the high temperature changes in ligand field strengths and (c) the glass matrix, the model reliably predicts the Rosseland radiation conductivity, with a chemical analysis of the glass as input only. K E Y W O R D Scolor< optical properties, glass forming melts< properties, thermal conductivity< glass forming melts

Far- and mid-infrared spectroscopy is presented as a powerful and nondestructive tool for estimation of the porosity of oxide ceramic films, which are typically employed as part of thermal barrier coatings. A radiative model that takes porosity into account and includes the optical response of thin films has been used. Provided that pore size is small enough as compared to the infrared wavelength, the porosity level can be included as an adjustable parameter of effective medium theories (EMT) such as Maxwell-Garnet or Bruggeman. Periodic interferential oscillations are found on radiative properties in spectral ranges where the coatings are semitransparent and do not scatter significantly infrared radiation. This information has been used to retrieve the porosity level of several semitransparent samples. Analysis of plasma-sprayed yttria-stabilized zirconia (YSZ) ceramic coatings shows that the radiative model is relevant, and the comparison between experimental and theoretical porosity values illustrates the level of accuracy of the technique.

There are many experimental situations in which infrared reflectivity spectra can be acquired only over a limited spectral range. It is therefore necessary to find computing procedures that allow the efficient analysis of such data. In this paper, we propose a new procedure labeled constrained finite range correction (CFRC) that can be advantageously substituted to multiply subtractive Kramers-Kronig relations. The constrained finite range correction is able to produce realistic results even when very little supplementary information is available. For semitransparent crystals, the hypothesis of the phase spectrum positiveness alone is often sufficient to compute satisfactory approximations of the optical functions. The efficiency of the new method is shown through the analysis of several synthetic and experimental spectra.

We report on temperature dependent TmMnO 3 far infrared emissivity and reflectivity spectra from 1910 K to 4 K. At the highest temperature the number of infrared bands is lower than that predicted for centrosymmetric P6 3 /mmc (D 6h 4 ) (Z=2) space group due high temperature anharmonicity and possible defect induced bitetrahedra misalignments. On cooling, at ~1600 ± 40 K, TmMnO 3 goes from non-polar to an antiferroelectric-ferroelectric polar phase reaching the ferroelectric onset at the ~700 K.

The effective refractive index (neff) of suspensions of subwavelength particles is calculated in resonant domains of the thermal infrared region. On account of strong cooperative effects, notable deviations arise from what is expected for small particles; these features include unusual activation of higher‐order multipoles despite the fact that the system can be homogenized and the manifestation of effective magnetic properties. The former feature leads to fundamental questions about the validity of the homogenization procedure, in particular regarding the meaning of the imaginary part of neff, that is absorption by the particles and therefore interrogates the degree to which the composite can be unrestrictedly described by an effective dielectric function. The latter feature offers interesting perspectives for the development of nanophotonic devices, based on dielectric subwavelength particles, exhibiting an effective magnetic response. Finally, the study of the coherent and incoherent decomposition of the field allows to demonstrate, counterintuitively, that a material can admit effective optical properties even in the presence of strong incoherent intensities and that the variance of the field over a statistical ensemble of configurations is a misleading indicator of scattering.

The main concern of this investigation was the relation between the representative volume element of the scattered flux from 2D random agglomerates, composed of sub-wavelength interacting particles, and the extraction of their effective electromagnetic properties. Using a constant ratio between agglomerates radius and wavelength, behaviors of the scattered flux mean value and standard deviation were studied as a function of the agglomerate surface, in the case of relevant particle permittivities. Using the mean or standard deviation functions, two criteria for homogenization were derived. Compared to the extraction of the effective refractive index, both criteria failed to predict the minimal surface needed for homogenization, questioning the existence of a link between the accuracy of the scattered flux evaluation and homogenization. However, it is shown that when no surface-plasmon resonances are excited, a single minimal representative surface for homogenization can be defined.