Ulrich Bismayer

HARD MODE SPECTROSCOPY : THE CONCEPT AND APPLICATIONS

Abstract Hard modes are, in the context of this review, optically active phonons which show systematic changes of their Raman and/or IR spectra when the structural properties of a material are changed (e.g. by heating, application of pressure or chemical reactions). As the characteristic length of high-frequency phonons is very short (the Ornstein–Zernike correlation length) the structural variations are measured on an atomic scale. This feature is a great advantage for the analysis of heterogeneous materials, e.g. exsolution pattern, disordered systems. The interpretation of frequency shifts, variations of the intensities and line width of optical spectra is largely based on symmetry arguments which show that the renormalization of phonon spectra is, in most cases, proportional to AQ 2 + BQ 4, where Q is a structural order parameter and A, B are numerical constants. Recipes for the analysis of phonon spectra including the use of reference spectra, profile analysis and the application of spectral autocorr...

Structural characterisation of X-ray amorphous calcium carbonate (ACC) in sternal deposits of the crustacea Porcellio scaber

Mineral deposits in land-based woodlice (crustacea, Porcellio scaber) were analysed by high-resolution X-ray diffraction, X-ray absorption spectroscopy (EXAFS) and infrared microscopy. Calcium carbonate is stored within the first four anterior sternites before changing the cuticle (shell). These deposits consist of fully X-ray amorphous calcium carbonate (primary particle size less than 100 A). The short-range order is comparable to crystalline calcium carbonate phases (first coordination shell), but there is increasing structural disorder beyond about 3 A. This high degree of structural disorder gives a high solubility, i.e. an easy mobilisation within the biological system. The results are compared with EXAFS data from the literature on other biogenic amorphous calcium carbonates.

Linear birefringence and X-Ray diffraction studies of the structural phase transition in titanite, CaTiSiO5

AbstractThe antiferroelectric phase transition in titanite characterised by a collinear displacement of Ti-atoms from their central octahedral position is investigated using linear optical birefringence and X-ray diffraction techniques. Both methods indicate a continuous transition near 496 K and extra contributions to δΔn and X-ray intensity signals at higher temperatures. The critical exponent of the macroscopic order parameter is found to be β = 0.14 ± 0.02 and the transformation is interpreted in terms of a two-dimensional quasi-spin model. Topological features of the structure agree well with the spatial distribution of the diffuse scattering of the superstructure reflection 40

Influence of lattice imperfections on the transition temperatures of structural phase transitions: The plateau effect

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Structural phase transition in titanite, CaTiSiO5: A ramanspectroscopic study

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High-pressure high-temperature in situ diffraction studies of nanocrystalline ceramic materials at HASYLAB

Abstract We report the direct experimental observation of a structural anomaly near 825 K in synthetic and natural titanite samples by high-temperature, hard-mode infrared spectroscopy. The anomaly in titanite, CaTiSiO5, is characterized by a break of the temperature dependence of the 562 cm-1 Si-O bending mode, the 675 cm-1 Ti-O band, and the 900 cm-1 Si-O stretching modes, and the rapid decrease of the IR signal at 873 cm-1. The order parameter, as determined from the temperature evolution of the frequencies of the absorption bands in the middle infrared (MIR) region follows a second-order Landau behavior with an order-parameter exponent β = ½. At T > 825 K, the Ti-O band shows further softening, whereas the Si-O bands at 562 and 900 cm-1 show hardening with increasing temperature. In natural samples, the effects of impurities on the high-temperature transition are weak. For pure titanite, the transition temperature, Tc, is about 825 K and increases with increasing impurity concentration.

Anti-phase boundaries and phase transitions in titanite; an X-ray diffraction study

Abstract The influence of defects on the apparent phase transition temperature has been studied over a large range of defect concentrations. An interval was found for low concentrations (around 1 mol%) in which the transition temperature depends only weakly on the number of defects and where defect tails occur. This interval is called the plateau. For higher defect concentrations we find that chemical mixing leads to strong variations of the transition temperature which are well described in the approximation of a homogeneous crystal.

Temperature dependence of IR absorption of hydrous/hydroxyl species in minerals and synthetic materials

Abstract The application of Raman spectroscopy for the investigation of phase transitions focused traditionally on the observation of soft modes in displacive systems. The present furthergoing study on displacive and order-disorder systems is based on the observation of systematic changes of the scattering profiles of hard modes during the phase transition. It reveals the temperature evolution of the order parameters, the role of order-parameter fluctuations and phonon density of states effects in model systems like improper ferroelastic Pb3(P1-x As x O4)2, lead diluted ferroelastic (Pb1-x Ba x )3(PO4)2, pure ferroelastic As2O5 and the relaxor ferroelectric Pb(Sc0.5Ta0.5)O3. Allied with supplementary experimental techniques hard-mode Raman spectroscopy (HMRS) is an ideal method for the investigation of order-parameter coupling effects and the characterization of structural phase transitions.