Monica Dapiaggi

Thermal expansion in cuprite-type structures from 10 K to decomposition temperature: Cu2O and Ag2O

The thermal expansion of two isostructural oxides, Cu 2 O and Ag 2 O, has been measured from 10K to their respective decomposition temperatures by means of high-resolution X-ray powder diffraction. The thermal behaviours of the two oxides are different. Cuprite has a negative thermal expansion up to about 200K, and above this temperature it becomes positive. Ag 2 O, on the other hand, has a negative thermal expansion up to its decomposition temperature. A comparison with EXAFS data in the same temperature range shows that the observed difference between the thermal expansion regimes of the two compounds can be ascribed to the vibrational behaviour of the Cu and Ag atoms and, in the ultimate analysis, to the different rigidities of the metaloxygen bonds.

Average and local atomic-scale structure in BaZrxTi1?xO3 (x = 0.10, 0.20, 0.40) ceramics by high-energy x-ray diffraction and Raman spectroscopy

High-resolution x-ray diffraction (XRD), Raman spectroscopy and total scattering XRD coupled to atomic pair distribution function (PDF) analysis studies of the atomic-scale structure of archetypal BaZrxTi(1-x)O3 (x = 0.10, 0.20, 0.40) ceramics are presented over a wide temperature range (100-450 K). For x = 0.1 and 0.2 the results reveal, well above the Curie temperature, the presence of Ti-rich polar clusters which are precursors of a long-range ferroelectric order observed below TC. Polar nanoregions (PNRs) and relaxor behaviour are observed over the whole temperature range for x = 0.4. Irrespective of ceramic composition, the polar clusters are due to locally correlated off-centre displacement of Zr/Ti cations compatible with local rhombohedral symmetry. Formation of Zr-rich clusters is indicated by Raman spectroscopy for all compositions. Considering the isovalent substitution of Ti with Zr in BaZrxTi1-xO3, the mechanism of formation and growth of the PNRs is not due to charge ordering and random fields, but rather to a reduction of the local strain promoted by the large difference in ion size between Zr(4+) and Ti(4+). As a result, non-polar or weakly polar Zr-rich clusters and polar Ti-rich clusters are randomly distributed in a paraelectric lattice and the long-range ferroelectric order is disrupted with increasing Zr concentration.

Phase transformations and reaction kinetics during the temperature-induced oxidation of natural olivine

Abstract This work describes the sequence of transformations and their reaction kinetics during the oxidation of olivine in air at high temperature. A natural olivine sample from the layered series of the Ivrea-Verbano igneous complex (Western Alps, Italy) was heated in the temperature range 25-1300 °C in air and investigated by in situ, real time powder X-ray diffraction (PXRD). The evolution of the peaks (measured integrated intensities) was followed in non-isothermal conditions using variable heating rates (b = 20, 22, 25, 27, and 30 °C/min). The total time of the experiments ranged from about 256 (b = 30 °C/min) to 277 (b = 20 °C/min) min including the time for the data collections. An additional isothermal run was performed at 800 °C. The analysis of the kinetic data was attempted with the use of different equations including the classical Avrami theory for solid-state reactions. The kinetic results were confirmed by independent experimental data from electron microscopy (SEM, TEM). In the transformation sequence, hematite appears at about 600 °C producing amorphous segregations. of silica that later recombine with forsterite to form pyroxene Hematite is stable up to 1130 °C where it is transformed into magnetite. The rate limiting step for the formation of hematite is a twodimensional diffusion with constant or decelerating nucleation rate and apparent activation energy of 15 kcal/mol. The concentration of Fe3+ in Fe-rich regions favors the heterogeneous nucleation of hematite, which may take place on existing defects or at the grain boundaries with impurity phases such as serpentine. At 1130 °C, magnetite is formed at the expenses of hematite, with a contracting volume interface-controlled reaction in two or three dimensions with an apparent activation energy of 30-31 kcal/mol. The hematite to magnetite transformation is direct, without a metastable amorphous intermediate. It is described by the “shrinking core model,” with the formation of a magnetite outer layer at the surface of the hematite particles that proceeds toward the core of the reacting hematite by diffusion of the oxygen throughout the newly formed magnetite layer. Its rate is limited by the advancement of the reaction front. The rate-limiting step for the formation of pyroxene is two-dimensional diffusion with decelerating nucleation rate with an apparent activation energy of 29 kcal/mol.

The thermal behaviour of cuprite: An XRD–EXAFS combined approach

Abstract Cuprite (Cu2O) is a low thermal expansion material with a negative thermal expansion coefficient below room temperature. Its peculiar thermal behaviour encompasses the increase of the shear modulus with increasing temperature, and the presence of rather intense symmetry-forbidden eeo reflections below room temperature. The thermal expansion of cuprite was studied at low temperature (between 5 and 298 K) by means of high-resolution (10−5 A) X-ray powder diffraction at European Synchrotron Radiation Facility (Grenoble, BM16) and extended X-ray absorption fine structure (EXAFS) (BM29). Negative thermal expansion is confirmed up to 200 K, by EXAFS as well as by XRD measurements, and no sign of transition was found in XRD data. The comparison between EXAFS and XRD results provides a valuable insight into vibrational behaviour of cuprite at low temperature.

Microscopic strain in synthetic pyrope-grossular solid solutions determined by synchrotron X-ray powder diffraction at 5 K: The relationship to enthalpy of mixing behavior

Abstract A series of synthetic pyrope-grossular garnets (Mg3Al2Si3O12-Ca3Al2Si3O12) were investigated by powder X-ray synchrotron radiation at 5 K to determine their microscopic structural strain, which may be responsible for the observed excess enthalpy of mixing for this binary. This substitutional solid solution provides an excellent system for investigating microscopic-macroscopic relationships and the physical nature behind non-ideal thermodynamic mixing behavior in silicates, because of the measurable nonidealities shown by its enthalpy and volume of mixing. An analysis of the X-ray refection profiles, based on theoretical considerations of X-ray line broadening, permits for the first time a direct experimental determination of crystallite size and the root-mean-square structural strain for a mineral solid solution. The measured microscopic strain shows positive and asymmetric deviations from linearity across the join with the largest excess in pyrope-rich compositions. There is a good correlation between the structural strain and the macroscopic enthalpy of mixing behavior for pyrope-grossular garnets as measured by calorimetry.

Kinetics of the Decomposition of Crocidolite Asbestos: A Preliminary Real-Time X-Ray Powder Diffraction Study

This work is a preliminary kinetic study of the crocidolite decomposition followed in situ at high temperature using real time conventional powder diffraction and DTA in the temperature range 720-795 °C. The data analysis using the Avrami models indicates that the rate limiting step of the reaction is monodimensional ion diffusion (n=0.5) with an activation energy of 129 (10)kcal/mole.

Negative (and very low) thermal expansion in ReO3 from 5 to 300 K

This paper reports the accurate measurement of the ReO3 cell parameter as a function of temperature. The thermal expansion is confirmed to be negative over most of the temperature range from 5 to 300 K. The main problems with the measurements are the very small variations (in the range of 10−5 A) in the cell parameter at each temperature, requiring tight control of the stability and reliability of instrumental effects. In particular, achieving monochromator stability over time might be challenging with the high energy and high beam current variations of a third-generation synchrotron facility. On the other hand, such effects are usually checked by the addition of silicon as an internal standard, but the accuracy (and precision) of the published thermal expansion (which is not certified) might not be sufficient for its use when dealing with very small cell parameter variations.

Local Structure of Si-Al-Ca-Na-O Glasses from Coupled Neutron and X-ray Total Scattering Data

Amorphous materials became significantly important and more widely studied during the last few decades, due the their increasingly widespread applications in materials science and technology. Their local structure seems to have a very strong bond with those properties: in this paper, the local structure of Si-Al-Ca-Na-O glasses is studied by means of total scattering. EPSR simulations, coupling neutron, and X-ray data have been used to study glass samples (as a function of composition), with a composition close to the one used in their technological applications (ceramic glazes), providing a consistent structural model. The disordered structure of these materials has been evaluated in terms of network-forming/modifier elements. The network-forming elements (silicon and aluminum) show coordination numbers and bond angles that are consistent with a tetrahedral arrangement. In contrast, network-modifying elements (Ca and Na, whose content is different in all samples) depolymerize the network, increasing the number of nonbridging oxygens. This structural information is required to rationalize many important technological properties of these materials, such as the glass transition temperatures and thermal expansion, that control their efficiency as glazes.

P-V and T-V equations of State of natural biotite : An in-situ high-pressure and high-temperature powder diffraction study, combined with Mossbauer spectroscopy

Abstract The P-V and T-V equations of state of a natural biotite sample (Mg/Fe ratio ≈ 1) have been studied using in-situ high-pressure (0.0001.11 GPa) synchrotron radiation powder diffraction at the European Synchrotron Radiation Facilities (ESRF) in Grenoble, France, and in-situ high-temperature (298.610 K) laboratory X-ray powder diffraction. A third-order Birch-Murnaghan model [V0 = 498.7(1) Å3, measured value] provides the following elastic parameters: K0 = 49(1) GPa, K’ = 8.1(5). The volume thermal expansion is satisfactorily described by a constant value resulting in 37(2) 10.6 K-1. Mössbauer spectroscopy proves that REDOX reactions have occurred upon heating, presumably 2(OH- + Fe2+) → 2O2- + 2Fe3+ + H2 ↑ and/or 4Fe2+ + 2OH- + O2 → 4Fe3+ + 3O2- + H2O. On the basis of the elastic and thermal parameters measured we have modeled the deformation contribution (Gdeform) to the Gibbs energy. The third-order Birch-Murnaghan model with V0 fixed at its experimental value and the model with refined V0 do not significantly differ from one another in terms of Gdeform. A comparison based on Gdeform between biotite and phlogopite shows a better compliance to P of the former, though balanced in mineral reactions by a difference of molar volume, i.e., V0(biotite) > V0(phlogopite).

Accuracy in quantitative phase analysis of mixtures with large amorphous contents. The case of zircon‐rich sanitary‐ware glazes

The accuracy of quantitative phase analysis (QPA) of samples with dominant amorphous content, reproducing zircon-rich sanitary-ware glazes, has been investigated. X-ray powder diffraction (XRPD) methods were applied using both conventional Cu Kα radiation and high-resolution synchrotron data. In this work, a combination of the reference intensity ratio (RIR) and Rietveld methods was applied to an artificial mixture (90 wt% glass, 10 wt% zircon), taking into account some of the most common effects that may affect the accuracy in amorphous quantification, such as the degree of crystallinity of the phases, microabsorption and sample preparation. Certified NIST SRM 676a (α-Al2O3) [Cline, Von Dreele, Winburn, Stephens & Filliben (2011). Acta Cryst. A67, 357–367] was used to quantify the amorphous content in zircon and in the different internal standards commonly used when a certified standard is not available or not applicable: the results show that all of the phases invariably contain amorphous material in the range 2.0–15.0 wt%. If the amorphous content of the standard is taken into account, the accuracy of the QPA of the artificial mixture is improved. It was observed that the Brindley correction for microabsorption does not significantly improve the results. Care must be applied if grinding time is increased, since this may increase the amorphous content in the sample. Finally, the sensitivity of the RIR–Rietveld method to the addition of a small amount of zircon (∼1 wt%) has been considered, showing that accurate results can be achieved if great care is taken in the sample preparation and refinement strategy.