1,721,044 research outputs found
Pair distribution function structural investigation: BaTi1-xCexO3 as a locally disordered perovskite
No full textNowadays barium titanate has become the most widely used functional ceramic material in electronics, finding wide applications in devices as microphones, ultrasonic and underwater transducers, multilayer ceramic capacitors and spark generators [1]. In order to tailor the properties for the specific applications, BaTiO3 is easily and usually doped to improve the material performance, for example modifying the long-range order and consequently the ferroelectric and dielectric properties. The solid solutions with other BaMIVO3 perovskites (M =Sn, Zr, Hf, Ce) show an almost continuous variation of the ferroelectric behaviour with composition, from conventional ferroelectric, via diffuse ferroelectric transition to a clear relaxor state and further to dipolar glass behaviour [2]. Different researches have investigated these systems, but the relation with the average crystallographic structure and the local order is still mostly unknown. Just few previous works, as for the BaTi1-xZrxO3 [3], have addressed the issue demonstrating the existence of a structural local disorder linked to the evolution of the polar behaviour. In order to improve our knowledge on the evolution of polar order in these systems more structural investigations are necessary.
In this work the case of BaTi1-xCexO3 is presented and the average and local structure variations as a function of composition and temperature are shown. Ce4+ (r =0.87 Å) is much bigger than Ti4+ (r = 0.605 Å), so this system represents a limit and interesting case also because this kind of substitution does not involve the creation of charge compensating lattice defects. In particular BaTi1-xCexO3 ceramic solid solutions with x = 0.05, 0.10, 0.20 have been investigated between 100 and 400 K. As indicated by dielectric permittivity measurements, the three samples under study correspond to a different polar behaviour: conventional ferroelectric (x = 0.05) but close to the so-called pinched transition, diffuse phase transition (x = 0.10) and non-ergodic relaxor (x = 0.20).
Total scattering data have been collected at ID22 high resolution beamline, ESRF (European Synchrotron Radiation Facility; Grenoble, France). This because there are subtle phase transitions in the studied temperature range, that need very high resolution data to be properly detected. Pair Distribution Function (PDF) refinements have been performed with the aim of understanding the induced differences between average and local structure. The results show a clear local disorder which exhibits displacements in atomic positions or even different crystallographic space group compared with the average structure. These local deviations are caused by the evolution of Ti-O and Ce-O bond distances with temperature and by the increase of the cerium amount.
[1] S. Yasmin, S. Choudhury, M.A. Hakim, A.H. Bhuiyan and M.J. Rahman J. Mater. Sci. Technol. 2011, 27(8), 759-763.
[2] V.V. Shvartsman and D.C. Lupascu J. Am. Ceram. Soc. 2012, 95 (1), 1-26.
[3] V. Buscaglia, S. Tripathi, V. Petkov, M. Dapiaggi, M. Deluca, A. Gajovic, Y.J. Ren J. Phys.: Condens. Matter 2014, 26, 065901
Negative (and very low) thermal expansion in ReO3 from 5 to 300 K
No full textThis 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 Å) 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
Average and local structural comparison of BaTi1-xCexO3 by pair distribution function
No full textBaTiO3 is considered as a typical ferroelectric material. It presents, with a temperature decrease, the classical phase transition from a paraelectric cubic structure to ferroelectric phases (progressively: tetragonal, orthorhombic and rhombohedral). The solid solutions with other BaMIVO3 perovskites (M=Sn, Zr, Hf, Ce) can strongly modify these transitions and the related polar behaviors. Some materials show, dependently from the dopant type and amounts, a variation from conventional ferroelectric, via diffuse ferroelectric transition to a clear relaxor state and further to dipolar glass behaviour [1]. Considering that BaTiO3-based solid solutions are environment-friendly dielectrics with similar performances as many toxic Pb-based electroceramics [2], the study of these compounds becomes a topic of great interest and a current matter. Different researches have investigated these systems, but the relation with the average crystallographic structure, the local order and electric properties is still mostly unknown. Just few previous studies, as for the BaTi1-xZrxO3 [3], have addressed the issue demonstrating the existence of a structural local disorder linked to the evolution of the polar behaviour. In this work BaTi1-xCexO3 ceramic solid solutions with different cerium amounts (x = 0.05, 0.10, 0.20) are presented. This system represents a limit and interesting case due to a large difference in the Ti and Ce atomic radius ( rCe4+ = 0.87 Å and rTi4+= 0.605 Å), and because this kind of substitution does not involve the creation of charge compensating lattice defects. As indicated by dielectric permittivity measurements, the three samples under study correspond to a different polar behaviour: conventional ferroelectric (x = 0.05) but close to the so-called pinched transition, diffuse phase transition (x = 0.10) and non-ergodic relaxor (x = 0.20). Total scattering data has been collected ID22 high resolution beamline, ESRF (European Synchrotron Radiation Facility; Grenoble, France) in a range between 100 and 400 K every 100 K to explore the phase transitions associated to each sample. Pair Distribution Function (PDF) refinements have been performed with the aim of understanding the induced differences between average and local structure as a function of composition and temperature. The Rietveld and average PDF results show a good agreement with the structural hypothesis suggested by the dielectric permittivity measurements. On the other hand, PDFs analysis also highlights a clear local disorder for all the samples. Often local and average structure have required different space groups to be fitted appropriately. PDFs comparison and structural refinements demonstrate in fact a clear local evolution of Ti-O and Ce-O bond distances with temperature and by the increase of the cerium amount.
[1] V.V. Shvartsman and D.C. Lupascu J. Am. Ceram. Soc. 2012, 95(1), 1-26.
[2] C. Ciomaga, M. Viviani, M.T. Buscaglia, V. Buscaglia, L. Mitoseriu, A. Stancu, P. Nanni J. Eur. Ceram. Soc. 2007, 27, 4061-4064.
[3] V. Buscaglia, S. Tripathi, V. Petkov, M. Dapiaggi, M. Deluca, A. Gajovic, Y.J. Ren J. Phys.: Condens. Matter. 2014, 26, 065901
Thermal expansion in cuprite-type structures from 10 K to decomposition temperature: Cu2O and Ag2O
No full textThe thermal expansion of two isostructural oxides, Cu2O and Ag2O, has been measured from 10 K 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 200 K, and above this temperature it becomes positive. Ag2O, 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 metal - oxygen bonds
Use of IR-spectroscopy and diffraction to discriminate between natural, synthetic and treated turquoise, and its imitations
No full textA newly developed high-temperature chamber for in-situ X-ray diffraction : setup and calibration procedures
No full textThe crystal chemistry of julgoldite-Fe3+ from Bombay, India, studied using synchrotron X-ray powder diffraction and Fe-57 Mossbauer
No full textThe crystal structure of julgoldite-Fe3+ from Bombay, India, was investigated by Fe-57 Mossbauer spectroscopy and synchrotron X-ray powder diffraction. Only ferric iron was detected in the Mossbauer measurements and it occurs at two different octahedral sites in the atomic ratio 20:80. Based on Rietveld refinements, the Fe3+ cations are located at the X- and Y-octahedral sites with atomic percentages of about 25% and 75%, respectively. The resulting chemical formula of the Bombay julgoldite sample is Ca-8(Fe2.73+Al1.1Mg0.2)(Fe-8.0(3+))Si12O42(OH)(14). The oxidation state of Fe is not the same as that arrived at through simple crystal-chemical considerations. Such analysis cannot give quantitative results for the valence state of mixed-valence cations in pumpellyite-type minerals and their intracrystalline partitioning behavior. Assignments of the Mossbauer absorption doublets and an analysis of Fe-intracrystalline partitioning behavior are discussed with reference to previous works on different pumpellyite-type minerals
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
No full textA 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
The crystal chemistry of julgoldite-Fe3+ from Bombay, India, studied using synchrotron X-ray powder diffraction and 57Fe Moessbauer spectrocopy
No full textThe crystal structure of julgoldite-Fe3+ from Bombay, India, was investigated by 57Fe Mössbauer spectroscopy and synchrotron X-ray powder diffraction. Only ferric iron was detected in the Mössbauer measurements and it occurs at two different octahedral sites in the atomic ratio 20:80. Based on Rietveld refinements, the Fe3+ cations are located at the X- and Y-octahedral sites with atomic percentages of about 25% and 75%, respectively. The resulting chemical formula of the Bombay julgoldite sample is crystal-chemical considerations. Such analysis cannot give quantitative results for the valence state of mixed-valence cations in pumpellyite-type minerals and their intracrystalline partitioning behavior. Assignments of the Mössbauer absorption doublets and an analysis of Fe-intracrystalline partitioning behavior are discussed with reference to previous works on different pumpellyite-type minerals
Shell microstructure and mineralogy of the mollusc species Anadara uropigimelana (Bory de Saint-Vincent, 1827), Tivela stefaninii (Nardini, 1933) and Oliva bulbosa (Röding, 1798)
No full textMollusc shells are composite structures build of crystals of calcium carbonate (calcite, aragonite, or both) and biopolymers, arranged in a great variety of microstructures. Shell formation is affected by environmental and physiological factors and differences in shell microstructures are believed to be of phylogenetic and adaptive biomechanical significance. Here, we performed a detailed characterization and illustration, through SEM and XRD analyses, of the shell microstructure and mineralogy of specimens belonging to the bivalves Anadara uropigimelana (Bory de Saint-Vincent, 1827) and Tivela stefaninii (Nardini, 1933), and to the gastropod Oliva bulbosa (Röding, 1798), collected in the Upper Holocene (Meghalayan) HAS1 settlement and in a shell midden in the Khor Rori Archaeological Park (Dhofar, Oman). Specimens of Anadara uropigimelana show an aragonitic shell with an outer crossed lamellar layer, an inner complex crossed lamellar layer and an irregular simple prismatic pallial myostracum; furthermore, periodic bands of dendritic nondenticular composite prisms occur in the outer part of the outer layer, reflecting seasonal changes in water temperatures. Specimens of Tivela stefaninii show an aragonitic shell with an outer composite prismatic layer, a middle crossed lamellar layer and an inner complex crossed lamellar layer, whereas specimens of Oliva bulbosa have an aragonitic shell characterised by an irregular alternation of crossed lamellar layers. With this investigation we aim to provide novel data on the shell fabric of these poorly known mollusc species, increasing the availability of useful characters for phylogenetic, evolutionary, palaeoenvironmental and crystallographic studies
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