1,721,089 research outputs found
New insight into the thermodynamic properties of high silica ZSM-5 zeolite: effect of adsorbed organic molecules
No full textIn situ high-temperature structural evolution of the Ca(Ti,Ge)O3 perovskite solid solution: looking for a new locked-tilt perovskite
No full textProgetto accettato dal Sincrotrone Elettra di Trieste, per la quale sono stati conferiti 6 giorni (144 ore) di accesso alla struttura per degli esperimenti. Il budget totale stimato del progetto è di 79500 € (mediamente vengono stimati 550€/h) compresi i vari rimborsi.A recent synchrotron structural investigation at high-pressure (HP) sets YAl0.25Cr0.75O3 orthorhombic perovskite as the prototype of the so-called "locked-tilt" perovskites. Placed at the boundary of the previously described structural evolution models for GdFeO3-type perovskites at HP, YAl0.25Cr0.75O3 represents the first finding of a perovskite characterized by the absence of changes in the octahedral tilting (as well as octahedral distortion), and a volume reduction with P exclusively controlled by an isotropic polyhedral compression. Although a theoretical modeling of a new locked-tilt perovskite can be done with a high degree of accuracy, the only way to confirm the possible occurrence of a perovskite belonging to this family is through new experiments at non-ambient conditions (HP or High Temperature, HT). The use of geochemical constrains, and the assessment of the "normalized cell distortion factor with pressure/temperature, dnorm(P/T)" for several perovskite solid solutions, allowed the identification of three possible locked-tilt perovskite formulations, i.e., La(Mn0.69Ga0.31)O3, Ca(Ti0.95Ge0.05)O3, and (Sc0.86Y0.14)AlO3, respectively. The aim of our proposal is to investigate at HT the above identified Ca(Ti,Ge)O3 perovskite solid solution which were previously characterized at ambient conditions through structural refinements from X-ray Powder Diffraction (XRPD). Besides to extend the locked-tilt perovskite family, this investigation will provide a deeper comprehension on the role of these compounds in view of their application as functional materials (e.g., multiferroics, layered perovskites). Furthermore, the structural modification at HT of a hypothesized locked-tilt perovskite where the cubic site has a lower formal charge than the octahedral site (i.e., the 2:4 Ca(Ti0.95Ge0.05)O3), will be extremely useful from a geophysical viewpoint to outline a more accurate Earth's mantle model
Lattice relaxation in solid solutions: long-range vs. short-range structure around Cr3+ and Co2+ in oxides and silicates
No full textThis work reports the results derived from the 3-year doctoral thesis project aimed at exploring some oxide and silicate structures as promising ceramic pigments with enhanced colorimetric properties with respect to the traditional colorants. Solid solutions of perovskite, alumoniobite, and melilite compounds were obtained by doping octahedral and tetrahedral coordination sites with transition metal ions (e.g. Cr3+, Co2+, and Zn2+) through a solid-state synthesis performed by means of an industrial-like process. The analytical techniques adopted to investigate the synthesized compounds allowed the determination of the “averaged” crystal structure, or the so termed long-range properties, and the short-range properties (i.e. the local structure around the substituting ions) through X-ray powder diffraction (XRPD) and electronic absorption spectroscopy (EAS), respectively. As stated by Geiger (2001) “an understanding of the microscopic, mesoscopic and macroscopic properties and of the behaviour of solid solutions under different conditions is a challenge for all disciplines concerned with the solid state”. Hence, the precise determination of a structure around impurities results fundamental to provide detailed information on their incorporation and on physical properties. For instance, in the case of the solid solutions here reported, the lattice incorporation of transition metal ions as impurities is the cause of their gradual coloration. Most of the times, such a coloration is more intense as greater is the impurity amount.
The final goal of this work was attained by calculating the structural relaxation coefficient for each studied solid solution by combining the mean with the local bond distances achieved by XRPD and EAS, respectively
Compressibility of orthorhombic perovskites. The effect of transition metal ions (TMI)
No full textInterest in perovskites evenly spans Materials Science and Geophysics. Due to their inimitably lattice flexibility enabling small as well as large ions to be accommodated, perovskites have become a base structure for new technological applications. Understanding the mechanisms governing their evolution at non-ambient conditions (such as high-pressure and high-temperature) is fundamentally important both for devising functional materials and in order to provide the most reliable possible deep-Earth model. With particular attention being paid to the chemical nature of the constituent ions, a suite of orthorhombic perovskites has been selected and contrasted using several parameterizations and models. A new perspective on the pressure-induced distortion of orthorhombic perovskite structures has enabled their compressional behaviour to be redefined
New insight into the thermodynamic properties of high silica ZSM-5 zeolite: effect of adsorbed organic molecules
No full textLattice relaxation in solid solutions: long-range vs. short-range structure around Cr3+ and Co2+ in oxides and silicates
No full textThis dissertation reports the results derived from the 3-year doctoral thesis project aimed at exploring some oxide and silicate structures as promising ceramic pigments with enhanced colorimetric properties with respect to the traditional colorants. Solid solutions of perovskite, alumoniobite, and melilite compounds were obtained by doping octahedral and tetrahedral coordination sites with transition metal ions (e.g. Cr3+, Co2+, and Zn2) through a solid-state synthesis performed by means of an industrial-like process. The analytical techniques adopted to investigate the synthesized compounds allowed the determination of the "averaged" crystal structure, or the so termed long-range properties, and the short-range properties (i.e. the local structure around the substituting ions) through X-ray powder diffraction and electron absorption spectroscopy (EAS), respectively. As stated by Geiger (2001) "an understanding of the microscopic, mesoscopic and macroscopic properties and of the behaviour of solid solutions under different conditions is a challenge for all disciplines concerned with the solid state". As a matter of fact, the precise determination of a structure around impurities results fundamental to provide detailed information on their incorporation and on physical properties. For instance, in the case of the solid solutions here reported, the lattice incorporation of transition metal ions as impurities is the cause of their gradual coloration. Most of the times, such a coloration is more intense as greater is the impurity amount.
The final goal of this work, was attained by calculating the structural relaxation coefficient for each studied solid solution by combining the mean with the local bond distances achieved by XRPD and EAS, respectively
La collezione Gasser nel Museo di Mineralogia dell'Università di Padova
No full textNel capitolo viene descritta la collezione mineralogica Gasser presente nel Museo di Mineralogia dell'Università di Padova, e comprende:
- le vicende storiche legate all'acquisto della collezione;
- il lavoro di riordino e di inventariazione della collezione;
- i campioni rappresentativi della collezione;
- i falsi mineralogici
La collezione Gasser nel Museo di Mineralogia dell'Università di Padova
No full textNel capitolo viene descritta la collezione mineralogica Gasser presente nel Museo di Mineralogia dell'Università di Padova, e comprende:
- le vicende storiche legate all'acquisto della collezione;
- il lavoro di riordino e di inventariazione della collezione;
- i campioni rappresentativi della collezione;
- i falsi mineralogici
Locked-tilt perovskites
No full textA recent synchrotron structural investigation at high-pressure (HP) sets YAl0.25Cr0.75O3 orthorhombic
perovskite as the possible prototype of the "locked-tilt" perovskites [1]. Placed at the boundary of the
previously described structural evolution models for GdFeO3-type perovskites at HP [e.g., 2, 3],
YAl0.25Cr0.75O3 represents the first finding of a perovskite characterized by the absence of changes in the
octahedral tilting and a volume reduction with P exclusively controlled by an isotropic polyhedral
compression. Although a theoretical modeling of a new locked-tilt perovskite can be done with a high
degree of accuracy, the only way to confirm the possible occurrence of a perovskite belonging to this
family is through a HP experiment. The use of geochemical constraints, and the assessment of the
"normalized cell distortion factor with pressure, dnorm(P)" for several perovskite solid solutions [1 ,3, 4],
has allowed the identification of three possible locked-tilt perovskite formulations, i.e., La(Mn0.69Ga0.31)O3,
Ca(Ti0.95Ge0.05)O3, and (Sc0.86Y0.14)AlO3, respectively.
The aim of our proposal is to investigate at HP the above identified perovskite samples which were
previously characterized at ambient conditions through structural refinements from XRPD. Besides to
extend the locked-tilt perovskite family, this investigation will provide a deeper comprehension on the role
of these compounds in view of their application as functional materials (e.g., multiferroics, layered
perovskites). Furthermore, the structural modification at HP of a hypothesized locked-tilt perovskite where
the cubic site has a lower formal charge than the octahedral site (i.e., the 2:4 Ca(Ti0.95Ge0.05)O3), will be
extremely useful from a geophysical viewpoint to outline a more accurate Earth' mantle compressional
model
Structural relaxation in tetrahedrally coordinated Co2+ along the gahnite-Co-aluminate spinel solid solution
No full textThe structural relaxation around the Co2+ ion along the gahnite (ZnAl2O4)-Co-aluminate (CoAl2O4) join was investigated by a combined X-ray diffraction (XRD) and electronic absorption spectroscopy (EAS) approach. Monophasic spinel samples (Zn1–yCoyAl2O4 with y = 0, 0.25, 0.5, 0.75, and 1 apfu) were obtained through solid-state reaction (1300 °C with slow cooling). The cobalt incorporation induces a linear increase of the unit-cell parameter (a) accompanied by an increasing inversion parameter (up to 0.07) so that the Co2+ for Al3+ substitution in the octahedral site is, at a first approximation, the cause of the lattice expansion. However, a careful consideration of T-O distances highlights the role played by an enhanced covalence degree of Zn-O bonds. The optical spectra are characterized by the occurrence of electronic transitions of Co2+ in tetrahedral coordination affected by a strong spin-orbit coupling, causing a threefold splitting of spin-allowed bands. Further complications stem from mixing of quadruplet and doublet states (leading to a consistent intensity gain of spin-forbidden bands) and vibronic effects (producing intense sidebands). Crystal field strength goes from 4187 to 4131 cm–1 with increasing cobalt amount, while the Racah B parameter is in the 744–751 cm–1 range (C ∼3375 cm–1). To achieve a reliable estimation of the local Co-O distance, the tetrahedral distance evolution was recast to eliminate the effects of the inversion degree. By this way, a relaxation coefficient as low as ε = 0.47 was obtained, i.e., significantly smaller than literature data for other spinel systems. The gahnite-Co-aluminate join seems to be constrained by the strong preference of Zn2+ for the tetrahedral site in which its enhanced covalency can be exerted, limiting the cation exchange between tetrahedral and octahedral sites as well as the lattice flexibility.The structural relaxation around the Co2+ ion along the gahnite (ZnAl2O4)-Co-aluminate (CoAl2O4) join was investigated by a combined X‐ray diffraction (XRD) and electronic absorption spectroscopy (EAS) approach. Monophasic spinel samples (Zn1–yCoyAl2O4 with y = 0, 0.25, 0.5, 0.75, and 1 apfu) were obtained through solid-state reaction (1300 °C with slow cooling). The cobalt incorporation induces a linear increase of the unit-cell parameter (a) accompanied by an increasing inversion parameter (up to 0.07) so that the Co2+ for Al3+ substitution in the octahedral site is, at a first approximation, the cause of the lattice expansion. However, a careful consideration of T-O distances highlights the role played by an enhanced covalence degree of Zn-O bonds. The optical spectra are characterized by the occurrence of electronic transitions of Co2+ in tetrahedral coordination affected by a strong spin-orbit coupling, causing a threefold splitting of spin-allowed bands. Further complications stem from mixing of quadruplet and doublet states (leading to a consistent intensity gain of spin-forbidden bands) and vibronic effects (producing intense sidebands). Crystal field strength goes from 4187 to 4131 cm–1 with increasing cobalt amount, while the Racah B parameter is in the 744–751 cm–1 range (C ∼3375 cm–1). To achieve a reliable estimation of the local Co-O distance, the tetrahedral distance evolution was recast to eliminate the effects of the inversion degree. By this way, a relaxation coefficient as low as ε = 0.47 was obtained, i.e., significantly smaller than literature data for other spinel systems. The gahnite-Co-aluminate join seems to be constrained by the strong preference of Zn2+ for the tetrahedral site in which its enhanced covalency can be exerted, limiting the cation exchange between tetrahedral and octahedral sites as well as the lattice flexibility
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