F. Congiu - Academia.edu (original) (raw)
Papers by F. Congiu
An entry from the Cambridge Structural Database, the world's repository for small molecule cr... more An entry from the Cambridge Structural Database, the world's repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
Hyperfine Interactions (C), 2002
A sample of nanocrystalline YI. S EUO.203 was examined by 151Eu Mossbauer spectroscopy. The degre... more A sample of nanocrystalline YI. S EUO.203 was examined by 151Eu Mossbauer spectroscopy. The degree of covalency of the Eu-O bond has been studied. The spectrum of the cubic Y1.8EUO.203 sample has been resolved into 2 contributions due to europium in the C 3i and C 2 sites, for the first time in 151Eu Mossbauer spectroscopy. The degree of covalency and the electric field gradient of
Zeitschrift für Naturforschung A, 2000
A series of europium doped Na2O-B2O3-SiO2, PbO-GeO2 and ZnOTeO2 glasses was investi-gated by mean... more A series of europium doped Na2O-B2O3-SiO2, PbO-GeO2 and ZnOTeO2 glasses was investi-gated by means of151 Mössbauer spectroscopy. The distortion of the Eu sites has been evaluated in the Na2O-B2O3-SiO2 glasses by means of the quadrupolar interaction parameter and the asym-metry parameter; the disorder has been estimated by the line width. The occupancy of the sites in the borosilicate glasses is discussed. The correlation of the isomer shift with the optical basicity of the glass is discussed in terms of the degree of covalence of the Eu-O bond.
Zeitschrift für Naturforschung A, 2002
A series of Fe2O3-SiO2 nanocomposites (25 weight % of Fe2O3) has been prepared using a sol-gel me... more A series of Fe2O3-SiO2 nanocomposites (25 weight % of Fe2O3) has been prepared using a sol-gel method. Samples showing different features were obtained by varying the evaporation conditions acting on the surface / volume ratio of the starting sol. The samples were investigated using Mössbauer spectroscopy at low temperature. The Fe2O3 nanoparticles have been identified as maghemite ( γ-Fe2O3) particles. The samples show a superparamagnetic behavior with a blocking temperature that depends on the average particle size.
Zeitschrift für Naturforschung A, 1996
The path of the reaction between Fe2O3 and Si, activated by high energy ball milling, has been in... more The path of the reaction between Fe2O3 and Si, activated by high energy ball milling, has been investigated by X-ray diffraction and Mössbauer spectroscopy. Hematite reduction involves oxygen transfer from Fe to Si with the formation of intermediate phases containing Fe(II), which are then reduced to Fe(0). A steady state is reached in the milling process where the reduction of stoichiometric amounts of Fe2O3 and Si is not complete and an amount of Fe(II) in an amorphous matrix still remains. The same intermediate compounds are also observed in the milling process of mixtures with higher Fe2O3 /Si molar ratio.
Physical Review B, 1996
The shape of the ␥ resonance absorption peak of the Eu 3ϩ ion in a disordered structure was inves... more The shape of the ␥ resonance absorption peak of the Eu 3ϩ ion in a disordered structure was investigated in some phosphate, borate, and silicate glasses by using 151 Eu Mössbauer spectroscopy. The quality of the fits was tested by using the Durbin-Watson d statistics. The observed full width at half maximum of the peak was resolved in a contribution of the broadening and a contribution of the quadrupole splitting, due to the distortion of the Eu site compared to a cubic symmetry. The Eu-O bond was found to have a covalent admixture with 6s character. The axial component of the electric-field gradient at the Eu site was found to be correlated with the optical basicity of the glass.
Zeitschrift für Naturforschung A, 2003
The static magnetic susceptibility of two different YBa
Materials Science Forum, 2006
Magnetization M as a function of applied magnetic field H up to 50 kOe was measured at low temper... more Magnetization M as a function of applied magnetic field H up to 50 kOe was measured at low temperature T = 5K, using an SQUID magnetometer, for HfCo2 and Hf2Co intermetallic compounds. Both samples showed paramagnetic behaviour with a weak ferromagnetic contribution due to impurities (0.01-0.001 wt%). The values of the mass susceptibilities χ were deduced from the measurements. In addition, calculations using full-potential linearized augmented plane wave (FP-LAPW) WIEN 97 programme package were done, in order to check their agreement with the experimental results.
The Journal of Physical Chemistry B, 2005
FeCo-Al 2 O 3 nanocomposite aerogels were studied by high-resolution electron microscopy, energy ... more FeCo-Al 2 O 3 nanocomposite aerogels were studied by high-resolution electron microscopy, energy filtered transmission electron microscopy, Mössbauer spectroscopy, and measurements of static magnetizations and hysteretic behavior. The combined use of such techniques provided insights on the formation of bcc FeCo nanocrystalline particles inside the alumina matrix, which is promoted by thermal treatment under hydrogen flow of the parent aerogel. Sample characteristics such as alloy composition and crystallinity, influence of the matrix on the structural evolution, and resulting magnetic properties were investigated as a function of the temperature and time of the reduction treatment.
physica status solidi (b), 2009
Journal of Physics and Chemistry of Solids, 1997
Amorphous Fe*Zr samples were obtained by mechanical grinding from the intermetallic crystalline p... more Amorphous Fe*Zr samples were obtained by mechanical grinding from the intermetallic crystalline phase and by mechanical alloying starting from the elements. We used X-my diffraction and 57Fe Mtissbauer transmission spectroscopy to monitor the amorphization process and fo investigate the alloys prepared by the two methods. The Miissbauer data have been analysed accounting for the disorder proper to a glass structure. The probability distributions of the quadrupole splitting and isomer shift in the amorphous Fe*Zr samples are discussed.
Journal of Non-Crystalline Solids, 1995
57Fe Mössbauer spectroscopy has been used to investigate the effect of an addition of an increasi... more 57Fe Mössbauer spectroscopy has been used to investigate the effect of an addition of an increasing amount of Fe2O3 on the iron environment in a number of sodium phosphate glasses of composition (1 - x)NaPO3 · xFe2O3 (x = 0.05, 0.1, 0.15), prepared in air under the same conditions. High-spin ferric and ferrous iron in octahedral coordination is present in
Journal of Non-Crystalline Solids, 1999
... The latter (MA sample) was obtained from 5 g of a powder mixture of crystalline Fe (Carlo Erb... more ... The latter (MA sample) was obtained from 5 g of a powder mixture of crystalline Fe (Carlo Erba, 99.999%) and Zr (Ventron 99.9 ... 10. M. Magini, G. Licheri, G. Piccaluga, G. Paschina, G. Pinna, X-ray Diffraction of Ions in Aqueous Solutions: Hydration and Complex Formation, CRC ...
Journal of Nanoscience and Nanotechnology, 2011
Stoichiometric magnetic nanosized ferrites MFe 2 O 4 (M = Mn, Co, Ni) were prepared in form of ne... more Stoichiometric magnetic nanosized ferrites MFe 2 O 4 (M = Mn, Co, Ni) were prepared in form of nearly spherical nanocrystals supported on a highly porous silica aerogel matrix, by a sol-gel procedure. X-ray diffraction and transmission electron microscopy indicate that these materials are made out of non-agglomerated ferrite nanocrystals having size in the 5-10 nm range. Investigation by Mössbauer Spectroscopy was used to gain insights on the superparamagnetic relaxation and on the inversion degree. Magnetic ordering at room temperature varies from superparamagnetic in the NiFe 2 O 4 sample, highly blocked (∼70%) in the MnFe 2 O 4 sample and nearly fully blocked in the CoFe 2 O 4 sample. A fitting procedure of the Mössbauer data has been used in order to resolve the spectrum into the tetrahedral and octahedral components; in this way, an inversion degree of 0.68 (very close to bulk values) was obtained for 6 nm silica-supported CoFe 2 O 4 nanocrystals.
Journal of Magnetism and Magnetic Materials, 2004
We present the study of the ferromagnetic behaviour of crystalline and amorphous samples with ide... more We present the study of the ferromagnetic behaviour of crystalline and amorphous samples with identical composition Fe 2 Zr: The crystalline sample was obtained by arc melting. The two amorphous samples were obtained by mechanical grinding from the intermetallic crystalline phase and by mechanical alloying starting from the elements, respectively. We investigated the magnetic properties of the alloy using M. ossbauer spectroscopy and superconducting quantum interference device magnetometry. The values of the M. ossbauer parameters are reported and discussed. We present the initial magnetization curves and hysteresis loops. The magnetization behaviour is marked by saturation at low field values with very small values of remanent magnetization and coercive field, indicating that the samples are soft ferromagnets; this feature is more sharpened in the crystal. The mechanically grinded sample is more similar to the crystalline compound than the mechanically alloyed sample.
Journal of Magnetism and Magnetic Materials, 2004
The static magnetic susceptibility of two different YBa 2 Cu 3 O 6.30 samples shows a peak near t... more The static magnetic susceptibility of two different YBa 2 Cu 3 O 6.30 samples shows a peak near to 45 K; the peak shows a dependence on the field and on the magnetic history. This feature has been explained by the combined effect of regions with a Curie-like behavior and superconducting regions. The origin of these regions in the material has been discussed.
Journal of Applied Physics, 1993
Photoluminescence (PL) of type III (high-purity wet synthetic) silica excited by a XeCl excimer l... more Photoluminescence (PL) of type III (high-purity wet synthetic) silica excited by a XeCl excimer laser (hν=4 eV) is reported. Time-resolved spectra show that a PL band peaked at 1.9 eV can be induced by the XeCl laser irradiation. This band exhibits a fast rise time (<20 ns) but a decay time of several microseconds. The possible mechanisms of photogeneration and photoexcitation of the defects related to this emission are discussed.
The European Physical Journal B, 2007
. Thin films of La0.7Sr0.3MnO3 were grown by molecular beam epitaxy on (001)LaAlO3 crystals. Hig... more . Thin films of La0.7Sr0.3MnO3 were grown by molecular beam epitaxy on (001)LaAlO3 crystals. High resolution X-ray diffraction analysis proves the presence of twins in the films at room temperature, showing that the twin structure of the substrate which forms at the ferroelastic transition at TF = 813 K served as a template for the film microstructure. Magnetic measurements indicate a thermomagnetic
Physics and Chemistry of Glasses-European Journal of Glass Science and Technology Part B, Aug 1, 2006
Abstract: Samples of zinc borate glasses, doped with europium or iron ions, were prepared and exa... more Abstract: Samples of zinc borate glasses, doped with europium or iron ions, were prepared and examined by 151 Eu and 57 Fe Mössbauer spectroscopy. The structural features of the rare earth and transition metal ion sites have been investigated with particular attention to the coordination number, distortion and degree of disorder. The relative concentrations of the different oxidation states have been also determined. The disorder of the sites was investigated by using the convolution method for the analysis of the spectra of glasses. ...
An entry from the Cambridge Structural Database, the world's repository for small molecule cr... more An entry from the Cambridge Structural Database, the world's repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
Hyperfine Interactions (C), 2002
A sample of nanocrystalline YI. S EUO.203 was examined by 151Eu Mossbauer spectroscopy. The degre... more A sample of nanocrystalline YI. S EUO.203 was examined by 151Eu Mossbauer spectroscopy. The degree of covalency of the Eu-O bond has been studied. The spectrum of the cubic Y1.8EUO.203 sample has been resolved into 2 contributions due to europium in the C 3i and C 2 sites, for the first time in 151Eu Mossbauer spectroscopy. The degree of covalency and the electric field gradient of
Zeitschrift für Naturforschung A, 2000
A series of europium doped Na2O-B2O3-SiO2, PbO-GeO2 and ZnOTeO2 glasses was investi-gated by mean... more A series of europium doped Na2O-B2O3-SiO2, PbO-GeO2 and ZnOTeO2 glasses was investi-gated by means of151 Mössbauer spectroscopy. The distortion of the Eu sites has been evaluated in the Na2O-B2O3-SiO2 glasses by means of the quadrupolar interaction parameter and the asym-metry parameter; the disorder has been estimated by the line width. The occupancy of the sites in the borosilicate glasses is discussed. The correlation of the isomer shift with the optical basicity of the glass is discussed in terms of the degree of covalence of the Eu-O bond.
Zeitschrift für Naturforschung A, 2002
A series of Fe2O3-SiO2 nanocomposites (25 weight % of Fe2O3) has been prepared using a sol-gel me... more A series of Fe2O3-SiO2 nanocomposites (25 weight % of Fe2O3) has been prepared using a sol-gel method. Samples showing different features were obtained by varying the evaporation conditions acting on the surface / volume ratio of the starting sol. The samples were investigated using Mössbauer spectroscopy at low temperature. The Fe2O3 nanoparticles have been identified as maghemite ( γ-Fe2O3) particles. The samples show a superparamagnetic behavior with a blocking temperature that depends on the average particle size.
Zeitschrift für Naturforschung A, 1996
The path of the reaction between Fe2O3 and Si, activated by high energy ball milling, has been in... more The path of the reaction between Fe2O3 and Si, activated by high energy ball milling, has been investigated by X-ray diffraction and Mössbauer spectroscopy. Hematite reduction involves oxygen transfer from Fe to Si with the formation of intermediate phases containing Fe(II), which are then reduced to Fe(0). A steady state is reached in the milling process where the reduction of stoichiometric amounts of Fe2O3 and Si is not complete and an amount of Fe(II) in an amorphous matrix still remains. The same intermediate compounds are also observed in the milling process of mixtures with higher Fe2O3 /Si molar ratio.
Physical Review B, 1996
The shape of the ␥ resonance absorption peak of the Eu 3ϩ ion in a disordered structure was inves... more The shape of the ␥ resonance absorption peak of the Eu 3ϩ ion in a disordered structure was investigated in some phosphate, borate, and silicate glasses by using 151 Eu Mössbauer spectroscopy. The quality of the fits was tested by using the Durbin-Watson d statistics. The observed full width at half maximum of the peak was resolved in a contribution of the broadening and a contribution of the quadrupole splitting, due to the distortion of the Eu site compared to a cubic symmetry. The Eu-O bond was found to have a covalent admixture with 6s character. The axial component of the electric-field gradient at the Eu site was found to be correlated with the optical basicity of the glass.
Zeitschrift für Naturforschung A, 2003
The static magnetic susceptibility of two different YBa
Materials Science Forum, 2006
Magnetization M as a function of applied magnetic field H up to 50 kOe was measured at low temper... more Magnetization M as a function of applied magnetic field H up to 50 kOe was measured at low temperature T = 5K, using an SQUID magnetometer, for HfCo2 and Hf2Co intermetallic compounds. Both samples showed paramagnetic behaviour with a weak ferromagnetic contribution due to impurities (0.01-0.001 wt%). The values of the mass susceptibilities χ were deduced from the measurements. In addition, calculations using full-potential linearized augmented plane wave (FP-LAPW) WIEN 97 programme package were done, in order to check their agreement with the experimental results.
The Journal of Physical Chemistry B, 2005
FeCo-Al 2 O 3 nanocomposite aerogels were studied by high-resolution electron microscopy, energy ... more FeCo-Al 2 O 3 nanocomposite aerogels were studied by high-resolution electron microscopy, energy filtered transmission electron microscopy, Mössbauer spectroscopy, and measurements of static magnetizations and hysteretic behavior. The combined use of such techniques provided insights on the formation of bcc FeCo nanocrystalline particles inside the alumina matrix, which is promoted by thermal treatment under hydrogen flow of the parent aerogel. Sample characteristics such as alloy composition and crystallinity, influence of the matrix on the structural evolution, and resulting magnetic properties were investigated as a function of the temperature and time of the reduction treatment.
physica status solidi (b), 2009
Journal of Physics and Chemistry of Solids, 1997
Amorphous Fe*Zr samples were obtained by mechanical grinding from the intermetallic crystalline p... more Amorphous Fe*Zr samples were obtained by mechanical grinding from the intermetallic crystalline phase and by mechanical alloying starting from the elements. We used X-my diffraction and 57Fe Mtissbauer transmission spectroscopy to monitor the amorphization process and fo investigate the alloys prepared by the two methods. The Miissbauer data have been analysed accounting for the disorder proper to a glass structure. The probability distributions of the quadrupole splitting and isomer shift in the amorphous Fe*Zr samples are discussed.
Journal of Non-Crystalline Solids, 1995
57Fe Mössbauer spectroscopy has been used to investigate the effect of an addition of an increasi... more 57Fe Mössbauer spectroscopy has been used to investigate the effect of an addition of an increasing amount of Fe2O3 on the iron environment in a number of sodium phosphate glasses of composition (1 - x)NaPO3 · xFe2O3 (x = 0.05, 0.1, 0.15), prepared in air under the same conditions. High-spin ferric and ferrous iron in octahedral coordination is present in
Journal of Non-Crystalline Solids, 1999
... The latter (MA sample) was obtained from 5 g of a powder mixture of crystalline Fe (Carlo Erb... more ... The latter (MA sample) was obtained from 5 g of a powder mixture of crystalline Fe (Carlo Erba, 99.999%) and Zr (Ventron 99.9 ... 10. M. Magini, G. Licheri, G. Piccaluga, G. Paschina, G. Pinna, X-ray Diffraction of Ions in Aqueous Solutions: Hydration and Complex Formation, CRC ...
Journal of Nanoscience and Nanotechnology, 2011
Stoichiometric magnetic nanosized ferrites MFe 2 O 4 (M = Mn, Co, Ni) were prepared in form of ne... more Stoichiometric magnetic nanosized ferrites MFe 2 O 4 (M = Mn, Co, Ni) were prepared in form of nearly spherical nanocrystals supported on a highly porous silica aerogel matrix, by a sol-gel procedure. X-ray diffraction and transmission electron microscopy indicate that these materials are made out of non-agglomerated ferrite nanocrystals having size in the 5-10 nm range. Investigation by Mössbauer Spectroscopy was used to gain insights on the superparamagnetic relaxation and on the inversion degree. Magnetic ordering at room temperature varies from superparamagnetic in the NiFe 2 O 4 sample, highly blocked (∼70%) in the MnFe 2 O 4 sample and nearly fully blocked in the CoFe 2 O 4 sample. A fitting procedure of the Mössbauer data has been used in order to resolve the spectrum into the tetrahedral and octahedral components; in this way, an inversion degree of 0.68 (very close to bulk values) was obtained for 6 nm silica-supported CoFe 2 O 4 nanocrystals.
Journal of Magnetism and Magnetic Materials, 2004
We present the study of the ferromagnetic behaviour of crystalline and amorphous samples with ide... more We present the study of the ferromagnetic behaviour of crystalline and amorphous samples with identical composition Fe 2 Zr: The crystalline sample was obtained by arc melting. The two amorphous samples were obtained by mechanical grinding from the intermetallic crystalline phase and by mechanical alloying starting from the elements, respectively. We investigated the magnetic properties of the alloy using M. ossbauer spectroscopy and superconducting quantum interference device magnetometry. The values of the M. ossbauer parameters are reported and discussed. We present the initial magnetization curves and hysteresis loops. The magnetization behaviour is marked by saturation at low field values with very small values of remanent magnetization and coercive field, indicating that the samples are soft ferromagnets; this feature is more sharpened in the crystal. The mechanically grinded sample is more similar to the crystalline compound than the mechanically alloyed sample.
Journal of Magnetism and Magnetic Materials, 2004
The static magnetic susceptibility of two different YBa 2 Cu 3 O 6.30 samples shows a peak near t... more The static magnetic susceptibility of two different YBa 2 Cu 3 O 6.30 samples shows a peak near to 45 K; the peak shows a dependence on the field and on the magnetic history. This feature has been explained by the combined effect of regions with a Curie-like behavior and superconducting regions. The origin of these regions in the material has been discussed.
Journal of Applied Physics, 1993
Photoluminescence (PL) of type III (high-purity wet synthetic) silica excited by a XeCl excimer l... more Photoluminescence (PL) of type III (high-purity wet synthetic) silica excited by a XeCl excimer laser (hν=4 eV) is reported. Time-resolved spectra show that a PL band peaked at 1.9 eV can be induced by the XeCl laser irradiation. This band exhibits a fast rise time (<20 ns) but a decay time of several microseconds. The possible mechanisms of photogeneration and photoexcitation of the defects related to this emission are discussed.
The European Physical Journal B, 2007
. Thin films of La0.7Sr0.3MnO3 were grown by molecular beam epitaxy on (001)LaAlO3 crystals. Hig... more . Thin films of La0.7Sr0.3MnO3 were grown by molecular beam epitaxy on (001)LaAlO3 crystals. High resolution X-ray diffraction analysis proves the presence of twins in the films at room temperature, showing that the twin structure of the substrate which forms at the ferroelastic transition at TF = 813 K served as a template for the film microstructure. Magnetic measurements indicate a thermomagnetic
Physics and Chemistry of Glasses-European Journal of Glass Science and Technology Part B, Aug 1, 2006
Abstract: Samples of zinc borate glasses, doped with europium or iron ions, were prepared and exa... more Abstract: Samples of zinc borate glasses, doped with europium or iron ions, were prepared and examined by 151 Eu and 57 Fe Mössbauer spectroscopy. The structural features of the rare earth and transition metal ion sites have been investigated with particular attention to the coordination number, distortion and degree of disorder. The relative concentrations of the different oxidation states have been also determined. The disorder of the sites was investigated by using the convolution method for the analysis of the spectra of glasses. ...