Ester Zannoni

Optical spectroscopy of Ca3Sc2Ge3O12 : Ni2+

Abstract The optical spectra of Ca3Sc2Ge3O12 (CaSGG) single crystals doped with Ni2+ are presented. Analysis of the absorption spectrum shows that the dopant substitutes mainly for Sc3+ in octahedral sites and, to a lesser extent, Ge4+ in tetrahedral sites. The optical transitions were assigned in the framework of the Ligand Field theory and the Dq, B and C parameters were obtained for both coordination geometries. Three emission transitions were observed in the green, red and infrared regions and were assigned to the 1T2(1D) → 3A2(3F), 1T2(1D) → 3T2(3F) and 3T2(3F) → 3A2(3F) transitions of Ni2+ in octahedral sites, respectively. No luminescence from Ni2+ in tetrahedral sites was observed.

Optical spectroscopy and fluorescence dynamics of Er3+ in Ca3Sc2Ge3O12 crystal

The absorption and luminescence spectra of Er3+-doped Ca3Sc2Ge3O12 single crystals have been measured at various temperatures from 10 to 298 K. The decay times of the lower-lying excited states, emitting in the visible and the near-IR regions, have been measured as a function of temperature. From the oscillator strengths of the absorption transitions, the Judd–Ofelt intensity parameters have been evaluated. The spontaneous-transition probabilities, the branching ratios, and the radiative lifetimes have been determined for the most important emitting states by use of the calculated intensity parameters, and the results have been compared with the experimental data. The stimulated-emission cross sections have been estimated for the most important laser transitions, indicating that this compound can be considered a suitable active medium for laser operation.

Vibrational properties of Ca3Sc2Ge3O12, a garnet host crystal for laser applications

The IR and the Raman spectra of the Ca3Sc2Ge3O12 (CaSGG) garnet crystal have been measured and discussed in terms of internal and external modes of the tetrahedral GeO4-4 moiety. Some important aspects of the electronic spectroscopy of these materials activated with luminescent ions, i.e. the multiphonon relaxation of the excited states and the vibrational structure of the optical bands, have been taken into consideration, and correspondences with the vibrational properties of the host lattice have been presented.

Optical properties of Cr-doped Ca3Sc2Ge3O12 single crystals

Abstract The absorption and luminescence properties of Cr-doped Ca 3 Sc 2 Ge 3 O 12 garnet crystals were measured at temperatures ranging from 10 to 298 K. It results from the absorption spectrum that the main optically active species is Cr 3+ substituting for Sc 3+ in octahedral low field sites. The emission properties of this material have been interpreted by considering 4 T 2 as the only emitting level at all temperatures. The 10 K luminescence spectrum is made up of a broadband extending in the NIR region with its maximum at 13210 cm −1 ; when the temperature is increased from 10 to 298 K, its intensity decreases to about 65% and its maximum shifts to 13000 cm −1 . The fine structure which is present both in low-temperature emission and absorption spectra has been assigned as a vibronic progression in two modes with energies ν 1 = 190–195 cm −1 and ν 2 = 290–295 cm −1 . The decay curves are single exponential with lifetimes ranging from 195 (10 K) to 130 (298 K) microseconds. The temperature behaviour of the intensities and the lifetimes indicates that the non-radiative processes are not very effective for this material.

Muon spin rotation and SQUID investigation of superconductivity in (NH3)xNaK2C60 (x∼0.7)

The family of superconducting fullerides (NH_3)_xNaK_2C_60 shows an anomalous correlation between T_c and lattice parameter. To better understand the origin of this anomaly we have studied a representative x=0.75 compound using SQUID magnetometry and MuSR spectroscopy. The lower critical field H_c1, measured by the trapped magnetization method, is less than 1 G, a very small value as compared with that of other fullerides. Muon spin depolarization in the superconducting phase shows also quite small local field inhomogeneities, of the order of those arising from nuclear dipolar fields. On the other hand, the 40 T value for H_c2, as extracted from magnetometry data, is comparable to that of other fullerides. We show that these observations cannot be rationalized within the framework of the Ginzburg-Landau theory of superconductivity. Instead, the anomalous magnetic properties could be interpreted taking into account the role played by polaronic instabilities in this material.

NMR studies of Li15C60 fulleride

The intercalation compound Li15C60 has been studied with 13C, 7Li NMR and X-ray diffraction in the low temperature phase (T<520 K). XRD data suggest the presence of two different stable structures: a tetragonal monomeric and an orthorhombic polymerised phase. Detailed 13C MAS NMR experiments in the latter phase, evidence sp3 bondings among the carbon atoms, whereas the relative (sp2/sp3) intensities, together with X-ray data, suggest a C60 polymerization as [2+2] cycloaddition. Multiple quantum NMR experiments on 7Li confirm the presence of lithium clusters, previously shown by X-ray diffraction in the high temperature phase (T<520 K), also at lower temperatures. Nevertheless, the inferred cluster size is significantly smaller than that suggested by the stoichiometry, probably because of cluster fragmentation in the low-T phase. Further evidence about this scenario is obtained also from preliminary measurements of line shapes and Tt relaxation times, which exhibit a multi-exponential recovery with very di...

Clustering and polymerization of Li15C60

The structural and electronic properties of lithium intercalated fullerides (of which Li15C60 is the most representative) are still puzzling and unclear. Above 520 K, x-ray/neutron diffraction shows an fcc phase in which the 15 Li atoms clusterize in the octahedral interstices. However, at lower temperatures, a change in the crystalline symmetry and also in the electronic properties takes place as observed from 13C, 7Li/6Li NMR and x-ray diffraction measurements. X-ray diffraction data suggest the presence of two different stable structures: a tetragonal monomeric and an orthorhombic polymerised phase. Detailed 13C magic angle spinning NMR experiments in the latter phase indicate sp3 bondings among the carbon atoms, whereas the relative (sp2/sp3) intensities, together with x-ray data, suggest the C60 polymerization to be a [2+2] cycloaddition. Multiple quantum NMR experiments on 7Li confirm the presence of lithium clusters, as observed by x-ray diffraction in the high temperature phase, also at lower temperatures. However, the inferred cluster size is significantly smaller than that suggested by the stoichiometry. The distortion in the low-T structure of L15C60 is supposed to induce the migration of Li atoms from octahedral to tetrahedral voids, thus accounting for the lower number of Li atoms in the clusters. Further evidence of this scenario is obtained also from preliminary measurements of line shapes and T1 relaxation times, which exhibit a multiexponential recovery with very different constants that are hardly compatible with a single family of Li atom sites.

Anomalous superconductivity in (NH3)xNaK2C60

Contrary to what is observed in many fullerides, the transition temperature of the superconducting (NH3)xNaK2C60 decreases as its cubic lattice parameter increases, when more ammonia is intercalated in the system. To understand the origin of this anomaly we measure the electronic spin susceptibility with SQUID magnetometry and NMR in two differently doped samples. The relation between Tc and the density of states at the Fermi level is found to be opposite to the Migdal-Eliashberg prediction. CP-MAS measurements of the isotropic part of the 13C Knight shift qualitatively confirms this result. 13C NMR relaxation measurements validate the interpretation of the spin susceptibility in terms of density of states, ruling therefore out the presence of strong antiferromagnetic correlations in the Fermi liquid. A possible explanation of the anomaly should take into account the non adiabatic nature of the superconductivity in this compound.