R. J. M. da Fonseca

Structure determination and a vibrational study for the hexagonal elpasolite Cs2NaGaF6:Cr3+

Single crystals of 0.5% Cr3+-doped Cs2NaGaF6 were studied by means of x-ray diffraction and polarized Raman scattering. The crystal exhibits a unique stacking interaction, with a hexagonal structure with Rm symmetry. Polarized Raman spectra recorded at 16 and 300 K reveal sidebands that are ascribed to the local vibrational of the [CrF6] coordination unit, which differ markedly from those of the Cs2NaGaF6 host lattice. The results are compared to findings for other elpasolite lattices, and the room temperature luminescence quantum yields are discussed in terms of a delicate balance between electron–phonon strength and lattice distortion.

Vibrational Analysis of the Elpasolites Cs2NaAlF6 and Cs2NaGaF6 Doped with Cr3+ Ions by Fluorescence Spectroscopy

Interest in 3d transition metal impurities in ionic crystals has increased due to their important role in the laser activity of these materials. Moreover, recent advances in tunable solid-state lasers and high-power semiconductor laser diode arrays have generated a strong interest in investigating new compounds that emit in the visible and near-infrared spectral regions. In particular, many optical studies have been devoted to Cr3+-doped fluoride crystals as a consequence of the high quality of some Cr3+-based laser materials. In the present investigation, the low-temperature emission spectra of Cr3+ ions in the hexagonal elpasolites Cs2NaAlF6 and Cs2NaGaF6 have been measured. Each compound has two crystallographically inequivalent octahedral sites for the Al3+ and Ga3+ ions that can be occupied by Cr3+ ions. For both materials, the luminescence spectrum presents two zero-phonon lines accompanied by a well-defined vibrational structure. The different peaks of the emission broad band are described in terms of phonons of the lattice and normal modes of the octahedral complex [CrF6]3−. A detailed analysis of the vibrational structure observed leads to the conclusion that the 2E and 4T2 excited states of the [CrF6]3− ion are displaced along the eg and a1g and probably the t2g coordinates.

Dependence of MgGa2O4:Co2+ photoluminescence on temperature and impurity concentration

Polycrystalline samples of MgGa 2 O 4 doped with 0.1 and 1.0% of Co 2+ ions were produced by ceramic methods and investigated by x-ray diffraction and luminescence spectroscopy. Emission data at room temperature and 77 K, as well as transition lifetimes obtained by the phase-shift method, are presented. The emission is attributed to 4 T 1 ( 4 P) → 4 A 2 ( 4 F) of Co 2+ ions in tetrahedral sites. The excitation spectra are associated with 4 A 2 ( 4 F) → 2 A 1 ( 2 G), 4 A 2 ( 4 F) → 2 E( 2 G) and 4 A 2 ( 4 F) → 4 T 1 ( 4 P) electronic transitions. The crystal field Dq and Racah parameter B were obtained from the spectra and Tanabe-Sugano energy level diagram. The highlights of the present work are the relatively simple sample obtention process as well as its reproducibility and the high photoluminescence quantum efficiency (near to 1.0) together with the intense and broad emission band which make the MgGa 2 O 4 a very attractive material for use as tunable media.

Fotoluminescência em amostras policristalinas de galato de magnésio dopado com Ni2

The photoluminescence of MgGasO4:Ni2+ are presented in this article. The observed bands are attributed to the 1T2 (1D) ® 3T2 (3F) and 1T2 (1D) ® 3T1 (3F) spin-forbidden transitions and located in the visible and infrared regions, respectively. Absorption spectra obtained from diffuse reflectance measurements show bands associated with 3A2 (3F) ® 3T1 (3F) and 3A2 (3F) ® 3T2 (3F) transitions and a weak structure assigned to 3A2 (3F) ® 1E (1D) transition. All the observed transitions are identified with energy levels of Ni2+ in octahedral sites of magnesium gallate. From the Tanabe-Sugano energy diagrams for a d8 configuration, the crystal field Dq and interelectronic repulsion B and C parameters were calculated. We also present, for comparison, previous results of Cr3+ (d3 electronic configuration), Fe3+ (d5) and Co2+ (d7) ions in this host lattice for samples prepared with the same ceramic method.

Temperature and impurity concentration effects on Mg(1−x)CoxGa2O4 photoluminescence

Ceramic materials doped with magnetic ions that present emission in the visible and near‐infrared spectral regions are very attractive due to their inherent tunability. Possible applications include their utilization as optoelectronic and display devices, as spintronic material, in signal transmission and information storage, in the fabrication of special papers, as dosimetric materials and room temperature solid state lasers. Materials doped with tetrahedrally coordinated Co2+ present wide bands originated from electronic transitions in the ionic unfilled 3d electronic shell. The Co2+ 3d electrons are outside of the ion core and therefore their optical properties are directly affected by static and dynamic properties of ligand anions. The magnesium gallate MgGa2O4 is a partially inverted spinel described as a AB2O4 material with two possible positions for A2+ and B3+ cations. Polycrystalline MgGa2O4 : Co2+ samples were produced by solid‐state reactions between ultra‐pure raw oxides MgO, β‐Ga2O3 and the d...

Laboratory teaching in optics: formation x information and the physics and engineering students

The way laboratory work should be conducted and the effectiveness of the two major usual approaches for doing that are discussed. The mapping of problems presented by both these methods is discussed in the context of their application in the Education of physicists and engineers at undergraduate level. The challenges posed by trying to teach as much as possible, in as little as possible time, for as much as possible people are analyzed and some solutions are proposed.