Centers In SrF2 Research Articles

Structure and Metastability of MF2 (M = Ca, Sr, Ba) Fine Powders Mechanochemically Doped with Er3+ Ions

In the paper thermal treatment investigations of the MF2 (M = Ca, Sr, Ba) fine powders mechanochemically doped with Er3+ ions using electron paramagnetic resonance and X-ray diffraction are presented. It is shown that the prepared samples are found in the nonequilibrium metastable state characterized by the high concentration of the cationic vacancies and prevalence of the cubic symmetry-doped Er3+ ion centers. Vacancies formed when the deformation exceeds the elastic limit serve both as the means for a nonlocal charge compensation and a route for mechanically activated diffusion. Annealing brings the powders to the ground state with the most of the vacancies healed and the trigonal symmetry of the impurity Er3+ centers in SrF2 and BaF2 due to the local compensation by the interstitial fluorine ion.

Ab initio calculations for the F-center transfer and R centers in SrF2

We have simulated the F-center transfer and R center in SrF2 crystal by using density functional theory (DFT) with a hybrid B3PW description of exchange and correlation. Our calculations show that the F-center diffusion barrier is equal to 1.84eV. During the F-center transfer, the trapped electron is more delocalized than that in the regular F-center case, and the gap between defect level and conduction bands (CB) in the α-spin state decreases. The formation energy calculations of R center show the trend of F centers to aggregate in SrF2. During the F-center aggregation, a considerable covalency forms between two neighboring fluorine vacancies with trapped electrons. Three incompletely paired electrons trapped in the R center have an up-down-up spin arrangement and induce three defect levels in the gaps between valence bands (VB) and conduction bands for both of α- and β-spin polarized band structures, respectively. More defect bands lead to more complex electron transitions, which were classified into two F- and four M-like transitions. The DOS calculations clearly reveal the components of defect bands.

EPR of trivalent iron centers in SrF2: Fe crystals

An orthorhombic paramagnetic Fe3+ ion center (concentration of iron ions is 0.1 at. %) was found using EPR in BaF2: Fe crystals irradiated by x rays. The EPR spectra recorded in the Q range at a temperature T = 77 K exhibit both the fine structure typical of a center with effective spin S = 5/2 and a superhyperfine structure (SHFS) indicating the SHFS interaction of the electronic moment of the center with the nuclear magnetic moments of its six ligands (F− ions). An analysis of the SHFS reveals that this center forms through the replacement of a Ba2+ cation by a Fe2+ cation, which transforms into a Fe3+ (6A1g) cation under x-ray irradiation and shifts into a neutral position along the C2 axis of the cubic coordination shell of the replaced host cation.

A method for estimating the local lattice distortions near a magnetic ion from ligand hyperfine parameters: Yb3+trigonal centers in SrF2 and BaF2

An empirical relationship describing the radial dependence of the nondipole part of the anisotropic constant for the ligand hyperfine interaction in alkaline-earth fluorides is proposed. This relationship is used for calculating the distances between the magnetic ion and the fluorine ions involved in its nearest environment. The results of the calculations are in good agreement with the values obtained by other methods for both cubic and tetragonal fluorine centers in these crystals. The distances from the magnetic ion to different groups of nonequivalent fluorine ions of the nearest environment in Yb3+ trigonal centers of SrF2 and BaF2 are determined. It is found that the Yb3+ ion is slightly displaced along the trigonal axis away from the compensating fluorine ion.

Local structure of titanium pair centers in SrF2 crystals: EPR and ENDOR spectroscopic studies

The local structure of titanium pair centers in SrF2: Ti crystals is investigated using electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy. It is found that titanium pair centers with spin moment S=2 and tetragonal symmetry of the magnetic properties are formed in SrF2: Ti cubic crystals under certain growth conditions and during annealing. The tensor components of the fine and ligand hyperfine structures in the EPR and ENDOR spectra are determined. A model of the Ti+-Ti3+ paramagnetic dimer is proposed. This model provides an adequate interpretation of both the ferromagnetic nature of the exchange interaction and the observed displacements of four ligands in the first coordination sphere of titanium impurity ions in directions perpendicular to the impurity ion-ligand bonds.

Hyperfine and quadrupole interactions of trigonal 157Gd3+ centers in SrF2 and BaF2. Analysis of distortions in the nearest atomic environment

Trigonal 157Gd3+ impurity centers in SrF2 and BaF2 were experimentally studied by EPR and double electron-nuclear resonance (DENR) techniques. Parameters of the hyperfine and quadrupole interactions between these centers were determined. Possible distortions of the nearest atomic environment of the impurity centers are estimated within the framework of a superposition model using the EPR and DENR data for the centers of cubic and trigonal symmetry in the crystals studied.

Colour Centres in Alkali Metal Doped SrF2

AbstractColour centres in SrF2:Na+ and SrF2:K+ are studied by optical spectroscopy, magnetic circular dichroism (MCD) and by EPR. It is shown that, in each of these two materials, there exist two kinds of FA centres, the weakly perturbed F and the more strongly perturbed F. The improperly named F2A centre is paramagnetic and its MCD has no similarity at all with the one of the F2 centre in SrF2. In SrF2:A+, non paramagnetic colour centres of the “F“ type are observed, as in CaF2:A+, but no defect corresponding to the f centre of CaF2:Na+. On the other hand, in SrF2:Na+ can be produced a centre which is called c', comparable to “F“, but with a spectral shift toward shorter wavelengths.

F-aggregate centers in uranium doped and undoped BaF2by neutron irradiation at low temperature

Abstract The aggregate centers in uranium doped (0.06 mol %) and undoped BaF2 are investigated by reactor irradiation at 20 K. The new bands at 778, 858 and 890 nm appear in undoped BaF2 at doses above 5 × 1016 n/cm2. From the thermal optical properties and the correlation 778 and 858 nm bands, it is proposed that the 778 and 858 nm bands are attributed to F2 and F+ 2 centers in BaF2, respectively. These aggregate centers behave as similar as that of the F2 and F+ 2 centers in SrF2.

Raman scattering of F centres in SrF2

Single-phonon Raman scattering induced by F centres in SrF2 has been measured at low temperatures. Polarisation properties and resonant enhancement have been studied. The spectrum with A1 symmetry has been calculated using a perturbed density of states projected onto the motion of both nearest and next-nearest neighbours; the calculated spectrum agrees reasonably well with experiment.