The defect distribution of charge compensated sites in Er3+:CaF2 crystals has been measured by laser spectroscopy over a concentration range from 10−4 to 1.0 mol %. The measurements show the transition from the low concentration region, where the site distribution is described by simple pairing of Er3+ and fluoride interstitial ions to a high concentration region dominated by clustering of the simple pairs. The simple pairs undergo a decrease in the absolute concentration in this region in direct conflict with all equilibrium models of solid state defect equilibria. A model is proposed which describes the measured site distribution and also explains the anomalous behavior observed in many other experiments. The model describes the distribution by requiring the cation equilibria to be frozen at a high temperature and the fluoride equilibria at a low temperature, in agreement with the temperature dependence of their mobilities. The excess fluoride interstitial ions are then ’’gettered’’ by the clustered pairs forcing dissociation of the simple pairs.

Defect equilibria in fluorite structure crystals

Alkaline earth fluoride crystals doped with trivalent rare earth contain both isolated and clustered rare earths. Electronic nonradiative relaxation for isolated rare earths proceeds solely by multiphonon relaxation while for clustered rare earths, it proceeds by both efficient nonresonant energy transfer and multiphonon relaxation. We have measured the rates of all the relaxation processes for representative sites in CaF2, SrF2, and BaF2 in order to obtain a comprehensive understanding of relaxation in the materials. Exponential dependences of the multiphonon and energy transfer rates on energy gap have been found. The rates do not appear to scale simply with the lattice phonon energies but depend upon the details of the phonon distribution and coupling. Temperature dependence of the rates indicate a complex mixture of phonons may contribute to the observed relaxation.

Multiphonon and energy transfer relaxation in charge compensated crystals

In this review the authors treat the study of electronic relaxation phenomena using Mossbauer spectroscopy. First, examples are discussed where it is possible to reproduce the spectral lineshape by use of a stochastic theory. The general case is then considered, including a full discussion on the application of ab initio theories. There then follows a detailed analysis of the microscopic mechanisms responsible for relaxation in insulators (e.g. spin-phonon and spin-spin interactions) and metals. Finally, the fast relation limit is considered.

http://iopscience.iop.org/article/10.1088/0034-4885/49/11/002/pdf

Mossbauer spectra in paramagnetic relaxing systems

Mössbauer study of 170Yb in some metals and metal-like compounds. Isomer shift measurements and intermediate valence states of ytterbium

We report here the study, at very low temperature, by means of emission Mossbauer spectroscopy on 170 Yb, of the electronic properties of the Yb 3+ ion diluted (100 ppm) in palladium. The observed two-line hyperfine spectra show that the ground state of Yb 3+ in Pd is the crystal field doublet Γ 7 , and that one of the lines has an important static broadening. The Yb 3+ paramagnetic relaxation rate was measured between 0.11 K and 2 K; at the lowest temperatures (T≤0.65 K), we computed a relaxation lineshape within the slow-relaxation or «secular» approximation, well suited to the case when the two lines have different static broadenings. The obtained thermal variation of the relaxation rate evidences an excited crystal field state which is quasi-degenerate with the ground state Γ 7 ; it is interpreted by assuming that the conduction electrons scattered by the impurity have a strong d character. An analysis of the static broadenings allowed us to determine that the first excited crystal field state is the Γ 8 quartet Les spectres hyperfins a deux raies observes montrent que l'etat fondamental de Yb 3+ dans Pd est le doublet de champ cristallin Γ 7 , et que l'une des raies possede un elargissement statique important. Mesure de la frequence de relaxation paramagnetique de Yb 3+ entre 0,11 K et 2 K; aux plus basses temperatures, on a calcule une forme de raie de relaxation dans l'approximation de relaxation lente ou «seculaire» bien adaptee au cas ou les deux raies ont des elargissements statiques differents

F. Varret

Papers

Mössbauer study of 170Yb in CaF2

Mössbauer study of ytterbium ethylsulphate