Alkaline Earth Fluoride Crystals Research Articles

Investigations of the new trigonalGd3+ESR center produced in irradiated alkaline-earth fluoride crystals

A detailed account is given of various properties of the new trigonal ${\mathrm{Gd}}^{3+}$ ESR center produced by x-ray irradiation in single crystals of Ca${\mathrm{F}}_{2}$ and Sr${\mathrm{F}}_{2}$ containing gadolinium ions. The fine-structure splitting of the new center is characterized (in units of ${10}^{\ensuremath{-}4}$${\mathrm{cm}}^{\ensuremath{-}1}$) by ${b}_{2}^{0}=\ensuremath{-}2905.7\ifmmode\pm\else\textpm\fi{}0.5$, ${b}_{4}^{0}=+12.6\ifmmode\pm\else\textpm\fi{}0.2$, ${b}_{6}^{0}=0\ifmmode\pm\else\textpm\fi{}0.3$, ${b}_{4}^{3}=\ensuremath{-}547\ifmmode\pm\else\textpm\fi{}10$, ${b}_{6}^{3}=+30\ifmmode\pm\else\textpm\fi{}15$, ${b}_{6}^{6}=+11\ifmmode\pm\else\textpm\fi{}15$ in Ca${\mathrm{F}}_{2}$; and ${b}_{2}^{0}=\ensuremath{-}2671.7\ifmmode\pm\else\textpm\fi{}0.5$, ${b}_{4}^{0}=+11.7\ifmmode\pm\else\textpm\fi{}0.2$, ${b}_{6}^{0}=0\ifmmode\pm\else\textpm\fi{}0.3$, ${b}_{4}^{3}=\ensuremath{-}346\ifmmode\pm\else\textpm\fi{}10$, ${b}_{6}^{3}=+22\ifmmode\pm\else\textpm\fi{}15$, ${b}_{6}^{6}=+24\ifmmode\pm\else\textpm\fi{}15$ in Sr${\mathrm{F}}_{2}$. Despite extensive experimentation, the new center has not been produced in Ba${\mathrm{F}}_{2}$ crystals.

Investigations of the dynamical coloration behaviour of cerium-oxide-doped alkaline earth fluoride crystals

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Investigations of the coloration behaviour of cerium-oxide-doped alkaline-earth fluoride crystals

Investigations of the coloration behaviour of cerium-oxide-doped alkaline-earth fluoride crystals

Verification of the Lifshitz Theory of the van der Waals Potential Using Liquid-Helium Films

Accurate measurements are presented of the thickness of helium films on cleaved surfaces of alkaline-earth fluoride crystals at 1.38 $^{\mathrm{o}}\mathrm{K}$. The films were measured between 10 and 250 \AA{} using an acoustic interferometry technique. The data are in excellent agreement with calculations based on the Lifshitz theory of van der Waals forces. Calculations of the film thickness on a variety of other substrates are also given.

Absorption investigations on alkaline earth fluoride mixed crystals

AbstractSome results are given of absorption investigations of alkaline earth fluoride crystals doped with impurity alkaline earth cations – CaF2:Sr, CaF2:Ba, SrF2:Ca, SrF2:Ba, and BaF2:Sr. These crystals were exposed to X‐rays at room temperature and at 84 °K. The absorption spectra recorded at these two temperatures differ considerably. The room temperature spectrum is similar to the spectrum of Li+‐, Na+‐, and K+‐doped CaF2 and SrF2 crystals, which were subtractively coloured at room temperature. The low‐temperature spectrum corresponds to a modified spectrum of the pure alkaline earth fluoride crystals after subtractive coloration. A new model of centres, the FAE centre, is suggested. In this centre one of the lattice cations, tetrahedrally surrounding the F‐centre electron, is replaced by an impurity alkaline earth cation.

Foreign ions with d1-states in alkaline earth fluoride crystals

Starting from the model of an F-centre disturbed by an impurity ion and another centre in which the impurity ion plays an active part it is tried to explain photochromic processes in alkaline earth fluorides doped with Y, La, Ce, Gd, and Tb. Experiments and model concepts of Kiss et al. [1 to 3] are discussed and existing models investigated. Other absorption bands are associated with a so-called disturbed F-centre. These theories are supported experimentally by dichroism and bleaching experiments and theoretically by calculations on the basis of a Coulomb model of the disturbed F-centre. It can further be shown that the symmetry of the afore-mentioned activator centre is probably lower than cubic. An electron transfer between two locally separated types of centres across the conductivity band has so far been assumed by all authors [1 to 4]. Also Heyman [4] interpreted the results of his photoconductivity investigations of CaF2:Gd crystals in this way. In contrast to this it is shown that important, experimentally observable properties can be better described using a model which assumes the association of the disturbed F-centre and the activator centre [5]. Ausgehend vom Modell eines durch ein Fremdion gestorten F-Zentrums und einem Zentrum, bei dem das Fremdion eine aktive Rolle spielt, wird versucht, photochrome Prozesse in Erdalkalifluoriden, die mit Y, La, Ce, Gd und Tb dotiert sind, zu erklaren. Auf Untersuchungen und Modellvorstellungen von Kiss et. al. [1 bis 3] eingehend, werden bestehende Modellvorschlage uberpruft. Es werden andere Zuordnungen von Absorptions-banden zu einem sogenannten gestorten F-Zentrum getroffen. Diese werden experimentell durch Dichroismus- und Bleichuntersuchungen und theoretisch durch Abschatzungen an-hand eines Coulomb-Modells des gestorten F-Zentrums unterstutzt. Auserdem kann gezeigt werden, das die Symmetrie des oben erwahnten Aktivatorzentrums wahrscheinlich niedriger als kubisch ist. Bisher wurde von allen Autoren eine Elektronenubertragung zwischen zwei raumlich getrennten Zentrenarten uber das Leitfahigkeitsband hinweg angenommen [1 bis 4]. Auch Heyman [4] interpretiert seine Ergebnisse der Photoleitfahigkeitsunter-suchungen an CaF2:Gd-Kristallen dahingehend. Im Gegensatz dazu wird gezeigt, das mit einem Modell, das die raumliche Vereinigung des gestorten F-Zentrums und des Aktivator-zentrums annimmt, wichtige, experimentell beobachtbare Eigenschaften besser zu beschreiben sind [5].

Polarized Luminescence of X‐Irradiated CaF2:Y and SrF2:Y Crystals and the Structure their Luminescence Centres

AbstractY‐doped CaF2 and SrF2 crystals coloured subtractively by X‐rays at room temperature exhibit a broad luminescence band at 86°K in the near infrared if they are irradiated with light of the three long‐wavelength bands of the four‐band absorption spectrum. The peak of luminescence lies at 1.32 μm for CaF2: Y and 1.46 μm for SrF2:Y. The luminescence is partially polarized, the luminescence centres are oriented along the threefold axes ([111]‐directions). The results are shown to be in contradiction to earlier conceptions of the origin of the atomic centres in Y‐doped alkaline earth fluoride crystals. They are however compatible with a new model consisting of an electron trapped in a fluorine vacancy which is disturbed by a nearest neighbour Y3+‐ion.

The colour centre absorption of X‐Ray coloured alkaline earth fluoride crystals

AbstractAbsorption by colour centres (i. e. the Ψ‐centre comprising an anion vacancy with a trapped electron) is observed in pure alkaline earth fluoride crystals after subtractive coloration at temperatures of 300, 84, and 37 °K. Colorability increases with decreasing temperature and increasing cation radius of the crystal hosts. The coloration spectra of the CaF2 and SrF2 crystals are similar; they consist of a strong double band, called the Ψ‐band, a weak ultraviolet band, probably caused by hole imperfections, called Ω‐centres, and an increase in absorption at 200 nm (Ω‐centre: collective term for all hole imperfections occuring in alkine earth fluoride crystals [8]). The peak of the Ψ‐band occurs at 375 nm for CaF2 and at 434nm for SrF2. BaF2 crystals exhibit two different coloration spectra depending on whether they are coloured at room temperature or at lower temperatures. Both the coloration spectra of BaF2 are dissimilar to the type of spectrum observed for CaF2 and SrF2. On heating the CaF2 and SrF2 crystals, strong discoloration is only observed at relatively high temperatures (between 125 and 150 °K); on the other hand, coloration increased when it is performed at 37 instead of 84 °K. Saturation of the coloration of the BaF2 crystals takes place at 84 °K whereas their discoloration commences already between 85 and 100 °K. The discussion of the results considers special models and uses published results for additive coloration spectra, electron spin resonance and theoretical calculations.