EPR spectra and crystal field of hexamer rare-earth clusters in fluorites

Abstract

Disordered fluorite-related solid solutions ͑AF 2 ͒ 1−x−y ͑LnЈF 3 ͒ x ͑LnЉF 3 ͒ y , where A = Ca, Sr, Ba; LnЈ = Er, Tm, Yb; LnЉ = Lu, Y; and x Ӷ y = 0.001-0.4, were studied by both optical detection and conventional electron paramagnetic resonance ͑EPR͒ techniques. The EPR spectra of paramagnetic rare-earth ions Er 3+ , Tm 3+ , and Yb 3+ in clusters of diamagnetic Y 3+ and Lu 3+ ions were recorded. It appears that the crystalline electric field at the sites of Ln ions in the clusters is of "nearly" tetragonal symmetry and provides for high values of factors g ʈ , approaching the theoretical limits, and small values of factors g Ќ ϳ 0 in the ground states of the paramagnetic Ln ions. It was assumed that all the clusters of Ln ions in the solid solutions appear to be similar in structure to the hexameric clusters, which are the basic structural units of the homologous series of fluoriterelated superstructures ͑AF 2 ͒ 1−y ͑LnF 3 ͒ y with compositions y =5/m, where m is an integer in the range of 13-19. The structure of "symmetric" hexameric clusters in CaF 2 , SrF 2 , and BaF 2 hosts was established by computer simulation. The crystalline electric field and the spectroscopic ground-state parameters for Er 3+ , Tm 3+ , and Yb 3+ ions in the hexameric clusters were calculated and found to be in agreement with the experimental data, being totally different from those known for the "isolated" simple cubic and tetragonal centers in the fluorite crystals.

Optical absorption and thermoluminescence studies in 100MeV swift heavy ion irradiated CaF2 crystals

“…However, the peak positions of these color centers remained unaltered with ion fluences. Literature on O 16 and Pb 208 ions irradiated [3] and neutron irradiated [15] CaF 2 crystals showed similar absorption spectra with a prominent absorption peak at $550 nm. The results in the present study are in good agreement with the literature.…”

Supercooling at Melt Growth of Single and Mixed Fluorite Compounds: Criteria of Interface Stability

“…Importantly, in the low T-interval there occurs a transition from behavior typical of single crystals (with a maximum in T-dependence) to behavior characteristic for glasses, with a monotonic decrease in thermal conductivity with Tdecrease. As firmly considered such transition to glasslike structures is connected with formation, accumulation, and agglomeration of clusters of oppositely charged defects, which leads to disturbance of short-range order with retention of long-range order [Fedorov, 1991[Fedorov, , 2000Kazanskii et al, 2005;Sobolev et al, 2003). Within the next compositional interval, x = 0.09-0.25, the thermal conductivity, was found out to grow up monotonically with T, which is typical behavior for any disordered systems but is completely inadmissible for crystals of simple stoichiometric compounds.…”

Synthesis of CaF2–YF3 nanopowders by coprecipitation from aqueos solutions

“…Such clusters fit in naturally in the fluorite crystal lattice (Fig. 2) and appear to be the dominant type of structural defects in Ca 1−x Y x F 2+x solid solutions at higher yttrium concentrations [14,16,28,29]. The heterovalent substitution mechanism for the formation of the aforementioned solid solutions (Fig.…”

“…The formed cationic and anionic defects associate among themselves, thus forming defect clusters [28,29]. Ca 1−x Y x F 2+x solid solution maintains its original fluorite-type structure within the 0 ≤ x ≤ 0.38 interval limits.…”