Relaxation of type-I dipolar complexes in calcium fluoride containing trivalent rare-earth impurities

Abstract

Ionic thermocurrent (ITC) measurements have been used to determine the relaxation parameters for the principal (type-I) dipolar complex in CaF, :R'+, where R is used here to represent Sc, Yb, Tm, Er, Y, Dy, Gd, Eu, Nd, Ce, and U. An activation energy (E) of about 0.38 eV and a reciprocal frequency factor (To) of about 2 X 10 ' sec was measured for Sc'+, the smallest impurity ion measured. For the five largest impurities studies, E was about 0.42 eV and To was about 3 &( 10 " sec. A uniform transition between these two sets of relaxation parameters was observed over a relatively small range of R'+ ionic radii. These results are compared with other recently reported data and possible models for the observed behavior are discussed.

Dielectric spectrum of rare-earth-doped calcium fluoride crystals

“…For ytterbium-doped crystals only one low-temperature relaxation has been found, which corresponds to the R I type, with 0.38-0.5 eV activation energies [8][9][10]14]. An additional relaxation (R A ) has been also revealed at 285-290 K characterized by high activation energy.…”

The incorporation site of Er in nanosized CaF2

“…Er 3+ substitutes for Ca 2+ in the structure and, to balance the excess of positive charge, it is associated to a charge balancing defect. The structural properties of trivalent dopants in CaF 2 single crystals have been investigated in the past using electron paramagnetic resonance [12], ionic thermo current [13,14], audio-frequency dielectric relaxation [15,16]. In the case of low-doped (⩽0.1 mol%) samples it has been established that the principal complex consists in (adopting the Kroger-Vink notation) a Er Ca • + F I ′ where F is placed in a nearest (called nearest neighbor configuration, NN) free cube center respect to Ca.…”

Photo-reversible charge transfer processes and thermoluminescence in CaF2:Ce

“…Trivalent rare-earth ions RE^+ can substitute for Ca2+ in this lattice. The interstitial Fion charge compensator, which preserves charge neutrality, is located in nearest-neighbour (m) positions, thus leaving this dipolar (RE3+-F-) complex with tetragonal (C,,) symmetry (Kitts and Crawford 1974). RE3+ ions which are non-locally charge compensated also exist.…”