A detailed spectroscopic study of ${\text{Yb}}^{3+}:{\text{CaF}}_{2}$ is made by using laser selective excitation, low-temperature time-resolved fluorescence, and Raman techniques. Three dominant types of ${\text{Yb}}^{3+}$ isolated ion centers are identified at very low $(0.03\text{ }\text{at}\text{.}\text{ }%)$ dopant concentration: two of them with cubic and tetragonal symmetries and the third one not clearly identified but likely of trigonal symmetry. Fluorescence decays strongly differ from one site to another. The overall measured positions of the Stark sublevels of the ${^{2}F}_{7/2}$ and ${^{2}F}_{5/2}$ multiplets are satisfactorily explained by the crystal-field calculations. On the other hand, at higher dopant concentrations, only one dominant class of ${\text{Yb}}^{3+}$ centers occurs which corresponds to some hexameric cluster in which the ${\text{Yb}}^{3+}$ ions site in nearly tetragonal crystal-field environments and whose energy-level positions are discussed and compared with those previously reported in the literature.

Spectroscopy of Yb 3+ :CaF 2 : From isolated centers to clusters

Tm3+/Yb3+ doped CaF2, SrF2, and cubic phase NaYF4 nanoparticles dispersed as colloids in water (H2O and D2O) or saline solutions have been directly prepared by a one-step hydrothermal technique, using citrate anions as capping agents, without the need for any postsynthesis reaction. The size monodispersed nanoparticles are directly dispersed in water. Comparison of the upconversion emissions at 800 nm (Tm3+ ions) among the CaF2, SrF2, and NaYF4 hosts indicates that the SrF2 host leads to the highest emission intensity, 2 orders of magnitude higher than the NaYF4 one. Alkali ions (Na+ or K+) counter cations of the citrate salts used as precursors can enter the fluoride host crystals as charge compensators and strongly influence the spectroscopic properties of the lanthanide ions. The Tm3+/Yb3+ doped SrF2 nanoparticles dispersed in a 0.4 g/L concentration solution show detectable upconversion with laser excitation intensities as low as 1 W/cm2.

Water (H2O and D2O) dispersible NIR-to-NIR upconverting Yb3+/Tm3+ doped MF2 (M = Ca, Sr) colloids: Influence of the host crystal

We present the spectroscopic properties and room-temperature cw tunable laser operation of Yb3+-doped CaF2, SrF2 and BaF2 single crystals grown and studied in the same conditions. Emission cross sections, lifetimes, laser thresholds, laser slope efficiencies and laser wavelength tuning ranges are compared. It appears that Yb3+-doped BaF2 might be more promising for diode-pumped high power laser operation.

Comparative spectroscopic and laser properties of Yb3+-doped CaF2, SrF2 and BaF2 single crystals

Upconversion luminescence dynamics of Er-doped fluoride crystals for optical converters

An output pulse energy of 17.3 mJ has been achieved with a diode-pumped Yb:CaF2 regenerative laser amplifier. The bandwidth of the output pulse spectrum was 7.3 nm, being seeded with femtosecond pulses stretched to 2.2 ns. In cw operation a tuning range of 80 nm has been observed. A maximum pulse energy of 44 mJ at a repetition rate of 1 Hz has been obtained in Q-switched mode. The laser damage threshold of a Yb:CaF2 crystal has been determined at a wavelength of 1064 nm and a pulse duration of 10 ns.

Broad-band regenerative laser amplification in ytterbium-doped calcium fluoride (Yb:CaF2)

In solid solutions of alkaline-and rare-earth fluorides with a fluorite structure, ions of most elements of the rare-earth (RE) row form hexameric clusters that assimilate the minor component of the solid solutions (fluorine) and build it into the cubic fluorite lattice without changing its shape. An analysis of the EPR spectra of paramagnetic RE ions (Er3+, Tm3+, Yb3+) in clusters of diamagnetic ions (Lu3+, Y3+) confirms their hexagonal structure, which was established when studying the superstructures of the compounds under study. In such a cluster, a RE ion is in a nearly tetragonal crystal field, with the parameters of this field differing radically from those of single cubic and tetragonal RE centers in crystals with a fluorite structure. In particular, this field causes high (close to limiting) values of the g∥ factors of the ground states of the paramagnetic RE ions. Computer simulation is used to determine the atomic structure of a hexameric cluster in MF2 crystals (M = Ca, Sr, Ba). The crystal field and energy spectrum of Er3+, Tm3+, and Yb3+ ions in such clusters are calculated, and the spectroscopic parameters of the ground states of these ions are determined. The calculations confirm the earlier assumption that the unusual EPR spectra of nonstoichiometric fluorite phases are related to RE ions in hexameric clusters.

Crystal fields of hexameric rare-earth clusters in fluorites

The structure of the mixed fluorites Ca1−xSrxF2 and Sr1−xBaxF2, as well as the structure of the Eu2+ impurity center in these crystals, is calculated within the framework of the virtual-crystal method realized in the shell model and pair-potential approximation. The phenomenological dependence of the position of the lower level of the 4f65d configuration of the Eu2+ ion on distance to the Eu2+-ligand is derived. The dependences of the Stokes shift and the Huang-Rhys factor on x are calculated for the yellow luminescence in Sr1−xBaxF2:Eu2+. The value of x at which the lower level of the 4f65d configuration of the Eu2+ ion in Sr1−xBaxF2:Eu2+ falls within the conduction band is found.

The structure of mixed fluorides Ca1−x SrxF2 and Sr1−x BaxF2 and the luminescence of Eu2+ in these crystals

The structural, vibrational, and elastic properties of La2CuO4 are calculated using a model for calculating the energy of the crystal based on interionic potentials with the multiparticle Jahn-Teller contribution included explicitly. The microscopic reasons for the structural instability of the La2CuO4 lattice relative to rotations of the oxygen octahedra are investigated. A structural phase transition from the orthorhombic phase (space group D

 2

 18
 ) into the tetragonal phase (space group D

 4

 17
 ) under hydrostatic compression of an La2−x
SrxCuO4 crystal is modeled. The (P,x) structural phase diagram for La2−x
SrxCuO4 is constructed.

Modeling of a structural phase transition in La2−x SrxCuO4

The structure and dynamics of the crystal lattice of MeF2 fluorites (Me = Ca, Sr, Ba, and Pb) under external hydrostatic compression (0–3.5 GPa) are calculated within the shell model in the pair potential approximation. The first-order structural phase transition from the cubic to the orthorhombic phase in these crystals under pressure is investigated. The effect of chemical pressure on the BaF2 crystal is analyzed by the simulation of mixed crystals, namely, Ba1−xCaxF2 and Ba1−xSrxF2. It is demonstrated that the supercell method, as applied to the simulation of mixed crystals, results in a lower lattice energy per formula unit as compared to the lattice energy obtained by the virtual-crystal method.

Structure and dynamics of the pure and mixed fluorites MeF2 (Me = Ca, Sr, Ba, and Pb)

The local crystal structure of Gd3+ and Eu2+ cubic impurity centers in cadmium fluoride is calculated within the shell model in the pair potential approximation. The local compressibility of the cationic and anionic sublattices of the host lattice is determined in the vicinity of the Gd3+ (Eu2+) impurity ion.

A. Yu. Zakharov

Papers

Crystal fields of hexameric rare-earth clusters in fluorites

The structure of mixed fluorides Ca1−x SrxF2 and Sr1−x BaxF2 and the luminescence of Eu2+ in these crystals

Modeling of a structural phase transition in La2−x SrxCuO4

Structure and dynamics of the pure and mixed fluorites MeF2 (Me = Ca, Sr, Ba, and Pb)

Local structure of Gd3+ and Eu2+ impurity centers in a CdF2 crystal