Additive colouring of CaF2:Yb crystals: determination of Yb2+ concentration in CaF2:Yb crystals and ceramics

When growing CaF₂ crystal doped with rare-earth ions, most of these ions are present in a trivalent state. However, due to contact with graphite crucible, a small proportion of a number of ions (Eu, Sm, Yb and Tm) are reduced to a bivalent state. A similar situation takes place during fabrication of CaF₂ ceramics doped with rare-earth metals. This fact is of particular importance for laser CaF₂:Yb crystals (ceramics), a promising material for short-pulse, high-power, high-energy diode-pumped solid state lasers since the presence of bivalent Yb ions can be a source of thermal losses. To date, there has been no technique to determine Yb²⁺ concentration in as-grown crystals. The proposed technique is based on a total reduction of Yb³⁺ ions via the heating of as-grown CaF₂ crystals with known concentration of Yb in the reducing atmosphere of metal vapour and determining the cross section of absorption bands of Yb²⁺ ions. The knowledge of these parameters allows estimation of the Yb²⁺ content in CaF₂:Yb crystals or ceramics by analysing their absorption spectra. Examples of using this technique are given. The technology of CdF₂ crystals reduction (an “additive colouring”) and features of colouring of crystals doped with rare-earth ions are considered.     

Thermal decay of photochromic color centers in CaF2, SrF2, and BaF2 crystals doped by La and Y impurities

The absorption spectra of photochromic centers in CaF₂, SrF₂, and BaF₂ crystals doped by La and Y impurities and thermal decay of the centers in the temperature range 80–600 K are investigated. Under low-temperature x-ray irradiation, ionized photochromic color (PC⁺) centers are generated in La- and Y-doped CaF₂ crystals and in a La-doped SrF₂ crystal. It is revealed that, upon heating of the CaF₂-LaF₃ crystal, PC⁺ centers are transformed into photochromic color (PC) centers. In the SrF₂-YF₃ crystal irradiated at room temperature, photochromic color centers are generated as well. All color centers decay at a temperature of approximately 600 K. After irradiation of the BaF₂-YF₃ crystal at a temperature of 80 K, absorption bands are observed at energies of 2.25 and 3.60 eV, which are related to neither PC centers nor PC⁺ centers.     

Thermoluminescence of proton- and electron-irradiated CaF2 crystals

Russian Physics Journal -     

Additive coloring of CaF2:Sm crystals

The study of the mechanism of additive coloring of calcium fluoride crystals, both pure and samarium-doped, demonstrates dynamical nature of processes that, in the course of coloring, tend to maintain equilibrium between Sm³⁺ and Sm²⁺ ions.     

Two types of additional absorption spectra in CaF2 crystals

A study of two types of additional absorption spectra observed in colored CaF₂ crystals is reported. The spectral shape for the additively colored crystal is governed by the experimental conditions; during photochemical coloring, it depends on the thermal prehistory of the sample. In all cases, a spectrum consisting of bands at 370 and 560 nm appears in crystals having a band near 200 nm. An additional band appears at 560 nm in the Smakula spectrum at high-irradiation doses; there are three stages in the increase of this band with increasing dose. A similar dependence on the radiation dose is observed for the 560-nm band in synthesized CaF₂ crystals. It is suggested that this band is due to defects produced in the lattice during the irradiation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, Vol. 12, No. 4, pp. 56–64, April, 1969.     

Optical spectra of CaF2(Tb) crystals

The luminescence and absorption spectra of fluorite crystals, alloyed with terbium fluorides, oxyfluorides, and oxides at concentrations of 0.01 to 5 mol.% were studied. The change in the optical spectra was studied as a function of the chemical composition of the alloy addition and also as a function of the concentration. It is proposed that the change with the concentration in the spectra of the crystals takes place as a consequence of mutual interactions between the ions with formation of cation pairs. It was observed that the ratio of the relative luminescence intensities of the crystals to the absorption coefficient changes nonlinearly with the concentration of the alloying addition. The problem of an interaction between an isomorphic admixture and crystal defects is discussed.In conclusion, the authors express their gratitude to E. V. Sobolev and M. V. Konovalova for help in the work.     

Absorption of CaF2 crystals containing lead and oxygen

The absorption in the vacuum and near ultraviolet region of CaF₂ crystals containing lead and oxygen has been measured. Two main absorption bands at 155 and 202 nm are presumably due to lead centres and the band at 206 nm to oxygen. The nature of the weak band at 166 nm remains obscure. The absorption below 145 nm is probably due to excitons.The authors would like to express their gratitude to Ing. E.Mariani, CSc., for his interest and helpful discussions during the course of this work.     

Giant pulses from CaF2: Dy2+crystals

These crystals produce 30 nsec pulses of peak power 1.4 MW in the single-pulse mode, or 40 nsec and 1 MW in the repetition mode (500 Hz repetition frequency). The beam produces a bright plasma when focused on the surface of a metal.     

ESR of X-ray induced centre in CaF2:Gd crystals

An ESR centre with orthorhombic symmetry was induced in CaF₂:Gd crystals by X-ray irradiation after heat treatment at 900°C. The observed ESR spectrum has resolved shfs in particular crystal directions. Spin Hamiltonian analysis indicates that the ESR centre is a hole trapped at an oxygen substituted for a fluorine site and interacts mainly with two nearby fluorine ions located along the crystal [001] direction. The determined spin Hamiltonian parameters are g_xx = 2.0036, g_yy = 2.0159, g_zz = 2.0306, A_xx = 22.5, A_yy = 5.6 and A_zz = 5.6 (G), respectively. A probable model is discussed by considering both the angular dependence of the ESR lines and the local charge neutrality around the ESR centre.     

Ionic conductivity study of CaF2: Nd single crystals

The electrical ionic conductivity of unirradiated and irradiated CaF₂: Nd crystals in the range of 60 to 800°C has been measured. The conductivity plot is basically divided into four parts, i.e., intrinsic and extrinsic unassociated, extrinsic associated, and extrinsic segregated regions. Activation energy (for unirradiated samples) in the extrinsic unassociated region is in the range of 0.69 to 1.20 eV depending on the doping concentration while for the intrinsic region, it is of the order of 1.89 eV. The conductivity in the extrinsic unassociated region increases with increase of Nd content in the sample. Also, the conductivity in the extrinsic region forγ-irradiated sample is higher than that for unirradiated one. In the intrinsic region, however, the conductivity is independent of dopant concentration orγ-irradiation. From these results it is surmised thatF ⁻ interstitials are the charge carriers in this region for CaF₂: Nd³⁺ system.     

Raman scattering in Li2B4O7 crystals with impurities

Raman spectra of an Li₂B₄O₇ crystal containing potassium and silver impurity ions with a concentration in a range of 0.1–0.2% were studied in detail. In the scattering geometries corresponding to manifestation of totally symmetric polar vibrations, the redistribution of spectral intensity for some lines in a region of 450–570 cm^?1 was observed. Using the model of weakly interacting oscillators, parameters of vibrations in this region were determined. On the basis of present-day concepts of the theory of light scattering by impurities, the position of impurity ions in a crystal lattice is discussed.