Exciton states in CaF2, SrF2, and BaF2

We have applied the pseudopotential theory to the calculation of exciton states and binding energies for the ionic crystals CaF₂, SrF₂, and BaF₂, using a transfer model. Because of the satisfying agreement between our results and the experimental data in each case, we assume this pseudopotential formalism will describe transfer exciton states and energies in other ionic crystals.     

Linewidth and auger effect in CaF2, SrF2, and BaF2

In BaF₂ crystals, the linewidth of certain core band-conduction band excitons shows strong temperature dependence, while the corresponding linewidths in CaF₂ and SrF₂ do not. Calculations of the broadening due to the temperature-independent Auger effect are in good agreement with experimental data.     

Study of Nd2+ absorption in x-irradiated CaF2, SrF2, BaF2 crystals

The absorption spectra of x-irradiated alkaline-earth fluoride (CaF₂, SrF₂, BaF₂) crystals doped with Nd³⁺ ions have been investigated. X-irradiation results in creating the absorption bands of inter-configuration 4fⁿ–4fⁿ⁻¹–5 d¹ transitions of Nd²⁺. The charge reduction of the neodymium by irradiation is not temperature-stable and the ions reoxidation (Nd²⁺ → Nd³⁺) occurs under heating to 570 K in CaF₂, 520 K in SrF₂ and 470 K in BaF₂.     

Lattice defects in CaF2 and SrF2 after proton irradiation

The time differential perturbed angular distribution method (TDPAD) has been applied to investigate the electric field gradient produced by radiation induced defects in the cubic lattices of CaF₂ and SrF₂. In mixed crystals of the alkaline-earth difluorides CaF₂ or SrF₂ and the rare earth trifluorides PrF₃, NdF₃ or SmF₃ the amplitudesA ₂₂ ^exp in the TDPAD spectra strongly depend on the rare earth trifluoride concentration. From the fact that the observed amplitudes are well described by a simple statistical model the conclusion can be drawn that the defects produced in CaF₂ and SrF₂ after proton irradiation are F^? interstitials.     

Effect of electron irradiation on fluorides of alkali-earth elements (CaF2, SrF2 and BaF2)

The effect of electron irradiation on fluorides of alkaline-earth elements CaF₂, SrF₂, and BaF₂ in the column of an electron transmission microscope is studied by means of elemental and diffraction analysis. It is established that when the current density of the electron beam exceeds the threshold value, we observe the desorption of fluorine and the binding of liberated metal ions by the oxygen of the residual vacuum to form oxide phase MeO (Me = Ca, Sr, and Ba) with the structure of rock salt NaCl.     

Change of hyperfine and transferred hyperfine interaction of Eu2+ in CdF2, CaF2, SrF2, and BaF2

ENDOR measurements of the hyperfine and transferred hyperfine interaction of Eu²⁺ in CdF₂, SrF₂, and BaF₂ were performed. Compared with measurements on CaF₂, there is a radial shifting of the next fluorine ligands in CdF₂∶ +5.4%, CaF₂∶ + 3.5%, and BaF₂∶ ?3.9%. Since the SrF₂ and EuF₂ lattice constants are approximately equal, a shifting in SrF₂ was not assumed. In both europium isotopes (151 and 153) a change of hyperfine fields at the nucleus was observed. This can be explained qualitatively by the difference in radial distribution of the different europium orbitals. Also noticed was a small change of the hyperfine anomaly, which indicates contributions of the zero-point vibration.     

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.     

First-principles calculations of the electronic structure of fluorite-type crystals (CaF2, BaF2, SrF2, and PbF2) containing Frenkel defects. Analysis of optical and transport properties

The electronic structure of the alkali-earth fluorides CaF₂, BaF₂, SrF₂, and PbF₂ with Frenkel defects is investigated in the tight-binding approximation by the LMTO method. The defect formation and migration energies are calculated. The electronic structure and optical excitations of a H center in a defective fluorite structure are examined. It is shown on the basis of calculations of the binding energies that CaF₂, BaF₂, and SrF₂ are ionic compounds, while the chemical bond in PbF₂ is partially covalent. Possible methods of displacement of interstitial fluorine atoms that lead to the observed optical spectra of an H center are investigated. Fiz. Tverd. Tela (St. Petersburg) 40, 2019–2025 (November 1998)     

EPR and optical absorption of manganese doped BaF2 and SrF2

Abstract

Mn⁺ ions in off-centre position along [100] have been produced in BaF₂:Mn by X-irradiation at LNT. Their EPR spectra are characterized by g-2. 0107. D = 265.8·10^?4cm^?1. A = 122.9·10^?4cm^?11 and a superhyperfine structure arising from 8 surrounding F^? nuclei. In the optical absorption spectrum they show up by a band at 411 nm. This behaviour is compared with the results found for SrF₂:Mn. After X-irradiation at room temperature beside a different Mn⁺ centre two species of Mn⁰ appear in non-cubic position. Both interact dominantly with two F^? neighbours. Possible models for these species are discussed.     

Optical absorption spectrum of X-irradiated CaF2:Cr and SrF2:Cr

The optical absorption spectra of X-irradiated CaF₂:Cr and SrF₂:Cr are studied. X-irradiation induces the formation of a prominent band in the u.v. region (centered at 277 nm in CaF₂ and 285 nm in SrF₂) as well as a weak absorption in the visible region. Combined EPR and optical absorption measurements together with some optical bleaching experiments suggest that the u.v. absorption bands are related with either Cr⁺ or Cr³⁺ ions. Several arguments are presented in order to tentatively associate these bands with an interconfigurational transition of Cr⁺ in a cubal environment.     

Ionic,electronic and ion-diffusion controlled relaxation processes in CaF2, BaF2 and LiBaF3 crystals

Abstract

The ionic, electronic and anion-diffusion controlled thermally stimulated relaxation (TSR) processes at 80—700 K in CaF₂ BaF₂ and LiBaF₃ crystals (X-ray irradiated or non-irradiated) have been investigated by means of ionic conductivity, ionic thermally stimulated (TS) depolarization current (TSDC); as well as current (TSC), luminescence (TSL) and bleaching (TSB) techniques. Above 250—290 K broad and overlapping anion TSDC peaks and correlated TSB stages are detected. The TSB kinetics is initiated and controlled by anion detrapping and interaction with the localized charges, i.e., the anion-diffusion controlled TSR processes take place in fluorides. The TSL and TSC data for LiBaF₃ indicate that the lifetime and drift of electrons at 80—250 K is very small because of deep retrapping. The main TSL peaks at 132K, 170K and 220 K are caused by V_k center detrapping and hole-diffusion controlled tunnel recombination within pairs like {D_n e^?…V_k }.     

Pressure and temperature dependence of the emission in BaF2:Eu and SrF2:Eu

The 4f⁶5d¹(e)→4f⁷ emission (normal) of Eu²⁺ in SrF₂ and normal and anomalous emissions of Eu²⁺ in BaF₂ are studied as a function of pressure and temperature. Although in BaF₂ both emissions (anomalous and normal) shift with pressure in the same spectral direction (a red shift), the anomalous emission converts to the normal emission when the pressure increases to 33 kbar. Considering the dependence of BaF₂:Eu²⁺ luminescence on the pressure and temperature we have found that spectral transformation takes place due to a smooth pressure-induced level crossing at approximately the same pressure as the phase transition.     

An EPR study of Ni+ centers in SrF2:Ni and BaF2:Ni

An EPR study of RT X-irradiated SrF2:Ni and BaF2:Ni has been performed. Different kinds of Ni⁺ centers showing small deviations with respect to a basic structure have been observed. This basic structure consists of a Ni⁺ ion displaced along a <100> direction from the cation substitutional position toward the center of a face of the cube of fluorines. These results are similar to those previously reported on CaF2:Ni.

The spin Hamiltonian parameters of the different centers obtained from the analysis of the corresponding rotational diagrams are also reported.