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.     

Effect of hydrostatic and chemical pressure on BaF2 and BaF2: Eu2+ crystals

The effect of hydrostatic pressure on a BaF₂ crystal was studied within the shell model in the pair-wise potential approximation. The structural phase transition from the cubic to orthorhombic phase was simulated. The behavior of the unit-cell parameters of the α-and β-BaF₂ phases under hydrostatic pressure (from 0 to 12 GPa) was investigated. The fundamental vibration frequencies of BaF₂ under hydrostatic pressure (0–3.5 GPa) were calculated for both phases. The effect of chemical pressure on the BaF₂ crystal was studied by simulating Ba_1?x Me_xF₂ mixed crystals (Me=Ca, Sr). It was shown that at impurity concentrations up to 15–20 at. % the lattice constant varies in the same way as it does when hydrostatic pressure increases to P _c , which corresponds to a phase transition to the orthorhombic phase. The effect of chemical and hydrostatic pressure on BaF₂: Eu²⁺ doped crystals was also studied. The dependence of the absorption and luminescence zero-phonon line shift on the Eu²⁺-ligand distance was calculated.     

Quantum-size shift in PbTe/BaF2MQW structure photoluminescence spectra

PbTe/BaF₂multi-quantum-well (MQW) structures were grown on cleaved (111) BaF₂substrates by the MBE method. The substrate temperature was 330 ^°C. The MQWs consist of 10 pairs of 10 nm thickness BaF₂barriers and PbTe wells with widths ranging from 5 to 20 nm. For this highly mismatched system we have measured photoluminescence spectra at 77 K and have demonstrated the quantum-size confinement consistent with the model of wells with infinite barriers. A remarkable deformation shift due to the thermal expansion coefficient difference is observed at cryogenic temperatures.     

Laser-excited nonlinear properties of BaF2 and its application in a single-shot spatially insensitive autocorrelator

A detailed investigation of laser-generated nonlinear processes of undoped barium fluoride is presented including two-photon-excited exciton fluorescence, color-center formation and damage-threshold measurements. A single-shot autocorrelator based on two-photon fluorescence of BaF₂ was constructed to measure the phase and amplitude of pulses provided by large apeture excimer lasers. The capabilities of the autocorrelator are demonstrated at 248 nm and 268 nm with pulses in the regime of 100 fs to 10 ps.     

Thermoluminescence and opitical absorption of BaF2 crystals

Abstract

Nominally pure and Dy-doped BaF₂ crystals were investigated concerning their optical absorption (OA) and thermoluminescence (TL) properties. Peaks at 120—150 and 200°C were observed for a heating rate of 1.7°C/s. The TL response for γ-rays and the TL emission spectra were obtained for these peaks. Except for the purest crystal, all BaF₂ crystals produced OA bands before irradiation typical of Ce³⁺ ions. After irradiation, Dy doped crystals showed bands due to Dy²⁺ ions. A nominally pure sample gave bands related to Ce²⁺ ions and photochromic centers of Ce³⁺ ions. and photochromic centres of Ce³⁺ ions. The correlation between some OA bands and TL peaks is discussed.     

Charge states of the activator and size effects in BaF2: Eu nanophosphors

ABSTRACT

According to the spectra of stationary X-ray excited luminescence (XEL) of BaF₂: Eu nanophosphors at 80 and 294 K, it was revealed that the thermal annealing of fine-grained nanoparticles (d?=?35?nm) in the range of 400–1000°C, which is accompanied by an increase of their sizes in the range of 58–120?nm, does not result in effective changes of the charge state of Eu^{3 +} → Eu^{2 +} activator, in contrast to CaF₂: Eu nanoparticles. The maximum light output of X-ray excited luminescence of BaF₂: Eu nanophosphors in the 590?nm emission band of Eu³⁺ ion was observed at an annealing temperature of 600°C with the average size of nanoparticles 67?nm. The subsequent growth of annealing temperatures, especially in the range of 800–1000°C, causes decrease in the light output of X-ray excited luminescence due to the increase of defect concentration in the lattice as a result of sharp increase of nanoparticle sizes and their agglomeration. In BaF₂: Eu nanoparticles of 58?nm size, according to the thermostimulated luminescence (TSL) spectrum, transformation of Eu³⁺ → Eu²⁺ under the influence of long-time X-ray irradiation was revealed for the peak of 151?K. Thus, X-ray excited luminescence spectra of BaF₂: Eu nanophosphors are formed predominantly due to the emission of Eu³⁺ ions, while emission of Eu²⁺ ions is observed in the TSL spectra.     

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.     

UV induced processes in pure and doped SrF2

The effects of UV irradiation on pure and Tb doped SrF₂ were investigated by TL methods, and compared to those of X irradiation. Excitation spectra were measured in the range 110–300 nm. In both the pure and doped samples excitation maxima appeared on the long wavelength tail of the first exciton band (at 125 nm), indicating an excitonic process for defect creation. The doped specimens showed additional maxima at lower energies, which were attributed to transfer of electrons to local impurity levels. Thermal activation energies were evaluated by various methods. TL emission spectra were also measured. The same spectral bands appeared in the glow peaks of SrF₂ : Tb after UV as after X irradiation. Results indicate that UV and X induced TL are due to similar processes.     

Study of VUV emission and γ-ray responses of Nd:BaF2 scintillaotor

Nd³⁺ 1%, 5% and 10% doped BaF₂ single crystals were grown by the micro-pulling down method. Photoluminescence properties, including excitation and emission spectra and luminescence decay were measured under synchrotron radiation excitation at the Superlumi station in HASYLAB at DESY (Hamburg, Germany). The Nd³⁺ related 5d-4f emission lines peaking around 180 nm, 230 nm, and 260 nm, identified as the 5d–⁴I_j, 5d–⁴F_j, and 5d–²G_j transitions, were observed under 140–168 nm excitation. In photoluminescence decay under the 160 nm excitation, the dominant component decay time is about 12, 2.5 and 1.2 ns for Nd³⁺ 1%, 5% and 10% concentration, respectively. The decay time shortening is explained by the concentration quenching effect. Transmittance of Nd1% sample is about 80% for wavelengths above 185 nm. Finally, gamma-ray responses, non-proportionality and energy resolution of Nd1% sample were compared with the undoped BaF₂ scintillator. The light yield of the Nd1%:BaF₂ is about 93% of that of undoped BaF₂. ^©2009 Elsevier Ltd. All rights reserved.     

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.     

Colloidal synthesis of BaF2 nanoparticles and their application as fillers in polymer nanocomposites

Nanoparticles of pure and Eu-doped BaF₂ have been prepared through sol-gel colloidal synthesis. In addition, BaF₂-filled PMMA polymer nanocomposites were fabricated and dielectric properties were measured. The as-synthesized pure and Eu-doped BaF₂ nanoparticles were analyzed by both X-ray diffraction and transmission electron microscopy and consisted of crystalline BaF₂ particles with an average diameter of 13.6 nm with a standard deviation of about ±2.4 nm. The photoluminescence properties of the pure and Eu-doped (2%, 4% and 8%) nanoparticles showed characteristic emission of Eu³⁺ (⁵D₀→⁷F _J (J=1–4) transitions). We also measured significantly enhanced dielectric breakdown strength of up to 30% for BaF₂ nanocomposites over the unfilled PMMA polymer. This study thus offers some promise of sol-gel synthesis of nanocomposite dielectrics with great potential for use as electrical insulation materials in cryogenic high-voltage applications.     

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)     

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.     

Theoretical Investigation of the Electronic and Spatial Structures of the La Impurity in a BaF2 Crystal

The results of theoretical investigations of the electronic structure of BaF₂ and LaF₃ crystals and BaF₂–LaF₃ complexes without and with La impurity are presented. Changes in the structure of the state density, electronic spectrum, and effective charges caused by defects are analyzed. It is demonstrated that the NNN position of the [La³⁺F_i ^-] defect in BaF₂ is more stable than the NN position.     

Tetragonally distorted Cr2+ ions in BaF2 and SrCl2 studied by ESEEM spectroscopy

Electron spin echoes on tetragonally distorted non-Kramers Cr²⁺ ions in chromium doped BaF₂ and SrCl₂ single crystals have been observed. The superhyperfine interaction (shf) with the nearest halide ions has been studied using the electron spin echo envelope modulation (ESEEM) signal. For Cr²⁺ in BaF₂ the nearest coordination shell consists of four equivalent fluorine ions in a plane perpendicular to the defect tetragonal axis, the Cr²⁺ ion being off-centre displaced towards the fluorine plane. The Cr²⁺ shf tensor for BaF₂ has been determined. For SrCl₂:Cr ESEEM signals associated to the shf interaction with nearest chloride ions were observed but the measurements did not allow us to give a definitive defect model in this case.     

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.     

Lattice dynamics from vibronic spectra: Examination of a generalized shell model for SrF2 and BaF2

On the basis of quantum mechanical ab initio theories practical shell-models are established. These models include a polarization as well as a radial and axial deformation of the lattice ions, and are built up in such a way that measured macroscopic quantities can be incorporated. The predicted dynamics is tested by means of vibronic spectra of Sm²⁺ in SrF₂ and BaF₂. It turns out that both the structural form and the positions and intensities of the predominant vibronic maxima are very sensible to the high frequency dielectric constant ?_∞. An excellent agreement with the experimental result is found if ?_∞ fictitiously is chosen lower than the measured value, whereas the agreement would be considerably worse, if the experimental value itself of ?_∞ is choosen. Possible sources of these discrepancies are discussed.     

Simultaneous energy and timing decisions in time differential perturbed angular correlation experiments using BaF2 scintillators

BaF₂ scintillation counters, coupled to constant fraction differential discriminatiors, permit simultaneous energy and timing decisions in recording delayed coincidence spectra. We explore the advantages and limitations of this innovation in time differential perturbed angular correlation experiments, and illustrate it for¹¹¹CdNi and¹⁰⁰RhNi.     

Nanostructure,conductivity, and reflectivity of thin iron and (Fe) X (BaF2) Y films

Fe and (Fe) _X (BaF₂) _Y films 10- to 160-nm-thick are grown on a polymer substrate by vacuum evaporation. The surface relief and nanostructure of the films are examined. The dependence of the conductivity on the film thickness is obtained, and a correlation between the conductivity and surface relief is found. The influence of the BaF₂ dielectric phase on the conductivity and reflectivity of the Fe films is studied. It is shown that the BaF₂ layer to some extent slows down oxidation and maintains the reflectivity of the Fe films compared with pure iron films exposed to air.     

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.