SrF2 Crystals Research Articles

Charge Transfer Luminescence in Ce-Doped Fluorides

New emission bands were found in Ce3☎ doped SrF2 and BaF2 crystals under excitation into the charge transfer region. The bands at 4.85 eV in BaF2 and 4.35 eV SrF2 were found in all crystals with Ce-concentration from 0.001 to 1 mol.%, most prominent in 0.01% samples. Decay of luminescence was simple exponential in BaF2, decay time decreased from 1.3 ms at 65 K to 0.41 μs at 523 K. The decay curve in SrF2 shows t−0.3 dependence at room temperature. All experimental results were naturally explained by assumption that new emission belongs to radiative charge transfer recombination in Ce2☎—Fio pairs with different distances between them.

Local structure of titanium pair centers in SrF2 crystals: EPR and ENDOR spectroscopic studies

The local structure of titanium pair centers in SrF2: Ti crystals is investigated using electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy. It is found that titanium pair centers with spin moment S=2 and tetragonal symmetry of the magnetic properties are formed in SrF2: Ti cubic crystals under certain growth conditions and during annealing. The tensor components of the fine and ligand hyperfine structures in the EPR and ENDOR spectra are determined. A model of the Ti+-Ti3+ paramagnetic dimer is proposed. This model provides an adequate interpretation of both the ferromagnetic nature of the exchange interaction and the observed displacements of four ligands in the first coordination sphere of titanium impurity ions in directions perpendicular to the impurity ion-ligand bonds.

Aggregation of defects in Ho3+-doped fluorite crystals

Low Ho3+ concentrations (∼0.01%) yield only the 1∣11 and 1∣12 (monomer) configurations of Ho3+–F– centres in both CaF2 and SrF2 crystals. The 1∣11 and 1∣12 configurations correspond to the charge compensating F– ion occupying the nearest neighbour and next nearest neighbour position respectively, relative to the Ho3+ ion. For CaF2:Ho3+ crystals, the two monomer centres occur with comparable densities while the 1∣11 centre is dominant in SrF2:Ho3+ crystals. At moderate Ho3+ concentrations (∼0.1%), Ho3+ dimer centres are dominant in both CaF2 and SrF2 and the population of monomer centres is significantly reduced. In CaF2:Ho3+, the 2∣21 and 2∣22 configuration dimer centres occur in comparable densities; these rhombic centres are formed by preferential aggregation of the 1∣11 and 1∣12 configuration centres, respectively. A third dimer centre, of the 2∣31 configuration, also appears in CaF2. Only the 2∣21 and 2∣31 configuration centres are present in similarly doped SrF2 crystals. The type and population of dimer centres that form in Ho3+–doped fluorite crystals depend on the monomer centre(s) favoured by the particular lattice. The crystalfield energy level structures of the 2∣21 and 2∣31 configuration centres are consistent with weaker crystal field effects, particularly in CaF2, compared to the parent 1∣11 centre; those of the 2∣22 and 1∣12 centres are comparable.

Local structure of bivalent copper centers in SrF2 crystals: An ENDOR spectroscopic study

Structural distortions of the SrF2 crystal lattice near the bivalent copper impurity Jahn-Teller center are investigated by the ENDOR method (v=9.3 GHz, T=4.2 K). The approximate directions and the magnitudes of displacements of a Cu2+ impurity ion and its surrounding F− ions are determined with respect to one of the anionic networks in the crystal matrix. The tensor components for the ligand hyperfine interaction (LHFI) with fluorine ions separated from the impurity by a distance R≤6.2 A are obtained from the angular dependences of the location of the ENDOR resonance lines. It is found that the parameters of magnetic interactions between the impurity and these ligands contain the contributions determined by the covalence of bonds in the impurity complex and the polarization of electron shells of the ligands.

Nonlinear Raman spectroscopy of the low-lying levels of theSm2+ ion doped in SrF2 and CaF2 crystals

We used coherent anti-Stokes Raman scattering (CARS) spectroscopy tostudy the symmetry of the low-lying levels of the Sm2+ ion doped in sites ofcubic (Oh) symmetry in the SrF2 and CaF2 matrices. Taking advantage of thepolarization sensitivity of the Raman spectroscopic techniques, we confirmthat the line at ≈652 cm-1 (that we could only observe in the SrF2 casebecause of wavelength limitations) corresponds to excitation from the ground(4f6 7F0) A1g level to the Stark T2g level originating from the splitting ofthe (4f6 7F2) multiplet. In the same SrF2 sample, the(4f6) 7F0→ 7F1 transition was observed to be weaker. In CaF2 matrices in which it is moreeasily detected, we checked its T1g symmetry.

Optical analysis of X-ray effects on Ag+-doped strontium–fluoride crystals

The optical and paramagnetic properties of X-irradiated silver doped SrF2 crystals were investigated. The freshly irradiated crystals show a complex absorption spectrum between approximately 200 and 650nm. Subsequently, systematic heat treatments were applied and absorption, photoluminescence and its polarization dependence, thermo- and radio-luminescence experiments have been undertaken. The resulting experimental data were mutually correlated with the aid of the factor analysis technique and six different origins of the observed spectra were identified. Models of the underlying silver-defect structures are discussed and crystal preparation is further presented.

Off-centre dynamic Jahn-Teller effect studied by electron spin relaxation of Cu2+ ions in SrF2 crystal

Temperature cw-EPR and pulsed EPR electron spin echo experiments were performed for a low concentration of Cu2+ ions in cubic SrF2 crystals. The well resolved EPR spectrum at low temperatures (below 30 K) with parameters g = 2.493, g = 2.083, A = 121, A = 8.7, A(19F) = 135, A(19F) = 33.0 (A-values in 10-4 cm-1) is transformed continuously into a single broad line above 225 K on heating, due to the g-factor shift and EPR line broadening. These data along with the angular variation EPR data are described in terms of a pseudo-Jahn-Teller effect of (T2g+A2u)(a1g+eg+t1u) type producing six off-centre positions of the Cu2+ ion in the fluorine cube. Above 30 K a two-step averaging g-factor process occurs and is governed by vibronic dynamics between potential wells of the off-centre positions. This dynamics governs the electron spin relaxation in the whole temperature range. The electron spin-lattice relaxation rate 1/T1 grows rapidly by six orders of magnitude in the temperature range 30-100 K and is determined by the Orbach-type process with excitations to two excited vibronic levels of energy 83 and 174 cm-1. For higher temperatures the relaxation is dominated by overbarrier jumps leading to the isotropic EPR spectrum above 225 K. The phase memory time TM has the rigid lattice value 3.5 µs determined by nuclear spectral diffusion and its temperature variation is governed by the vibronic dynamics indicating that the excitations between vibronic levels produce a dephasing of the electron spin precessional motion.

Excited-state absorption spectroscopy of Nd3+:SrF2 crystals in the 1280 — 1320 nm spectral wavelength range

The excited-state absorption (ESA) in Nd3+:SrF2 crystals was investigated in the 1280 — 1320 nm spectral wavelength range by selective laser excitation. It was found that, on excitation at wavelengths of 743 and 800.5 nm, there is no ESA in the ranges 1304 — 1309 and 1295 — 1297 nm. Weak ESA bands are observed in the 1297 — 1304 and 1309 — 1320 nm ranges and the differential ESA cross section does not exceed 0.3 × 10-21 cm2 . The results indicate that the ESA does not preclude the use of an Nd3+:SrF2 crystal as the amplifying medium in the wavelength range 1280 — 1320 nm.

Time-resolved spectroscopy of self-trapped excitons in fluorides of alkaline-earth metals under pulsed electron irradiation

Nanosecond-resolution absorption spectroscopy at room temperature was used to study the laws governing the creation and evolution of the primary defect structure in CaF2, SrF2, and BaF2 crystals exposed to an accelerated electron pulse. It is shown that the spectral-kinetic characteristics of self-trapped excitons created in undamaged parts of the crystal lattice are qualitatively similar. Partial polarization of the absorption of self-trapped excitons is observed in CaF2. The structure of the transient absorption spectra becomes more complex in the sequence CaF2, SrF2, BaF2 because of the formation of excitons trapped in phase inclusions of homologous cationic impurities. The spectral characteristics of excitons trapped in undamaged parts of the CaF2 and SrF2 lattice and in their phase inclusions in BaF2 are the same although the latter have a considerably shorter relaxation time. Short-lived (τ⩽100 ns) absorption of unknown defects was observed in the spectral range ⩾5 eV.

EMPIRICAL PARAMETERIZATION OF INTER-IONIC POTENTIALS FOR ALKALINE EARTH FLUORIDES

When the functional forms of non-coulombic inter-ionic short-range potentials have been defined, the bulk properties of ionic crystals can be calculated. The parameters presented in potential functions are determined by fitting procedure, to reproduce as colsely as possible the measured crystal data. In the present study, the shell model is employed for the investigations of CaF2, SrF2 and BaF2 ionic crystals. Remarkable feature of the present fitting procedure is that shell charges are sure to be negative and recent experimental data are adopted. The anion (F-) charge, which is assumed to be common to the three fluorides, is adjusted to 0.97e. In so doing, the obtained model parameter set is able to produce a satisfactory agreement between the calculated results and the experimental data, such as lattice formation energies, elastic constants, dielectric constants, ionic polarizabilities, transverse optic and Raman frequencies, for the alkaline earth fluorides.

EPR of trivalent chromium centers with monoclinic symmetry in SrF2 crystals

Cr3+ centers with monoclinic symmetry in SrF2:Cr crystals are studied by EPR. Based on an analysis of the EPR data and a study of the conditions under which the observed samples were formed, a model is proposed which explains the low symmetry of the magnetic properties in terms of the simultaneous effect on the impurity ion state of interactions with the interstitial F− ion and with normal vibrations of the crystal lattice in a trigonal symmetry.

EPR characterization of O? and trapped electrons in 20% SrF2/SmOF

An X-ray diffraction study of the 20% SrF2/SmOF catalyst has revealed the presence of cubic SrF2 and rhombohedral SmOF crystal phases. The lattice of the SrF2 crystal phase was distorted. Cationic and anionic exchanges may have occurred to cause such deformity. When the 20% SrF2/SmOF sample was treated in helium at 1073 K, EPR signals corresponding to O– centres were observed. Hydrogen or ethane treatment of the sample at 973 K or above resulted in the detection of superhyperfine structures corresponding to the presence of trapped electrons. No similar EPR signals were observed on the SmOF sample. We suggest that these EPR-detectable species were located in the SrF2 lattice and existed largely in the bulk. The trapped electrons were stable in oxygen up to 673 K while O– centres were stable in ethane or hydrogen up to a temperature as high as 873 K. We speculate that the migration of O– centres and trapped electrons is more arduous in fluoride than in oxide. Hence the temperature for the combination of O– centres and trapped electrons as well as for the migration of O– centres and trapped electrons from the bulk towards the surface is higher in the former than in the latter.

Optical and thermally stimulated emission studies on γ-irradiated SrF2: Pb single crystals

Optical absorption (OA), fluorescence, and thermally stimulated emission (TL) studies are made on a Pb-doped (0.3 wt%) SrF2 crystal. Unirradiated crystals show a prominent OA band around 6.05 eV. This band increases after γ-irradiation and also a broad band around 4.77 eV appears which can be attributed to Pb3+ ions. Fluorescence emission is observed at 5.35 eV when the crystal is excited in the Pb2+ absorption region which is around 6.05 eV. Thermoluminescence glow shows a very intense broad peak at 653 K. TL emission shows only the characteristic Pb2+ emission at 5.35 eV. Also the OA of heavily irradiated crystals shows the presence of Pb centres. The probable TL mechanism is that the trapped electron from a Pb centre when thermally released can recombine with Pb3+ ions to give the characteristic Pb2+ emission. All these results are discussed.

Red-to-blue up-conversion spectroscopy of Tm3+ in SrF2, CaF2, BaF2 and CdF2

A spectroscopic study of the blue emission from different Tm3+ centres of SrF2, CaF2, BaF2 and CdF2 crystals for CW red excitation by a single dye laser is presented. Our observations suggest that the cubic Tm3+ centres are excited by an avalanche process rather than by a sequential two-photon process.

High-resolution spectroscopy and optically detected nuclear magnetic resonance studies of Ho3+ ions in SrF2 crystals containing Ba2+ and Ca2+ impurities

Optical-RF double-resonance techniques have been used to measure the superhyperfine transition frequencies of two Ho3+ centres of C4v symmetry present in SrF2:0.001%Ho3+ crystals containing about 0.2% of Ba2+ and Ga2+ as inadvertent impurities. One centre has a charge-compensating F- ion located in the nearest-neighbour interstitial position along the (100) axis while the other has an additional Ba2+ ion also located on this axis. The superhyperfine transition frequencies are about 60% smaller than those reported for CaF2:Ho3+ and these decreases are attributed principally to the decreased Ho3+ ground-state magnetic moment resulting from the increased energy separation between the ground and first excited state Ho3+ electronic energy levels. Another contribution arises from the larger host lattice dimension for SrF2. The two sets of nearest-neighbour F- ions have almost identical resonance frequencies for both SrF2 centres while the interstitial F- ion has a substantially reduced resonance frequency in the Ba2+ modified centre. These trends correlate with those in the relevant crystal-field parameters. Some qualitative conclusions are drawn about the positions of the Ho3+ and F- ions in these centres.

Luminescence Discontinuity and Local Crystal Field Continuity in Mixed Crystals of SrF2 and BaF2 Doped with Eu2+ Monitors

Discontinuity in photoluminescence has been investigated contrasting with the continuity in local crytal field in mixed crystals or solid solutions Sr1-xBaxF2 with the fine mixed ratios between SrF2 and BaF2. The signal from the doped Eu2+ ions has been monitored by means of optical measurements as well as electron spin resonance (ESR).

Polarization study and crystal-field analysis of the laser-selective excitation spectra of Ho3+ ions in CaF2 and SrF2 crystals.

The polarized fluorescence of a total of 12 Ho 3+ ion centers present in CaF 2 - and SrF 2 -type crystals and having a variety of Ho 3+ ion symmetries has been analyzed to give possible model assignments. In contrast to the earlier results for SrF 2 :Er 3+ , which has a dominant trigonal symmetry center, the principal center in SrF 2 :Ho 3+ is determined to have tetragonal symmetry

Fluorescence lifetimes of Pr3+ centers in mixed CaF2: SrF2 crystals

Using polarized laser selective excitation, sets of energy levels have been established for the principal new Pr3+ centers in four mixed alkaline earth fluoride systems: CaF2:Pr3+ containing up to 1% of either SrF2 or BaF2 and SrF2:Pr3+ containing up to 1% of either CaF2 or BaF2. The three principal centers for SrF2 in CaF2:Pr3+ are modified centers derived from the tetragonal C4v symmetry center of the parent crystal: two with an off-axis Sr2+ ion and one with an on-axis Sr2+ ion. Fluorescence lifetimes determined for mixed crystal centers are reported and related to their respective Pr3+ energy level patterns. Some results of high resolution scans of the respective absorption transitions of these centers are also presented.

Temperature variations of the elastic constants of CaF2 and SrF2 crystals.

We have investigated the temperature variations of the second-order elastic constants of ${\mathrm{CaF}}_{2}$ and ${\mathrm{SrF}}_{2}$ crystals by means of a three-body-interaction potential model which earlier has successfully described such variations exhibited by the other members of the fluorite family [Phys. Rev. B 39, 13493 (1989)]. Our results, obtained from the three-body-interaction potential model, have shown reasonably good agreement with their experimental neutron-scattering data. On the basis of these successful descriptions, we have concluded that three-body interactions and lattice anharmonicity play an important role in revealing the anomalous behavior and temperature variations of the elastic constants of fluorites.

Superconducting YBa2Cu3O7−x thin films on alkaline earth fluorides

Alkaline earth fluorides have low dielectric constants and large band gaps. In addition, they are chemically compatible with and several can be grown epitaxially on silicon and compound semiconductors. We have deposited YBa2Cu3O7−x thin films (200–500 nm thick) directly on (001) SrF2, (001) BaF2, (001) MgF2, and unoriented CaF2 single crystals by laser deposition. The substrate temperature was about 720 °C and the vacuum chamber was at 350 mTorr of oxygen. For this work, no additional annealing outside the deposition chamber was done after deposition. The deposited YBa2Cu3O7−x films were superconducting and exhibited transition temperatures Tc (R=0) of 83, 73, and 74 K on MgF2, SrF2, and BaF2, respectively. Structural studies by x-ray diffraction showed that YBa2Cu3O7−x films on MgF2, SrF2, and BaF2 were polycrystalline and exhibited some (00L) textures. Deposition on CaF2 substrates resulted in yellowish, insulating films with Ca- and Ba-rich interfacial reaction products.