BaFCl Crystals Research Articles

Photostimulated and persistent luminescence of samarium ions in BaFCl

Storage phosphors with efficient photostimulated luminescence (PSL) and/or persistent luminescence (PersL) are in great demand for a range of applications. In this paper, PSL, PersL and thermoluminescence (TL) of samarium ions doped in a macroscopic and sub-micron crystals of BaFCl are reported, and it is demonstrated that the co-precipitated BaFCl:Sm3+ sub-micron crystals become an efficient photostimulable X-ray storage phosphor upon annealing in the temperature range of 200–500 °C. The dependence of storage phosphor properties on annealing temperature was investigated and it was found that the X-irradiated co-precipitated BaFCl:Sm3+ crystallites (~400 nm size) showed maximum PSL and PersL from both Sm2+ and Sm3+ ions when annealed at 500 °C for 4 h. From a comparison with data for the macroscopic crystal (~3 mm) and ball-milled nanopowders (~30 nm), it also followed that PSL and PersL in the BaFCl:Sm system is affected by the crystal size. TL from Sm2+ of the macroscopic crystal is compared with the TL of sub-micron powders annealed at different temperatures.

Light-induced valence-state switching in BaFCl:La and SrFCl:La

A tetragonal La[2+] center (symmetry C[4v]) was identified in single crystals of BaFCl and SrFCl doped with lanthanum with the aid of electron paramagnetic resonance (EPR)/electron-nuclear double resonance (ENDOR). This center forms a donor-acceptor couple with initially present F(F-) centers. Switching takes place by illumination of appropriate wavelength. The kinetics of the process was monitored by EPR as La[2+] and the unswitched F center are paramagnetic. The results of our experimental investigation of this kinetics are presented. A foregoing spectroscopic characterization of the La[2+] center allowed one to identify a d-d (the B[1]-E) transition, a charge-transfer band (for BaFCl at 10 940 cm[-1] and at 17 890 cm[-1], respectively) and to obtain a value of 710 cm[-1] for the spin-orbit coupling constant in the ground state. In order to narrow the choice of possible acceptor-donor partners a detailed EPR/optical search was further done to identify a number of lattice defects and oxygen centers—in addition to a La-oxygen molecular structure.

Laser excitation-induced structure distortion and persistent spectral hole burning of Eu 3+ in BaFCl

Persistent spectral hole burning (PSHB) has been demonstrated in the inhomogeneously broadened 7 F 0→ 5 D 0 transition of Eu 3+ ions in crystals of BaFCl at temperatures below 77 K. It was shown that the observed PSHB effect was due to laser excitation-induced site conversion. In the lattice of BaFCl, multiple Eu 3+ sites formed due to charge imbalance and the presence of defects. Eu 3+ population at one site could be depleted and converted to another site. This process was not optically reversible and holes were erased only when temperature was increased to 150 K. Persistent holes burned at 3 K had an average width of 500 MHz while hyperfine transient holes burned at the same temperature were only 25 MHz wide. Although cycling of sample temperature did not affect the shape of the persistent hole, hole width and peak position were quadratic functions of temperature.

Converting Eu^3+ between defect sites in BaFCl for persistent spectral hole burning

Persistent spectral hole burning (PSHB) is demonstrated in the (7)F(0)-->(5)D(0) optical transition of Eu(3+) doped into crystals of BaFCl. For Eu(3+) ions at two different lattice sites, persistent holes can be burned at temperatures below 77 K. The characteristics of the hole-burning process suggest that the observed PSHB effect is due to laser-excitation-induced site-to-site conversion. One type of Eu(3+) site is converted into another type of defect site. This process is not optically reversible, and holes can be erased only when the temperature increases to 150 K.

Anharmonic behaviour of BaFCl using Raman scattering

Raman scattering from oriented single crystals of BaFCl was recorded at various temperatures from 20 to 1073 K for the first time. The Raman spectra, corrected for phonon population, were fitted to the sum of four Lorentzian peaks. The peak frequencies and full width at half maximum (FWHM) of the peaks were obtained from the fit. The FWHM is accounted for by cubic and quartic anharmonic processes. The quartic anharmonicity of the mode increases with the mode frequency. The quartic anharmonicity of the fluorine mode is exceptionally high. The peak frequencies decrease linearly with the increasing temperature. Fluorine mode frequencies decrease more than the internal mode frequencies do. The LO-TO splitting of the fluorine modes and that of the internal modes increases with temperature indicating the increase of the ionic bonding character. The results are discussed.

Antiferroelectric-to-paraelectric transition in BaFCl

Thermoanalytical investigations are carried out on the flux-grown BaFCl crystals. An endothermic peak at 725 $\ifmmode^\circ\else\textdegree\fi{}$C and 1.2% weight loss in the temperature range from 730 to 860 $\ifmmode^\circ\else\textdegree\fi{}$C is observed in differential thermal analysis and thermogravimetric analysis measurements. Heating rate dependency of the endotherm peak position and that of weight loss onset temperature indicate that these two processes are independent from one another. Endotherm is attributed to the antiferroelectric to paraelectric transition in BaFCl. High-temperature Raman scattering, high-temperature x-ray diffraction (XRD) measurements and thermomechanical analysis show that BaFCl has no structural transition from 25 to 800 $\ifmmode^\circ\else\textdegree\fi{}$C. Our results indicate order-disorder type transition. We propose the origin of the dipole to be the displacement of a Ba ion center from the nonpolar site, viz. anionic cage charge center. Weight loss is attributed to the evolution of halides with the activation energy of 1.57 eV. The weight loss results in the shrinkage of the unit cell as determined by XRD, leading to the shifting of the endotherm peak to 823 $\ifmmode^\circ\else\textdegree\fi{}$C. The signature of defects produced by the evolution of halide is seen in optical absorption spectra.

Powder diffraction data of BaFCl

X-ray powder diffraction data of BaFCl are reported for 2θ from 5° to 150° at 25 °C. Strain-free and dendrite-free single crystals of BaFCl, grown by flux method with KCl as flux, were powdered for recording this data. BaFCl is tetragonal, space group P4/nmm, with a=4.3964(1) Å and c=7.2315(3) Å.

Oxygen defects in BaFBr and BaFCl.

Oxygen in oxide form is a common contaminant of BaFBr and BaFCl crystals. Upon x-irradiation of oxide-containing crystals of both materials, ${\mathrm{O}}_{\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ centers are formed, as has been shown previously by electron paramagnetic resonance (EPR) methods. It is shown here by EPR that upon x-irradiation ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ centers are also formed on the ${\mathrm{Br}}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{Cl}}^{\mathrm{\ensuremath{-}}}$ sites, respectively. With electron nuclear double resonance spectroscopy, hyperfine interactions with the nearest $^{19}\mathrm{F}$ neighbors have been determined for ${\mathrm{O}}_{\mathrm{Br}}^{\mathrm{\ensuremath{-}}}$ in BaFBr. Their interpretation leads to the conclusion that the ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ ion is displaced along the c axis from the ${\mathrm{Br}}^{\mathrm{\ensuremath{-}}}$ site by about 1 \AA{}, in qualitative agreement with previous theoretical calculations. The EPR hyperfine data for ${\mathrm{O}}_{\mathrm{Cl}}^{\mathrm{\ensuremath{-}}}$ centers in BaFCl indicate that a similar displacement occurs. Also by EPR, (${\mathrm{O}}_{2}$${)}_{\mathrm{Cl}}^{\mathrm{\ensuremath{-}}}$ centers could be identified in x-irradiated BaFCl crystals containing ${\mathrm{O}}^{2\mathrm{\ensuremath{-}}}$ enriched with the magnetic $^{17}\mathrm{O}$ isotope.

Optical spectra of oxygen defects in BaFCl and BaFBr crystals

Oxygen impurities enter BaFCl and BaFBr crystals as oxygen-vacancy centers. As in other ionic crystals, these defects in the barium fluorohalide crystals have several absorption bands and one photoluminescence band. The defects also are involved in aggregation and photochemical reactions. We have found two kinds of oxygen-vacancy centers in barium fluorohalide crystals and denoted them as centers of types I and II. On the basis of the growth conditions, absorption spectra and other data one may conclude that centers of the first type consist of oxygen substituting at F − sites and neighboring Cl − or Br − vacancies. Oxygen-vacancy centers of the second type seem to be completely on either the Cl or the Br sublattice.

Exciton emission in BaFBr and BaFCl crystals

The emission of self-trapped excitons produced by vacuum ultraviolet excitation as well as X-ray excitation or photostimulation in BaFCl and BaFBr crystals was studied. The exciton emission bands at 4.2 and 5.15 eV in BaFBr and at 3.3 eV in BaFCl were observed upon irradiation into the Cl or Br exciton absorption bands. The 5.15 eV emission band in BaFBr thermally transforms to the 4.2 eV band with increasing temperature within the range 50-80 K. The 3.3 eV emission in BaFCl increases in the range 20-50 K in BaFCl, while the lifetime of the emission decreases. These facts indicate rather clearly the potential barrier between two different exciton configurations. The low-temperature exciton configuration was tentatively assigned to the Vk+e type and the other to F+H type.

Optical detection of Sm2+ nuclear quadrupole resonances in SrFCl and BaFCl

The authors report the first observation of nuclear quadrupole resonances in Sm2+, detected optically for both naturally occurring isotopes with nuclear spin (Sm147 and Sm149) in SrFCl and BaFCl crystals. Quadrupole coupling constants P are determined in the 7F0 electronic ground state and estimates are given for the ratios of shielding parameters (1- gamma infinity )/(1- sigma 2). Comparison of the two crystals demonstrates the sensitivity of quadrupolar coupling to Sm2+-ligand interactions. Anomalously small splittings in BaFCl ( mod P mod <0.05 MHz) are ascribed to the mutual cancelling of two electric field gradient components (4f shell and direct crystal field terms).

A computer simulation study of oxygen defect centers in BaFBr and BaFCl

Atomistic simulation techniques are used to examine several oxygen trapped-hole centers resulting from X-irradiation of BaFBr and BaFCl crystals. The calculations employ recently derived interatomic potentials for the oxide ion-host anion interactions. Particular attention is focused on the sites occupied by the oxide impurity and the energetics of ionization. Our results show the defect model involving O − substitutional at a Br − Cl − site to be a favorable trapped-hole center, in accord with the assignment proposed from EPR measurements. The defect simulations find a large energy barrier to oxide interstitial formation from the oxide precursor at a substitutional site, which suggests that conversion from O x − to O int − is highly unlikely as has been demonstrated in EPR experiments. Ion displacements following lattice relaxation about the defect are also examined. The position of O − substituted for Br − in BaFBr is computed to be displaced from the regular lattice site by 0.53 Å, along the c axis towards the Ba 2+ ion plane, in agreement with models derived later from ENDOR experiments, while O − substituted for F − remains on the lattice site in agreement with experiment.

Electrical Properties of Alkaline Earth Fluorohalide Crystals

AbstractThe ionic conductivity of undoped BaFCl, BaFBr, BaFI, SrFCl, and SrFBr single crystals is measured. The activation energies for a variety of anion vacancy mechanisms and the Schottky energy are defined. They are found to be in good agreement with Baetzold's theoretical data. Partial measurements of the static dielectric constant point out that these materials show an important dipolar contribution which is connected with their antiferroelectric character.

Thermoluminescence of rare earth doped BaFCl crystals

The defects and the mechanism of thermoluminescence (TL) have been systematically studied for X-irradiated cerium, samarium, europium and terbium doped BaFCl crystals. The experimental results reveal that the origin of glow peaks are as follows: V k-centre (at 122 and 166 K), some shallow electron trap defects (194, 215, 254 and 344 K), F(Cl) centre (384 K) and F(F) centre (412 K). The TL spectra show the characteristic emissions of the dopant rare earths. On the basis of this study and the X-ray-induced valence changes as previously reported by us, we were able to formulate a consistent model of the TL process for these crystals.

Optical Bleaching of F‐Centers in BaFCl Crystals after X‐Irradiation at Room Temperature

Optical Bleaching of F‐Centers in BaFCl Crystals after X‐Irradiation at Room Temperature

Optical Bleaching of BaFCl Crystals X‐Irradiated at Room Temperature

Optical Bleaching of BaFCl Crystals X‐Irradiated at Room Temperature

Characteristic parameters of the Eu2+S-state splitting in low-symmetric centres with F-and CI-ligands

A theory for Eu2+ ground S-state splitting in low-symmetric crystals analogous to that developed earlier for the iso-electronic Gd3+ ion is proposed. The characteristic parameters of the contributions to splitting influenced by the ligand point charges are calculated. The phenomenological values of the characteristic parameters of the contributions to the splitting dependent on the metal-ligand exchange effects are found. The Eu2+ S-state splitting in CaFCl, SrFCl, BaFCl crystals is interpreted.

Infrared Lattice Vibration, Dielectric Dispersion, and Lattice Dynamics of BaFCl

AbstractThe reflectivity of single crystals of BaFCl is measured in the wave number range from 30 to 5000 cm−1 at room temperature using a Fourier transform infrared spectrometer. The recorded reflection spectra are evaluated by means of the classical oscillator model and the Kramers Kronig analysis in order to get the TO and LO frequencies and the optical constants. The calculated effective dynamical charges of the involved ions are found to be in good agreement with the results obtained using a shell model.

Effect of Flux on thermoluminescence in Flux‐grown BaFCl crystals

AbstractBaFCl crystals have been grown using BaF2 and BaCl2 by flux technique. Glow curves, optical absorption, and TL emission spectra of x/r — irradiated crystals are studied. The results have been compared with those BaFCl crystals grown from NaF flux so as to study the effects of flux on these properties. It is found that crystals grown from BaF2 flux are relatively purer. An additional TL glow peak at 460 K, an optical absorption band at 775 nm and TL emission band at 485 nm have been obtained in the presently grown crystals. The additional glow peak, optical absorption band have been attributed to F(¯F) aggregate centers, whereas the 485 nm TL emission band to impurity centers.

Decay of X/r-ray induced luminescence in BaFCl crystals

Decay of X/r-ray induced luminescence in BaFCl crystals