Alkaline Earth Fluoride Crystals Research Articles

Spectral Luminescent Properties of Alkaline Earth Fluoride Crystals Implanted with Silver Ions

Spectral Luminescent Properties of Alkaline Earth Fluoride Crystals Implanted with Silver Ions

Simulation and demonstration of glass-like heat-transfer equations in rare-earth doped alkaline earth fluoride crystals

The intriguing phenomenon of glass-like heat-transfer behavior in rare-earth (RE) doped alkaline-earth fluoride (AEF) systems has captivated researchers' attention in fundamental science. The heat-transfer dynamics of AEF crystals exhibit transitions between crystalline-like and glass-like states as the species and concentration of doped RE ions are altered. Here, the applicability constraints of glassy thermal models in RE-AEF systems are elucidated by optimizing and adjusting parameters within the governing equations. Building upon insights derived from defective phonon-scattering calculations, the diverse factors that influence glass-like heat-transfer behavior in AEF crystals are comprehensively analyzed. The variation mechanism of the thermal conductivity models in RE-AEF crystals is subsequently simulated and explicated, employing an in-depth investigation into the parameters embedded within thermal equations. This endeavor ultimately establishes a solid theoretical foundation for the thermal investigation of disordered crystals.

Fine structure of 4f–5d absorption spectra of Me[formula omitted]-Yb[formula omitted] in the vacuum ultraviolet region under synchrotron excitation

Narrow dips were found in the excitation spectra of crystals of alkaline earth fluorides doped with ErF3 or/and YbF3. The positions of the dips are in good agreement with the positions of the narrow absorption lines Er3+, Yb3+ measured earlier. The absorption lines of the crystals were calculated from the excitation spectra. The observed dips in the excitation spectra are due to the absorption lines of 4f–5d transitions in lanthanide ions.

Emission of Tm2+ in alkaline-earth fluoride crystals

The luminescence spectra of 4f-4f and 5 d-4f transitions and absorption spectra of 4f-4f transitions of Tm2+ ions were studied in crystals of alkaline earth fluorides CaF2, SrF2, BaF2. The 5 d-4f emission of Tm2+ ions was detected in the 680–800 nm region with a decay time in the micro-millisecond time interval. The 4f-4f emission is thermally quenched at temperatures of 250, 300, and 350 K in BaF2, SrF2, and CaF2, which correlates with the distance from the excited level to the bottom of the conduction band.The splitting of the atomic F7/22, F5/22 levels of Tm2+ by the cubic field decreases linearly with increasing interion distance. The splitting tends to zero at a distance between fluorines of about 4 Å. The positions of the Tm2+ bands in alkaline earth fluorides are well described by ab initio quantum chemical calculations.

Modelling and analyzing the glass-like heat transfer behavior of rare-earth doped alkaline earth fluoride crystals

Based on the “three-stage” heat transfer behavior of rare-earth doped CaF2 crystals, numerical models at different stages for the thermal conductivity of rare earth doped fluorides are proposed to study the mechanism of glass-like heat conduction.

Optical absorption spectra of X-ray irradiated alkaline earth fluoride crystals doped with divalent rare-earth ions studied by thermal bleaching

The absorption spectra of X-ray irradiated alkaline earth fluoride (CaF2, SrF2, BaF2) crystals doped with Sm, Нo, Er, Tm ions are investigated by thermal bleaching of the sample in the 300–700 K temperature range. X-ray irradiation creates absorption bands of inter-configuration 4fn-4fn−1-5 d1 transitions of divalent impurity ions and F3− and (F2−)ii hole centers. The thermal oxidation of the divalent RE ions have been observed. Two possible mechanism of thermal oxidation of divalent lanthanide ions in alkaline-earth fluorides are discussed.

Spectroscopy of divalent samarium in alkaline-earth fluorides

Optical spectra (absorption, luminescence, decay of emission, spectra with time resolution) of Sm2+ ions in alkaline-earth fluoride crystals (CaF2, SrF2, BaF2) in the wavelength range 120–900 nm at 7–300 K were studied. By additive coloration of about 60% of the original Sm3+ is converted to Sm2+.One component of the decay of the Sm2+ emission was observed in BaF2 crystals with a lifetime of 16 ms (a slow component) and in CaF2 crystals with a lifetime of 1.3 μs (fast component) at low temperature. In SrF2 crystals, two decay of luminescence components are found in the millisecond and nanosecond time range. The luminescence spectra of the slow and fast components in CaF2, SrF2, BaF2 are similar, which confirms their attribution to forbidden 4f-4f and allowed 5d-4f transitions, respectively. The results obtained made it possible to clarify the position of the excited levels of Sm2+ 4f(5D0) and 5d(eg) relative to the bottom of the conduction band of alkaline-earth fluorides.

Divalent rare-earth ions Pr, Sm, Нo, Er, Tm, and Yb in crystals of alkaline-earth fluorides

The absorption spectra of radiation-colored CaF2, SrF2, and BaF2 crystals activated by trivalent Pr, Sm, Нo, Er, Tm, and Yb (rare-earth, RE) ions are studied. It is shown that ionizing radiation reduces the impurity ions to the divalent state. The temperature resistance of divalent RE ions of radiation-colored CaF2 crystals correlates with the chemical stability of the compounds with divalent RE ions. The photochromic centers are produced in CaF2-Pr crystal colored by radiation at room temperature and heated to 200°C.

Formation of H a - hydrogen centers upon additive coloration of alkaline-earth fluoride crystals

The mechanism of coloration of alkaline-earth fluoride crystals CaF2, SrF2, and BaF2 in calcium vapors in an autoclave with a cold zone is studied. It was found that the pressure in the autoclave upon constant evacuation by a vacuum pump within the temperature range of 500–800°C increases due to evaporation of metal calcium. In addition to the optical-absorption bands of color centers in the additively colored undoped crystals or to the bands of divalent ions in the crystals doped with rare-earth Sm, Yb, and Tm elements, there appear intense bands in the vacuum ultraviolet region at 7.7, 7.0, and 6.025 eV in CaF2, SrF2, and BaF2, respectively. These bands belong to the Ha - hydrogen centers. The formation of hydrogen centers is also confirmed by the appearance of the EPR signal of interstitial hydrogen atoms after X-ray irradiation of the additively colored crystals. Grinding of the outer edges of the colored crystals leads to a decrease in the hydrogen absorption-band intensity with depth to complete disappearance. The rate of hydrogen penetration inside the crystal is lower than the corresponding rate of color centers (anion vacancies) by a factor of tens. The visible color density of the outer regions of the hydrogen-containing crystals is several times lower than that of the inner region due to the competition between the color centers and hydrogen centers.

Theoretical study of Ce2+ cubic centres in alkaline earth fluoride crystals

In this paper we present theoretical study of Ce2+ impurity centres in alkaline earth fluoride crystals (CaF2, SrF2). Only cubic configurations of centres were considered. Electronic levels and related properties were studied using CASSCF/CASPT2 approach within embedded-cluster formalism including scalar relativistic corrections and spin-orbital interaction. Calculated absorption spectra for Ce2+ in CaF2 and SrF2 are in good agreement with experimental data. For both crystals the ground state of Ce2+ ion has predominantly 4f15d1 singlet character.

Spectroscopy of divalent rare earth ions in fluoride crystals

We have studied the absorption spectra of x-ray irradiation-induced Ce2+ and Pr2+ ions in crystals of alkaline-earth fluorides. We have calculated absorption spectra of divalent praseodymium ions in SrF2 crystals doped with Pr2+ for the first time. The calculated spectra agree rather well with the experimental data. In crystals containing induced Ce2+ ions we have found strong electron–phonon coupling. In BaF2, we do not observe bands corresponding to divalent Ce or Pr ions.

Divalent cerium and praseodymium ions in crystals of alkaline-earth fluorides

We have studied the absorption spectra of radiation-induced divalent cerium and praseodymium ions in crystals of alkaline-earth fluorides. Using ab initio quantum-mechanical methods, we have calculated absorption spectra of divalent praseodymium ions in CaF2 crystals for the first time. The theoretical spectrum agrees rather well with the experimentally registered spectra.

5d-4f Emission of Nd$^{3+}$, Sm$^{3+}$, Ho$^{3+}$, Er$^{3+}$, Tm$^{3+}$ Ions in Alkaline Earth Fluorides

Emission and excitation spectra of CaF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , SrF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , BaF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> doped with Nd <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> , Sm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> , Ho <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> , Er <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> , Tm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> excited by vacuum ultraviolet photons or by x-rays were studied. Decay times of the Nd <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> emission near 180 nm at 7 K were 17.7, 15.3, 12.3 ns in CaF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , SrF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , BaF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> respectively. 4f <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> -1 5d <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> -4f <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> luminescence bands were found at 170-200 nm for Sm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> ions and at 140-190 nm for Ho <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> . No excitation of 5d-4f emission of Nd <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> and Tm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> was observed in the region of band-to-band transitions below 127 nm (BaF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ), 120 nm (SrF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) and 115 nm (CaF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ). Emission spectra of Nd and Er doped alkaline-earth fluoride crystals under x-ray excitation were studied with concentration of impurity from 0.01 to 10 molar %. For the Nd <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> and Er <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> 5d-4f emissions in SrF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> the light yields were estimated as 30-40 ph/Mev.

5d-4f luminescence of Nd3+, Gd3+, Er3+, Tm3+, and Ho3+ ions in crystals of alkaline earth fluorides

The vacuum ultraviolet emission spectra of alkaline-earth fluoride (CaF2, SrF2, BaF2) crystals with rare earth impurity ions (Nd, Gd, Er, Tm, Ho) have been investigated. The main luminescence bands are described well by the transitions from the lowest excited 5d state to different 4f levels of rare earth ions.

Luminescence of singlet self-trapped excitons inMgF2

Fast (τ∼1.7 ns) broadband (fullwidth at half-maximum = 1.1 eV) vacuumultraviolet (VUV; hν = 8.4 eV)luminescence from MgF2 crystals has been detected at low temperature(T<80 K) and analyzed with techniques of cathodoluminescence and time-resolved VUV spectroscopyusing synchrotron radiation. The VUV emission discovered has been attributed to theluminescence of spin-singlet self-trapped excitons (STEs). In contrast to the case for alkalineearth fluoride crystals with fluorite structure, the luminescence of the singlet STEs inMgF2 shows smaller Stokes shift than that of the triplet STEs, which is similar to ‘typical’behavior observed for alkali halide crystals.

Phonon contribution to the hyperfine coupling constant

The temperature dependence of the hyperfine coupling constant A(T) of Mn2+ in MgO is analysed by means of the generalized phonon theory, obtained by treating the phonon wavelength without approximation over the entire phonon spectrum. A comparison is also made of the results obtained with both the generalized and the long-wavelength models as applied to A(T) for Mn2+ and Eu2+ in the alkaline earth fluoride crystals. This comparison suggests that local mode effects and not covalency may be the dominant mechanism required to improve the theoretical fit to the data. Contributions from optical phonons to A(T) of Mn2+ and Eu2+ in MgO, CaF2, SrF2, and BaF2 crystals are also tabulated and discussed.

Charge-transfer bands in crystals of alkaline earth fluorides with Eu3+ and Yb3 + 1

The absorption, luminescence, and excitation spectra of CaF2, SrF2, and BaF2 crystals with EuF3 or YbF3 impurity have been investigated in the range 1–12 eV. In all cases, strong wide absorption bands (denoted as CT1) were observed at energies below the 4fn-4fn − 15d absorption threshold of impurity ions. Weaker absorption bands (denoted as CT2) with energies 1.5–2 eV lower than those of the CT1 bands have been found in the spectra of CaF2 and SrF2 crystals with EuF3 or YbF3 impurities. The fine structure of the luminescence spectra of CaF2 crystals with EuF3 impurities has been investigated under excitation in the CT bands. Under excitation in the CT1 band, several Eu centers were observed in the following luminescence spectra: C4v, Oh, and R aggregates. Excitation in the CT2 bands revealed luminescence of only C4v defects.

Charge-transfer bands in alkaline-earth fluoride crystals doped by Eu 3+ or Y b 3+ ions

Absorption, emission and excitation spectra of CaF 2, SrF 2, BaF 2 crystals doped by Y bF 3 or EuF 3 impurities were studied in 1–12 eV spectral region. The intensive absorption broad bands (denoted as CT 1) were observed in all cases just below the 4f–5d absorption region. Less intensive absorption bands, denoted as CT 2, having energies 1.2–1.5 eV lower than those of CT 1, were observed in CaF 2, SrF 2 crystals doped by EuF 3 or Y bF 3 impurities. High resolution emission spectra of Eu-doped CaF 2 and SrF 2 crystals excited into CT 1 and CT 2 bands were measured. Under excitation into CT 1 bands, all present Eu sites (C 4v, O h and some aggregates) were observed in emission spectra. While under excitation into CT 2 bands, only the emission of C 4v sites was observed.

Triplet luminescence of cadmium centres in alkaline-earth fluoride crystals

The emission and excitation spectra as well as decay of emissions of alkaline-earth fluoride crystals dopedwith CdF2 were investigated in the 2–24 eV range at temperatures in the range 8–300 K. Emission bands at4.2 and 3.5 eV, respectively, were found under excitation into the Cd absorption region inCaF2–Cd and SrF2–Cd crystals at low temperatures. Both emission bands have slow luminescencedecay times of microsecond timescale. No Cd-related emission was found inBaF2–Cd crystals.The calculations of the geometrical configurations of excited tripletCd2+ centres and the Cd-related electron transitions were carried out by using the ab initio Hartree–Fockmethod. The results of experiments and calculations lead us to the conclusion that the observedCd2+ emission bands are due to the triplet–singlet transitions from the Cd s-states to the nearestfluorine ions. The calculated energies are in good agreement with experimentally observedones.

Exciton interaction with impurity in barium fluoride crystals

Optical absorption excitation and emission of alkaline-earth fluoride crystals doped with impurities, which did not introduce a new absorption band up to 8eV, were investigated. Comparison of emission of BaF2–LaF3, BaF2–KF and doubly doped BaF2–LaF3, KF shows that interstitial fluorines are not the main exciton suppressors as thought before.The two stages of exciton suppression on the plot of exciton intensity against impurity concentration were observed. The first stage has the interaction length near 130Å, while the interaction lengths of second stage are 10–30Å, depending on impurity.