LiCaAlF6 Research Articles

Band Gap Calculations for Thorium‐Doped LiCAF

AbstractThe large band gap insulator LiCaAlF6 (LiCAF) has been proposed as a possible host crystal for future realizations of a solid‐state based thorium‐229 nuclear clock, due to its excellent optical transmission in the vacuum ultraviolet range. To enable direct optical manipulation of the thorium isomeric state, the band gap has to remain larger than the nuclear excitation energy upon crystal doping. Here, a systematic search for possible charge compensation mechanisms, defect locations, and the emergence of other compounds, using density functional theory, is presented. Out of 535 optimized structures, the energetically most favorable arrangement is with an estimated band gap of 11.4 eV. Evaluating relevant uncertainties of the used methods suggests that the doped material remains transparent at the 229Th isomer energy for all investigated configurations.

Design optimization for a wearable, gamma-ray and neutron sensitive, detector array with directionality estimation

In this study, we report on a constrained optimization and tradeoff study of a hybrid, wearable detector array having directional sensing based upon gamma-ray occlusion. One resulting design uses CLYC detectors while the second feasibility design involves the coupling of gamma-ray-sensitive CsI scintillators and a rubber LiCaAlF6 (LiCAF) neutron detector. The detector systems’ responses were investigated through simulation as a function of angle in a two-dimensional plane. The expected total counts, peak-to-total ratio, directionality performance, and detection of 40K for accurate gain stabilization were considered in the optimization. Source directionality estimation was investigated using Bayesian algorithms. Gamma-ray energies of 122 keV, 662 keV, and 1332 keV were considered. The equivalent neutron capture response compared with 3He was also investigated for both designs.

First-principles calculations of electronic and optical properties of LiCaAlF6 and LiSrAlF6 crystals as VUV to UV solid-state laser materials

We report the density functional calculations of the electronic and optical properties of perfect LiCaAlF6 (LiCAF) and LiSrAlF6 (LiSAF) crystals. The calculations are based on the Perdew-Burke-Ernzerhof (PBE) functional employing 35% exact exchange. Using optimized unit crystal volumes and equilibrium lattice constants, both LiCAF and LiSAF are found to have indirect band gaps of 12.23 and 11.79 eV, respectively. The band gap energies of these fluoride crystals are also observed to increase upon application of pressure by uniform volume compression. Moreover, their bulk moduli are determined to be 108.01 (LiCAF) and 83.75 GPa (LiSAF), while their static dielectric constants are 1.27 (LiCAF) and 1.26 (LiSAF). Considering the dielectric functions, refractive indices, and absorption coefficients, both perfect LiCAF and LiSAF crystals are viable vacuum ultraviolet (VUV) to ultraviolet (UV) laser host media. With knowledge of the different optical transitions and pressure dependence, our results yield helpful insights on the use of these fluoride compounds as effective solid-state laser materials in the VUV region.

LiCaAlF6 scintillators in neutron and gamma radiation fields

Intentionally doped LiCaAlF6 (LiCAF) single crystals are prospective scintillators, especially for thermal neutron detection through the 6Li(n,t)4He nuclear reaction. Four different LiCAF scintillator samples were tested in various neutron and gamma fields. Two of the tested samples were LiCAF:Eu and LiCAF:Eu,Na single crystals, and another two samples were made of LiCAF:Eu micro crystals dispersed in transparent rubber, with different rubber dimensions. All LiCAF samples contain lithium enriched to[Formula: see text]Li. A plutonium–beryllium source was used as a neutron source. The neutron spectrum was modified by moderator and filter to get different ratios between thermal, epithermal and fast neutron fluence rates. The MCNP code was used for calculations of the fluence rates for different configurations. Radionuclides [Formula: see text]Cs and [Formula: see text]Co were applied as gamma radiation sources. The light signal from the scintillator was evaluated with a photomultiplier and a multichannel analyzer. The purpose of this work was to study the characteristics of LiCAF scintillators, especially the ability to discriminate signals from neutron and gamma radiation, which is the basic scintillator condition for neutron detection in mixed neutron-gamma radiation fields. Generally, the discrimination can be done by the pulse height and/or the pulse shape of the evaluated signals. Both methods can be used for a LiCAF scintillator. However, only the pulse height discrimination method is discussed in this paper. The possibility of fast neutron detection with LiCAF scintillators was also tested.

Evaluation of Ce3+ and alkali metal ions Co-doped LiSrAlF6 crystalline scintillators

High scintillation efficiency of Eu-doped LiSrAlF6 (LiSAF) and LiCaAlF6 (LiCAF) codoped with alkali metal ions has been reported in our recent studies. Thus in this paper, we demonstrated the scintillation properties of 1% Ce-doped LiSAF crystals with 1% alkali metal ions co-doping to increase the light yield and understand the scintillation mechanism. The crystals showed intense emission band corresponding to the 5d-4f transition of Ce3+, and their light yields under thermal neutron excitation were higher than that of the Ce only doped crystal. Especially, the light yield of Ce–Na co-doped crystal exceeded about two times that of Ce only doped one.

Electronic structure, small polaron, and F center in LiCaAlF6

LiCaAlF6 (LiCAF) is a promising inorganic neutron scintillator material. Here, we report the electronic structure as well as properties of the small hole polaron and F center in LiCAF based on first-principles calculations. The results suggest that lowering the Fermi level by acceptor doping will stabilize F+ against F− and reduce the reabsorption of Ce emission by F centers in LiCAF:Ce3+, thereby increasing light yield and radiation resistance.

Crystal Growth and Scintillation Properties of Ce<sup>3+</sup> and Mg<sup>2+</sup> Co-Doped LiCaAlF<sub>6</sub> Single Crystal

Ce and Mg Co-Doped LiCaAlF6 (LiCAF) Single Crystals Were Grown by Micro-Pulling-down (μ-PD) Method and the Optical and Scintillation Properties of Obtained Crystals Were Measured. The Grown Crystals Were Crack-Free and Had 30-50 mm in Length and 2 Mm in Diameter. Absorption Coefficients Were Proportional to the Ce Nominal Concentrations. the Radioluminescence Spectra under 5.5 Mev Alpha-Ray Irradiation Showed Emission Peak at 290 and 310 nm due to Ce3+ 5d-4f Transitions, and Were Agreement with those of Ce Single-Doped LiCaF. The Highest Light Yield under 5.5 Mev Alpha-Ray Irradiation of Ce 4% and Mg 2% Co-Doped LiCaF Was Found to Be 0.6 Times than that of Lithium Glass Scintillator GS-20. Comparing the Light Yield of Licaf Samples with the Fixed Ce Nominal Concentration and Different Mg Co-Doped Concentrations, the Light Yield Increased with an Increase of Mg Nominal Concentration. the Alpha-Ray Excited Decay Times Were Constantly 45-50 ns.

Effect of Eu and Pb doping on the dosimetric properties of LiCAF

LiCaAlF6 (LiCAF) crystals doped with two different ions (europium and lead) have been investigated as potential new dosimetric materials. The stability of thermally stimulated luminescence (TSL) glow peaks in LiCAF:Eu was evaluated by means of the initial rise technique. The decay times at room temperature of the traps related to the dosimetric glow peaks were found to range between 40 and 2 × 104 years confirming the good dosimetric characteristics of this crystal. The glow curve of LiCAF:Pb is dominated by a peak at approximately 300 °C emitting in the UV region (3P0,1–1S0 transition of Pb2+) superimposed to a very broad structure at lower temperature (20–200 °C) featuring recombination at an intrinsic defect centre. The anomalous behavior of the low temperature structure during thermal cleaning procedures prevented any reliable numerical analysis of the TSL glow peak at 300 °C.

Crystal Growth and Luminescence Properties of Pr-Doped LiYF4 and LiCaAlF6

Pr-doped LiYF4 (YLF) and LiCaAlF6 (LiCAF) single crystals were grown by the micro-pulling-down (µ-PD) method. The crystals were transparent, 2.0 mm in diameter, and 40–50 mm in length. Neither visible inclusions nor cracks were observed. Transmittance, reflectance, and X-ray-excited emission spectra of Pr-doped YLF and LiCAF were measured. For both crystals, approximately 80% transmittance and 3% reflectance were achieved at visible wavelengths. Emission peaks due to Pr3+ 5d–4f transitions at approximately 220–250, 270, 330, and 410 nm were observed for both crystals.

Rare earth doped LiCaAlF6 as a new potential dosimetric material

LiCaAlF6 (LiCAF) crystals doped with two different rare earth (RE) ions (europium and cerium) have been investigated as potential new dosimetric materials. Thermally stimulated luminescence (TSL) measurements have been carried out and a quantitative comparison has been performed with a standard LiF dosimetric material. Wavelength-resolved TSL measurements have revealed that the TSL spectrum is dominated by RE-related emissions with no significant contributions from defect centers. The glow curve of LiCAF:Eu displays a dosimetric peak at about 240°C with a good linearity versus imparted dose. Specifically, the material appears more sensitive than LiF at doses lower than 100mGy (with a sensitivity limit of 10−5Gy). Concerning LiCAF:Ce, the glow curve is more complicated, dose memory effects appear and no peaks with good dosimetric properties are found. Therefore, LiCAF:Eu seems to be more promising in the search for a new dosimetric material.

High efficiency UV solid state lasers based on Ce:LiCaAlF6 crystals

We report the spectroscopy and laser results of cerium-doped colquiriite LiCaAlF6 (LiCAF) single crystals. High quality samples have been grown using the Czochralski technique and characterized by means of X-ray Laue diffractometry, absorption and emission spectra. Moreover, we measured the lifetime and deduced the emission cross-sections of the laser transition. Using this crystal we realized two lasers devices: a high-efficiency laser emitting at 289 nm with a repetition rate (RR) of up to 2 kHz and a widely tunable laser (from 280 nm to 317 nm) with an RR of up to 4.3 kHz. The high quality of the crystal made it possible to obtain the highest slope efficiency (49%) ever reported to our knowledge for a Ce:LiCAF laser.

LiCaAlF6 and LiSrAlF6: tunable solid state laser host materials

Tunable all-solid-state lasers find more and more applications in different fields. Chromium-doped ternary colquiriites, LiCaAlF6 (LiCAF) and LiSrAlF6 (LiSAF), show a tunable laser activity in the region of 720–840nm and 780–1010nm, respectively. Other interesting properties enable them to be used as scintillators and window materials. Single crystals of LiCAF and LiSAF have been grown by various methods, including the Czochralski, Bridgeman, zone melting and top-seeded solution growth techniques. Vertical gradient freezing (VGF) is proposed for producing high optical quality crystals.

Energy storage in Ce-doped LiCaAlF6 and LiSrAlF6 crystals

Absorption, emission and thermoluminescence (TL) of as-grown and X-irradiated pure and Ce-doped LiCaAlF6 (LiCAF) and LiSrAlF6 (LiSAF) crystals have been analyzed. It is shown that the energy storage is defined by the dissimilarity of basic matrix properties, intrinsic defect structure, preferred Ce3+ ion disposition and charge compensation defect type. This is the reason of higher colorability and TL efficiency of as-grown Ce:LiSAF compared to Ce:LiCAF. Pre-filling of ultra-deep traps leads to enormous increases in the TL response. Due to these properties Ce:LiSAF and Ce:LiCAF are promising thermoluminescent dosimetric materials.

X-ray damage characterization in BaLiF3,KMgF3 and LiCaAlF6 complex fluorides

Two-inch sized KMgF3,BaLiF3 and LiCaAlF6 (LiCAF) single crystals were grown by the Czochralski method under a CF4 atmosphere. X-ray irradiation was used to carry out a comparative study of induced optical absorption phenomena and colour centre creation in the ultra-violet and visible spectral regions. The integral of the induced absorption spectra is significantly lower in LiCAF with respect to the other studied materials. It is found that the amplitude of the F-absorption band is suppressed more than a factor of 3 by Mg-doping. For Mg-doped crystals, the optimum doping concentration is about 0.2mol% of Mg2+.

Growth of LiCaAlF6 Single Crystals with an Extended Diameter and Their Optical Characterizations.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Induced Absorption Phenomena, Thermoluminescence and Colour Centres in KMgF3, BaLiF3 and LiCaAlF6 Complex Fluorides

KMgF3, BaLiF3 and LiCaAlF6 (LiCAF) single crystals were grown by the Czochralski method under CF4 atmosphere. X-ray irradiation was used to carry out a comparative study of induced optical absorption phenomena and colour centre creation in the vacuum-ultra-violet (VUV), ultra-violet and visible spectral regions. In the case of LiCAF, the X-ray induced absorption peak at 262 nm can be ascribed to F centre. The integral of the induced absorption spectra is significantly lower in LiCAF with respect to the other studied materials; moreover, LiCAF shows stable VUV transmission characteristics and the most resistant surface toward what is regarded as the damage due to reaction with air atmosphere.

Growth of LiCaAlF6 single crystals with an extended diameter and their optical characterizations

LiCaAlF6 (LiCAF) single crystals with an extended diameter were grown by the Czochralski (CZ) technique under CF4 atmosphere. Modifying the growth conditions enabled the growth of LiCAF single crystals with 3- and 4-inch diameters. After considering reflection loss, LiCAF showed transmittance over 95% at 157 nm. A precise measurement of the refractive index showed that the birefringence of LiCAF vanished at around 152 nm. The fabrication of a LiCAF lens was demonstrated.

Energy levels and symmetry of Ce3+ in fluoride and oxide crystals

Single crystal samples of Ce3+-doped fluorides [LiCaAlF6 (LiCAF), LiYF4 (LYF), NaCaYF6 (NCYF)], and oxides [Ca2Al2SiO7 (CASM), CaYAlO4 (CYA)] have been grown in inert and reducing atmospheres, with the intent of studying the effects of symmetry and ligand coordination on the dopant energy levels. The optical absorption spectra of Ce3+ in these crystals consist, at most, of five overlapping bands. The luminescence is also a broadband due to strong electron–phonon interaction in the 5d excited state. The energies of the lowest absorption band and the luminescence band decrease along the sequence LiCAF, LYF, NCYF, CASM, and CYA. This trend may be explained by the nephelauxetic effect and lowering symmetry of the anion-coordinate polyhedra of Ce3+ in these crystals. The lowest energy peak in the absorption spectrum of Ce3+ observed in CASM is coincident with the third harmonic (355 nm) of a Q-switched pulsed YAG laser.