MgF2 Single Crystals Research Articles

Understand anisotropy dependence of damage evolution and material removal during nanoscratch of MgF2 single crystals

To understand the anisotropy dependence of the damage evolution and material removal during the machining process of MgF2 single crystals, nanoscratch tests of MgF2 single crystals with different crystal planes and directions were systematically performed, and surface morphologies of the scratched grooves under different conditions were analyzed. The experimental results indicated that anisotropy considerably affected the damage evolution in the machining process of MgF2 single crystals. A stress field model induced by the scratch was developed by considering the anisotropy, which indicated that during the loading process, median cracks induced by the tensile stress initiated and propagated at the front of the indenter. Lateral cracks induced by tensile stress initiated and propagated on the subsurface during the unloading process. In addition, surface radial cracks induced by the tensile stress were easily generated during the unloading process. The stress change led to the deflection of the propagation direction of lateral cracks. Therefore, the lateral cracks propagated to the workpiece surface, resulting in brittle removal in the form of chunk chips. The plastic deformation parameter indicated that the more the slip systems were activated, the more easily the plastic deformation occurred. The cleavage fracture parameter indicated that the cracks propagated along the activated cleavage planes, and the brittle chunk removal was owing to the subsurface cleavage cracks propagating to the crystal surface. Under the same processing parameters, the scratch of the (001) crystal plane along the [100] crystal-orientation was found to be the most conducive to achieving plastic machining of MgF2 single crystals. The theoretical results agreed well with the experimental results, which will not only enhance the understanding of the anisotropy dependence of the damage evolution and removal process during the machining of MgF2 crystals, but also provide a theoretical foundation for achieving the high-efficiency and low-damage processing of anisotropic single crystals.

Luminescence of MgF2-WO3 ceramics synthesized in the flux of 1.5 MeV electron beam

The results of comparative analysis of optical properties of MgF2 single crystals and MgF2 ceramics, non-doped and doped with tungsten trioxide obtained in the process of high-speed radiation synthesis in air under a flux of 1.5 МеV electrons, are presented.The synthesized ceramics are found to have a high degree of crystallinity, their elements composition is close to stoichiometric, and the phase composition is a rutile lattice.Using time-resolved spectroscopy, it was shown that the initial defectiveness of as-synthesized ceramics samples is represented by emission centers belonging to WO3 complexes and radiation defects of F2- types with spectral-kinetics parameters similar to those in irradiated by ionizing radiation MgF2 single crystals. The ratio between these types of defects is determined by the temperature and luminescence excitation method and the ratios between dopant and co-dopant (LiOH).In MgF2 ceramics samples, being exposed to ionizing radiation, two radiative channels of relaxation of electronic excitations were found: T-So (triplet-singlet) emission transition in self-trapped excitons (STE) at 3.2 eV and recombination of F–H Frenkel pairs accompanied with the emission at 4.0 eV. The temperature dependences of these channel efficiency in the range of 5–300 K are similar to those ones observed in MgF2 single crystals.

Crystal growth of ultra-large MgF2 and LiCaAlF6 single crystals by a double-crucible Czochralski technique

We report the successful growth of ultra-large sized magnesium fluoride (MgF2) and lithium calcium aluminum fluoride (LiCaAlF6) single crystals using the Czochralski growth method with a double-layer crucible. The 8-inch diameter MgF2 crystal was grown in argon atmosphere at a growth rate of 2 mm/h up to a length of 10 cm while the 4-inch diameter LiCaAlF6 crystal was grown in carbon tetrafluoride atmosphere at a growth rate of 1 mm/h up to a length of 30 cm. The 8-inch diameter satisfies the required size for the moderator that slows down neutrons in Boron Neutron Capture Therapy (BNCT), a noninvasive radiation therapy that selectively targets only the cancer cells. The successful growth of these ultra-large single crystals will not only advance BNCT but will also open new possibilities for optical materials such as lenses and windows in the vacuum ultraviolet region.

Unravelling the intertwined atomic and bulk nature of localised excitons by attosecond spectroscopy

The electro-optical properties of most semiconductors and insulators of technological interest are dominated by the presence of electron-hole quasi-particles, called excitons. The manipulation of excitons in dielectrics has recently received great attention, with possible applications in different fields including optoelectronics and photonics. Here, we apply attosecond transient reflection spectroscopy in a sequential two-foci geometry and observe sub-femtosecond dynamics of a core-level exciton in bulk MgF2 single crystals. Furthermore, we access absolute phase delays, which allow for an unambiguous comparison with theoretical calculations. Our results show that excitons surprisingly exhibit a dual atomic- and solid-like character, which manifests itself on different time scales. While the former is responsible for a femtosecond optical Stark effect, the latter dominates the attosecond excitonic response. Further theoretical investigation reveals a link with the exciton sub-femtosecond nanometric motion and allows us to envision a new route to control exciton dynamics in the close-to-petahertz regime.

Thermal annealing of radiation defects in MgF2 single crystals induced by neutrons at low temperatures

Primary radiation defects in ionic solids consist of Frenkel defects – pairs of defects - anion vacancies with trapped electrons (F-type centers) and interstitial ions. Upon temperature increase after irradiation, the electronic F-type centers are annealed due to recombination with mobile interstitials. Analysis of the recombination (annealing) kinetics allows us to obtain important information on the interstitial migration. At high radiation doses more complex dimer (F2-type) centers are observed in several charge states, which are well distinguished spectroscopically. We analysed here available experimental kinetics of the F2-type center annealing in MgF2 in a wide temperature range (300–1000 K). The results of the electron, neutron and ion irradiation are compared.The phenomenological theory of the recombination kinetics of radiation defects takes into account the interstitial ion diffusion and recombination with the F2-centers as well as mutual sequential transformation with temperature growth of three types of experimentally observed dimer centers: F2 (1); F2 (2), F2 (3) (which differ tentatively by effective charges with respect to the host crystalline sites: 0, +1, +2).

Scintillation, TSL and RPL properties of MgF2 transparent ceramic and single crystal

We have prepared MgF2 transparent ceramics by spark plasma sintering (SPS) and investigated the scintillation and dosimeter properties compared with those of a single crystal. Under X-ray irradiation, a transparent ceramic showed scintillation with a broad feature over 300–600nm with decay times of 310 and 1940μs while the single crystal did not show such emission. It was found that MgF2 show radio-photoluminescence (RPL), which is a generation of new luminescence center by ionizing radiation. The RPL was only observed in the transparent ceramic samples, which appeared as two emission bands peaking around 415 and 700nm under 340±40nm excitation. During the PL reading, the emission band at 415nm effectively increased over 50s. This result suggested that the RPL in MgF2 shows a build-up phenomenon. From PL emission/excitation characteristics and decay time, it was suggested that the RPL peak around 415nm was due to M(C2h) center.

Growth of MgF2 optical crystals and their ionic conductivity in the as-grown state and after partial pyrohydrolysis

MgF2 single crystals have been grown from melt by the Bridgman technique in a fluorinating atmosphere. To control the presence of oxygen impurity, it was first suggested to measure the ionic conductivity in MgF2 crystals by impedance spectroscopy. The characteristics of ionic conductivity of “as grown” (i.e., without thermal treatment) crystals and crystals obtained by commercial vacuum technology practically coincide: the volume conductivity σv = 1.4 × 10−7 S/cm at 773 K and the ion-transport activation energy Ea = 1.40 ± 0.05 eV. Annealing MgF2 crystals during electrophysical studies upon heating from 293 to 823 K in vacuum (residual pressure ∼1 Pa) for 4 h led to their partial pyrohydrolisis. The influence of this thermal treatment of MgF2 crystals on their optical transmission is studied in the wavelength range of 115–300 nm.

Impedance and modulus spectroscopy of MgF2 single crystals

The electrical and dielectric properties of MgF2 single crystals were investigated in the temperature range of 11–800°C and the frequency range of 20 Hz–10MHz using impedance and modulus spectroscopy. Impedance analysis revealed that MgF2 shows intrinsic dielectric properties below ~500°C. A thermally activated relaxation at higher temperatures was observed, which was ascribed to be a Debye-type relaxation caused by the mobility of fluorine interstitials. The result indicates that MgF2 holds great promising applications as gate dielectric, substrate materials, and buffer layer in silicon technology.

Dual-mode temperature compensation technique for laser stabilization to a crystalline whispering gallery mode resonator.

Frequency stabilization of a diode laser locked to a whispering gallery mode (WGM) reference resonator made of a MgF2 single crystal is demonstrated. The strong thermal dependence of the difference frequency between two orthogonally polarized TE an TM modes (dual-mode frequency) of the optically anisotropic crystal material allows sensitive measurement of the resonator's temperature within the optical mode volume. This dual-mode signal was used as feedback for self-referenced temperature stabilization to nanokelvin precision, resulting in frequency stability of 0.3 MHz/h at 972 nm, which was measured by comparing with an independent ultrastable laser.

β-Ray angular distribution from purely nuclear spin aligned 20F

The alignment correlation term in the β-ray angular distribution from purely nuclear spin aligned 20F has been measured to test the G-parity conservation law which is one of the fundamental symmetries in the weak nucleon current. We utilized the hyperfine interaction of 20F in an MgF2 single crystal and successfully created the pure alignment from the polarization by means of the spin manipulation technique based on the β-NMR method.

CaF2, MgF2, SiO2, Al2O3, SiC, LiF, BaF2, and ZrO2 optical single crystals used in studies in the VUV spectral region

Spectral characteristics of single crystals—fluorite, magnesium fluoride, quartz, sapphire, silicon carbide, lithium fluoride, barium fluoride, and fianite—used as materials for optical windows in studies in the VUV spectral region are presented. Spectral and photometric studies of single crystals in the spectral range 30–700 nm were performed on a BM-70 vacuum spectrophotometer. This instrument is equipped with a vacuum chamber for measuring the light transmission and reflection at radiation angles of incidence between 10° and 70°; this allows both characteristics to be measured sequentially in time without breaking vacuum. Replaceable radiation sources (with and without a window) were used to perform studies in the spectral range 30–700 nm, including measurements in flows of atmospheric and inert gases (Ne, He, Ar, N, and H).

Swelling and creation of color centers in MgF2 single crystals irradiated with energetic heavy ions

Single crystals of MgF2 were irradiated with different ions of specific energy between 1.4 and 11.1MeV/u. Using UV–Vis absorption spectroscopy, the creation of color centers was analysed as a function of the ion fluence and energy loss. The concentration of single F- and F2-centers versus fluence exhibits a nonlinear evolution similar as in LiF. The creation efficiency of color centers and their thermal stability deduced from isochronal annealing experiments are discussed in terms of aggregation as well as recombination of electron and hole centers. In addition, profilometry measurements were performed quantifying ion-induced volume changes as a function of the beam parameters.

Recent progress in the development of concave Fresnel lenses for neutrons

The performance of a Fresnel-type compound refractive lens for focusing cold neutrons was evaluated. The lens consists of 44 pieces of compound Fresnel-type refractive lenses made of MgF2 single crystals. The intensity profile of the diffracted neutrons measured over the wavelengths of 9–14 Å was compared with the results of numerical simulations. Good neutron transmission of 0.85 for 9 Å neutrons and a focal length of 3.4 m for 16 Å neutrons were achieved.

X-ray photoemission study ofMgB2

A c-axis oriented thin film and a high density sintered pellet of MgB2 have been studied by x-ray photoemission spectroscopy, and compared to measurements from MgO and MgF2 single crystals. The as-grown surface has a layer which is Mg-rich and oxidized, which is effectively removed by a nonaqueous etchant. The subsurface region of the pellet is Mg-deficient. This nonideal near-surface region may explain varied scanning tunneling spectroscopy results. The MgB2 core level and Auger signals are similar to measurements from metallic Mg and transition metal diborides, and the measured valence band is consistent with the calculated density of states.

The temperature dependence of the A1g-mode Raman line of MgF2

The first-order Raman spectrum for the A1g mode of an MgF2 single crystal was measured at various temperatures from 301 to 973 K. The frequency shift and linewidth change significantly with increasing temperature. In order to analyse the temperature dependence of the frequency shift, first the authors calculated the anharmonic force constants of an MgF2 crystal. Then applying the lattice-dynamical perturbative treatment to the third- and fourth-order proper self-energy, they evaluated each contribution from the cubic and quartic anharmonic terms to the whole shift and could explain the frequency shift throughout the whole range of temperatures. They found that the cubic as well as quartic anharmonic terms play an important role in the temperature dependence of the Raman frequency shift for the A1g mode.

Color centers in irradiated MgF2

Abstract Color centers in rutile-structured MgF2 single crystals irradiated at 20K/360K by reactor neutrons are investigated spectroscopically at LNT. Four different types of the F-F vacancy bond in MgF2 are possibly identified to the observed absorption bands as due to the F2 centers; instead of previous assignments, the 300nm band to the F2(D2h), the 325nm band to the F2(C1), the 355nm band to the F2(C2v), and the 400nm band to the F2(C2h) centers.

Anisotropy effects on fracto-emission from MgF2 single crystals

An important component of fracto-emission is the emission of electrons (EE) accompanying the fracture of materials. In this letter we present measurements of EE accompanying the fracture of MgF2 single crystals loaded in three-point bend. In particular, we examine the effect of changing the crystal orientation relative to the loading direction on emission intensity. We find that fracture surfaces with different crystal orientations yield significantly different EE. We propose that this is due to differences in the density of defects produced by such a fracture.

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.

Remarks on extinction effects in MgF2 single crystals

Extinction effects are very often severe in ionic single crystals. Corrections for these effects depend mainly on theoretical models which describe primary extinction effects only poorly. In order to reduce the primary extinction, different thermal treatments were applied to MgF2 samples. γ-ray rocking curves were recorded to visualize the effects of these thermal treatments. The aptitude of a sample to give the best results in neutron and X-ray intensity measurements was decided on the basis of the shape of the γ-ray rocking curve profiles. The isotropic mosaic spread extinction parameter values obtained from γ-ray, X-ray and neutron techniques were of the same order of magnitude.

Cross-relaxation processes of polarized?-active nuclei in various crystalline solids

Cross-processes of neutron activated, polarized nucleiI with surrounding stable host nucleiS are observed in diverse solids, using an in-beam NMR technique where the polarizedI nuclei are produced by capture of polarized thermal neutrons. The polarization is determined by measuring the 0°–180° asymmetry of theβ-decay radiation of these nuclei. The following cross-processes are studied:8Li−6Li innatLi metal;8Li−7Li in7Li metal;8Li−6Li innatLiF (single crystal);8Li−93Nb in7LiNbO3 (single crystal);20F−19F in MgF2 (single crystal). Two applications are presented: a) Determination of the temperature dependence of the electrical quadrupole coupling constant of93Nb in LiNbO3; b) Observation of radiation induced point defects in MgF2 and their thermal annealing. All measurements are performed without any radiofrequency irradiation.