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The influence of erbium ion concentration on the optical properties of BaF2:ErF3 crystals was investigated. Four ErF3 concentration (0.05, 0.08, 0.15 and 0.5 mol% ErF3)-doped BaF2 crystals were obtained using the Bridgman technique. Room temperature optical absorption in the 250–850 nm spectral range was measured, and the photoluminescence (PL) and decay times were also investigated. The Judd–Ofelt (JO) approximation was used, taking into account four absorption peaks (at 377, 519, 653 and 802 nm). The JO intensity parameters, Ωt (t = 2, 4, 6), were calculated. The influence of the ErF3 concentration on the JO parameters, branching ratio, radiative transition probability and radiative lifetime were studied. The obtained results were compared with measured values and with those reported in the literature. Under excitation at 380 nm, the well-known green (539 nm) and red (668 nm) emissions were obtained. The calculated and experimental radiative lifetimes were in millisecond range for green and red emissions. The intensity of the PL spectra varied with the Er3+ ion concentration. The emission intensity increased linearly or exponentially, depending on the ErF3 concentration. Under excitation at 290 nm, separate to the green and red emissions, a new UV emission band (at 321 nm) was obtained. Other research has not reported the UV emission or the influence of ErF3 concentration on emission behavior.

Pure and PbF2 doped CaF2 crystals were obtained in our crystal research laboratory. The influence of PbF2 concentration on the color centers induced by γ-and x-rays irradiation were studied. Two broad absorption bands with maxima peaked at 410 and 730 nm appear in the optical absorption spectra; the spectra are quite similar in band structure for all samples and for the both radiation. Different bands showed different responses to the irradiation and to the long-term recovery. The most stable color centers are (Fi0) and the dimer P-P (Pb+(1)-Pb2+). These absorption bands remain stable even for trace amount of Pb2+ ions, so CaF2:PbF2 is not a good material for applications in which the new optical absorption bands are undesirable.

In the last decades, fluorites crystals (MeF2, Me = Ca, Ba, Sr, Pb) doped with trivalent rare-earth ions (RE3+) have attracted attention of research community due to their various applications such as: lasers, quantum electronics, scintilators or photosensors in PET (positron-emission-tomography) scanner. The goal of this work is to investigate the spectoscopic properties of gamma irradiated BaF2:CeF3 crystal. In order to investigate the quality of the as-grown crystal the dislocation density was estimated. The dislocation density is around 1.3×105 disloc./cm2. After irradiation with gamma rays, the absorption spectra reveal the appearance of a new absorption band in the 400-600nm region. The induced absorption bands depend on the gamma rays dose and the peaks intensities increase with the dose of radiation.In the last decades, fluorites crystals (MeF2, Me = Ca, Ba, Sr, Pb) doped with trivalent rare-earth ions (RE3+) have attracted attention of research community due to their various applications such as: lasers, quantum electronics, scintilators or photosensors in PET (positron-emission-tomography) scanner. The goal of this work is to investigate the spectoscopic properties of gamma irradiated BaF2:CeF3 crystal. In order to investigate the quality of the as-grown crystal the dislocation density was estimated. The dislocation density is around 1.3×105 disloc./cm2. After irradiation with gamma rays, the absorption spectra reveal the appearance of a new absorption band in the 400-600nm region. The induced absorption bands depend on the gamma rays dose and the peaks intensities increase with the dose of radiation.

The spectroscopic properties of the gamma irradiated BaF2:Er3+ crystals were studied. BaF2:0.2 mol% ErF3 crystal was grown using the vertical Bridgman method. The Judd-Ofelt intensity parameters, Ωt (t = 2,4,6) for f-f transitions of Er3+ ions were determined from optical absorption spectra recorded before and after gamma irradiation at various doses. The influence of gamma irradiation dose on the Judd-Ofelt parameters and on the radiative transition probability, on the branching ration and on the radiative lifetime was also investigated. The predicted radiative lifetimes were compared with those measured and with those reported by other authors. Apart from the green and red emissions, arising under excitation at 378 nm, a new UV emission band around 321 nm was observed under excitation at 290 nm. This emission was not reported before. Using the Fuchtbauer-Ladenburg method, the emission cross section and optical gain parameter of the red and green emission band were calculated.

ErF3 doped BaF2 crystals were obtained using the vertical Bridgman technique. Five crystals, with 0.05, 0.08, 0.1, 0.2 and 0.5 mol% ErF3 added to the starting material, have been investigated. The optical absorption spectra reveal the characteristic bands of the Er3+ ions. The distribution of Er3+ ions along the crystals was studied using the optical absorption spectrum of every samples (14–18 slices) cleaved from the as-grown crystals. The Er3+ ions are not uniform distributed along the BaF2 crystals. The segregation coefficient, k, was calculated using the so-called optical absorption method and the Scheil relationship between the dopant concentration and crystal growth conditions. The calculated segregation coefficient of the Er3+ ions ranges from 0.76 to 0.98, for ions with trigonal (C3v) sites, and from 0.6 to 0.86 for clusters, depending on the ErF3 concentration. In the investigated temperature range, only one type of dielectric relaxation has been observed. This relaxation, with activation energy of 0.54 eV is associated with trigonal NNN type (C3v) centers. The charge compensating defects were discussed taking into account both the optical absorption spectra and the calculated number of NNN dipoles whose relaxation were observed.

BaF2 crystals were obtained in our Crystal Research Laboratory using Bridgman method. Various raw materials were used to test the influence of the trace amount of Pb2+ in BaF2 crystals on the radiation damage. BaF2:1 mol % PbF2 crystal was also obtained in order to use it as comparative sample. The optical absorption spectra after irradiation are quite similar in band structure for all samples, except the intensities of the absorption bands. After gamma rays irradiation many new absorption bands have been observed in the entire investigated spectral domain (200 ÷ 900 nm) and these absorption bands remain stable during the time even for trace amount of Pb2+ ions. The magnitude of the induced absorption bands depends on the oxygen content in BaF2. The most intense bands is at ∼220 nm that corresponds to Pb3+ ions and ∼470 nm that corresponds to Pb+ ions and F center. The BaF2 crystals are used as fast scintillator involving emission in the UV domain around 195–220 nm and as window over a wide wavelength domain. The absorption band in the UV can attenuate the fast component. Thus, the study of the sensitivity to the radiation damage of the BaF2 crystals are practical importance. The lead ions need to be carefully investigated, because they can be in the raw material and during the crystal growth process are incorporated in BaF2 crystals, changing the optical properties of the crystal. The radiation damage depends more on the level of the oxygen content than on the trace amount of PbF2.

YbF-doped BaF crystals with various concentrations of YbF (0.05, 0.1, 0.17, 0.2 mol) have been grown using the conventional Bridgman method. Dielectric relaxation is used to study the charge compensating defects in BaF crystals with low YbF concentration. The optical absorption spectra reveal the existence of both Yb (in UV domain) and Yb (in IR domain) ions. In the investigated temperature range (150–320 K) we have observed only one type of dielectric relaxation. This relaxation with activation energy eV is associated with trigonal-type () centers. The charge compensating defects are discussed while taking also into account the optical absorption spectra and the calculated number of NNN dipoles whose relaxation is observed.

A small diameter Bridgman equipment was used to investigate the influence of two crucible materials (fused silica and graphite) and two coatings materials (Si3N4 and CaCl2) on the quality of the multicrystalline silicon ingots, with focus on the ingot detachment from crucible, grain size and orientation and precipitate formation. It was shown that using a CaCl2 encapsulating substance can lead to grains larger in size and more oriented along the axial growth direction for both the graphite and the fused silica crucible, although the detachment of the crystal from the crucible is only possible if a fused silica crucible was used. The layer of the SiC precipitates at the graphite crucible wall was found to be thinner if the CaCl2 encapsulant was used.

A comparative study of the gamma irradiated BaF2 and CaF2 crystals obtained under different growth conditions, using the Bridgman technique, is presented. A special attention was attributed to the influence of trace amount of three impurities: lead, ytterbium and oxygen ions, on the γ–rays induced color centers. The small amount of Pb ions comes from the different raw materials used. The oxygen content is due to the low vacuum level during the growth process. The crystals with Yb content were intentionally doped with a small amount of YbF3. The CaF2 crystal is more sensitive to a trace amount of lead ions than the BaF2 host. The oxygen, in addition of the lead ions in the BaF2 host, is more harmful than the trace amount of PbF2. The small amount of YbF3 is useful in eliminating the color centers in visible range. The induced color centers are stable in time. The lead ions contaminated BaF2 and CaF2 crystals are not good materials for applications in which the radiation induced absorption bands are undesirable.

Microcrystalline BaSn1−xTixO3–0.5mol%GeO2, x = 0, 0.1, 0.3 (BSTx–0.5Ge) ceramics (0.1–0.75 μm) were prepared by a conventional solid-state reaction method in glazed alumina crucibles. XRD was used to examine the crystalline structure and SEM technique to analyze the microstructure. The density was measured by the Archimedes’s method. The sintered ceramics exhibit at room temperature, a cubic symmetry, with a gradual decrease of unit cell dimensions. GeO2 addition improves crystallization and densification, such as for BS–0.5Ge, at the sintering temperature of 1190 °C, the relative density is 82.7%. Higher titanium doping will lead to denser ceramics: 94.5% for BST3–0.5Ge. For the dielectric constant, a steady value is recorded for all the compositions in a wide range of frequencies (50–500 KHz) and temperatures (150–250, 300–450 K).

Microcrystalline BaTi1−xSnxO3–0.5 mol%GeO2 x = 0, 0.1, 0.3, 0.5 (BTSx–0.5Ge) and BaTiO3 (BT) ceramics (1–0.5 μm) were prepared by a conventional solid-state reaction method. The crystalline structure of the samples was examined using XRD, the microstructure was analyzed by means of electron microscope and the density was measured by the Archimede’s method. The sintered ceramic disks have a tetragonal symmetry for BT, pseudo cubic for BTS1–0.5Ge and cubic symmetry for the other studied materials, with a gradual increase of unit cell dimensions. Small addition of GeO2 can improve the density of BT ceramics: 97.9% for BT–0.5Ge, and 96.21% for pure BT. The highest degree of densification in the case of tin doping is achieved for BTS1–0.5Ge (96.93%). The formation of a liquid phase can lead to an anomalous grain growth, and in the case of BT–0.5Ge the grains are completely surrounded by a frozen eutectic melt. For the dielectric constant, while increasing the Sn concentration, the TC gradually shifts towards lower temperatures, and the peak of this transition becomes broader. The lowering of TC is mostly due to the concentration of tin ions and in a much delicate way to Ge ions. Anomalies are noticed for the orthorhombic transition, where the permittivity is higher than the same transition of the matrix (BT), with a shift towards higher temperatures. The BTS3–0.5Ge and BTS5–0.5Ge are the most stable compositions in terms of dielectric behavior, since in the temperature range 200–400 K, e′r is almost constant. Therefore, these compositions can be used for devices that operate over a wide range of temperatures and frequencies.

Calcium fluoride crystals doped with YbF3 and codoped with NaF, LiF were grown using the vertical Bridgman method. The effect of Na+ and Li+ ions on the optical and dielectric spectra was studied. The relaxation parameters (the activation energy for dipole reorientation and relaxation time constant) and the number of the dipoles, ND, that contribute to the dielectric relaxation have been determined from the dielectric spectra. The correlation between ND and the optical spectra is also discussed. A comparison of our results with those obtained by other authors is also given. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Calcium fluoride crystals doped with ErF 3 and PbF 2 have been grown using the vertical Bridgman method. Temperature and frequency dependence of the complex dielectric constant have been measured at seven audio frequencies (1–100 kHz) over the temperature range of 150–300 K. The activation energy for dipole reorientation and the relaxation time constant, τ 0 , has been determined from the observed relaxation processes. The effect of Er 3+ , Pb 2+ ions and the X-ray irradiation on the optical and dielectric spectra was studied. A comparison of our results with those obtained by other authors is also given.

Calcium fluoride crystals doped with YbF 3 and PbF 2 -, NaF- and LiF-codoped were grown using the vertical Bridgman method. Transparent, high quality, with various high Yb 2+ contents in the as-grown crystals has been obtained using a special procedure. The optical absorption spectra reveal the characteristic ultraviolet (UV) absorption bands of the divalent Yb ions. Influence of the codoping with Pb 2+ , Li + and Na + ions on the absorption spectra and on the Yb 2+ ions content has been studied. High-intensity emission bands in the near-UV spectral region, not reported before, have been observed for excitation by 230 nm. A comparison of our results with those obtained by other authors is also given.

Transparent colorless Ca1−xYbx F2+x (x=0.0007–0.016) crystals were grown using the vertical Bridgman technique. In order to obtain efficient Yb3+–Yb2+ conversion in the as-grown crystals, a special procedure has been developed. Room temperature optical absorption spectra reveal the characteristic UV absorption bands of the Yb2+ ions in the as-grown crystals, with intensities more than 10 times higher than those reported by other authors using various conversion procedures. The influence of YbF3 content and of codoping with Pb2+ ions on the absorption and emission spectra has been studied. Room temperature emission bands in the near UV (not reported before) and in the visible spectral domain have been observed. The emission intensity depends on the dopant concentration. A comparison of our results with those obtained by other authors is also given.

Calcium fluoride crystals doped with YbF 3 were grown using the Bridgman technique. The optical absorption spectra reveal the characteristic UV-absorption bands of divalent Ytterbium. The absorption coefficient of divalent Yb is more than ten times higher than that of trivalent Ytterbium. This shows that the developed crystal growth process achieves a good Yb 3+ –Yb 2+ conversion. The concentration distribution of the Yb 2+ and Yb 3+ ions among two of the samples was investigated using the optical absorption method. Taking into account the relationship between the optical absorption coefficient and the concentration of the impurities in the sample, the segregation coefficient of the two types of ytterbium ions has been calculated. From our calculations, we obtained the results that the segregation coefficient of the Yb 3+ ions in a CaF 2 host is ≈1, and for Yb 2+ ions is

The chemical homogeneity of CaF2:Pb crystals, grown by using the vertical Bridgman method, is numerically investigated. The FIDAP commercial code is used to study the heat transfer, convection, and...

Calcium fluoride crystals doped with PbF2 and codoped with LiF, NaF have been grown using the vertical Bridgman method. The frequency (1-100 kHz) dependence of the real and imaginary parts of the complex dielectric constant, e1 and e2, have been studied in the temperature range 160-300 K; the activation parameters of the relaxation process have been determined for various doped and X-irradiated calcium fluoride crystals. The optical absorption spectra of the crystals are also investigated. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature and frequency dependence of the complex dielectric constant give information about the relaxation processes and permits the determination of the activation energy and the reciprocal frequency factor of the relaxation time. Pure and various concentration PbF2-doped CaF2 crystals have been grown using the vertical Bridgman method. The thin disks cut from the crystals have been X- ray irradiated. Room temperature optical absorption spectra were recorded for pure, doped and x-ray irradiated samples. Capacitance and dielectric loss measurements were performed on the samples over the temperature range 150-300 K at 1-100 kHz, and then the imaginary part of the dielectric constant has been calculated. The dielectric properties have been measured before and after X-irradiation, the activation energy and the reciprocal frequency factor have been calculated in order to characterize the observed relaxations.

In shaped crystals besides the usual structural defects, such as dislocations, faceting, and grain boundaries, so-called “voids” have been observed almost always; this defect affects especially the optical quality of the crystals. The influence of growth conditions on size and distribution of trapped gas bubbles in shaped sapphire crystals has been studied both experimentally and by numerical simulation of the fluid flow in the meniscus. The influence of the pulling rate, of the ambient temperature and of the shaper design has been taken into account.

Calcium fluoride crystals doped with ErF 3 and YbF 3 have been grown using the vertical Bridgman method. The real and imaginary parts of the complex dielectric constant, e' and e, have been measured at five audio-frequencies (1-100 kHz) over the temperature range 150-300K. From the observed relaxation processes the activation energy for dipole reorientation and the relaxation time constant. τ 0 , have been determined. A comparison of our results with those obtained by other authors is also given.

Ribbon and rod sapphire pulling has been performed in three different crystal growth equipments in order to study the effect of the installation, of the atmosphere, of the die shape, of the feed material and of the pulling rate on the distribution, number and diameter of the characteristic voids (micro-bubbles) in the crystals. The location of the bubbles in the crystals depends on the die geometry; however, in most cases they are essentially located close to the crystal periphery and then can be efficiently removed by lapping. After statistical analysis of the results, it is demonstrated that the number of gas moles incorporated in the crystals, inside the voids, is totally independent of any growth parameter. It is also shown that the bubble diameter depends only on the pulling rate. Consequently, for a given pulling rate, the number of bubbles auto-adjusts in order to satisfy the constant molar gas incorporation. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Shaped sapphire crystals obtained by the edge-defined film fed growth (EFG) method almost always contain a typical defect, the so-called “voids”, which affect the optical quality of the crystals. The origin of the trapped voids is not yet clarified. One of the hypotheses is concerned with gas formation in the crystal growth setup chamber. The goal of this paper is to study the species of the gases resulting from various chemical reactions between the different components of the crystal growth apparatus, such as the charge (Al 2 O 3 ), the crucible and the shaper material (Mo or W) and the screens and other details made from graphite (C). In order to obtain the partial pressure of the gas species in the growth chamber the thermodynamic calculations were performed using the complex equilibrium code—Gemini2—that minimizes the free enthalpy formation. Carbon monoxide (CO) becomes the most important gas species when one of the growth setup components is made from graphite. The various gas species resulting from different reactions are trapped in the crystal during the growth process and affect the crystal quality. In order to eliminate the contamination of the crystals, this type of calculation should be profitable for a better choice of the crucible, shaper and other set-up detail material.

The influence of the die geometry and various growth conditions on the fluid flow and on the solute distribution in EFG method has been studied using numerical simulation. The commercial FIDAP software has been used in order to solve the momentum and mass transfer equations in the capillary channel and in the melt meniscus. Two types of shaper design are studied and the results are in good agreement with the void distribution observed in rod-shaped sapphire crystals grown by the EFG method in the various configurations.

The effect of Bridgman furnace configuration on the temperature field, melt convection and the solute distribution in the resulting crystal are experimentally and numerically analyzed for the semiconductor diluted alloy solidification. The governing equations of the heat and mass transfer are solved by using the finite element method with help of the commercial software FIDAP ® . Two different solidification experiments of Ga 1− x In x Sb ( x =0.01 and 0.04) are simulated in order to compare the numerical results for thermal, velocity and solute fields. The central objective of the work is to give the conditions for which a more uniform distribution of the solute in the crystal can be obtained. It is found that crystals obtained in conditions of a strong convective regime in the vicinity of the solid–liquid interface are more homogeneous radially and on a significant length than the crystals for which solidification occurred in a quasi-diffusive regime. The results, in terms of axial and radial segregation, are compared to experimental chemical analysis.

Various concentrations of YbF3 –doped and NaF or PbF2 co-doped CaF2 crystals were grown using the conventional Bridgman method. Transparent colorless crystals were obtained in graphite crucible in vacuum using a shaped graphite furnace. The crystals have been cooled to room temperature using an established procedure. Room temperature absorption spectra have been obtained using a Shimadzu 1650PC spectrophotometer. Photoluminescent properties in IR spectral range were analyzed using a spectrofluorimeter Horiba Fluorolog 3. An IR laser diode at 932 nm was also used an directly injected in the equipment. The emission spectra are influenced by the concentration of co-dopant added to the melt, and the excitation wavelength. The high emission peak at 979 nm overlaps with the absorption peak. The highest intensity in the IR emission (around 1029 nm) is obtained for CaF2:0.72 mol% YbF3 crystal by excitation at 932 nm (diode lamp).

The effect of physical parameters and geometrical configurations on the interface shape in semi-transparent BaF 2 crystals grown by the vertical Bridgman process are studied both experimentally and numerically. Values of thermo-physical parameters are suggested in the case of BaF 2 .

The objective of this work is the study of the effect of thermal oscillations on the interface velocity in the vertical Bridgman configuration. In order to study this effect, a transient numerical simulation of the heat transfer and melt convection is performed in a simplified geometrical model. The relation between the thermal signal amplitude and the amplitude of the velocity oscillations is investigated. When the oscillation period is varied, an asymptotic evolution of the velocity oscillation amplitude, with a cut-off period, is observed. It is shown that latent heat of solidification has a huge effect on the velocity amplitude, but not on the cut-off frequency.

Shaped GaSb crystals (ribbons and rods) were grown by pulling from the melt using a non-wetted floating shaper to control the crystal cross-section shape and size. Graphite, which does not react chemically with molten gallium antimonide, has been chosen as shaper and crucible material. Etch pit density and electrical properties of the ribbons were determined.

The characteristics of the melt flow in the meniscus in edge-defined film-fed growth have been studied using a 3-D model. The influence of the growth conditions (pulling rates and ambient temperature distribution) on the void distribution in rod sapphire crystals has been studied modeling the melt flow in the meniscus. The flow behavior obtained can explain the observed void distribution.

Three-dimensional convection caused by a slightly tilted crucible and a small deviation from a constant temperature gradient furnace during the vertical Bridgman growth is analysed using a novel three-dimensional finite-volume code (STHAMAS 3D). The obtained results show that three-dimensional flow is induced by these deviations from a perfect axisymmetry. It is also shown that these effects are stronger for the semi-transparent materials than for the opaque ones.

The temperature distribution and the solid-liquid interface shape during semi-transparent crystal growth have been studied by modelling a vertical Bridgman technique, using a pseudo-transient approximation in an ideal configuration. The heat transfer equation and the boundary conditions have been solved by the finite-element method. It has been pointed out that the optical absorption coefficients of the liquid and solid phases have a major effect on the thermal field, especially on the shape and location of the crystallization interface.

The effect of some growth conditions on the solidification process of the semitransparent and opaque materials grown by the vertical Bridgman method are studied using the finite-element analysis.

BaF2:0.17 mol%YbF3 crystal was grown using the conventional Bridgman technique. The optical absorption spectra reveal the existence of both Yb2+ and Yb3+ ions with the characteristic bands of the trivalent ions in the near-IR region and of the divalent ions in the UV spectral domain. The concentration distribution of the Yb2+ and Yb3+ ions among the sample was investigated using the optical absorption method. The segregation coefficient of the two types of Yb ions has been determined. From our calculations the segregation coefficient of the Yb3+ ions in BaF2 host is ≈ 0.7, and of the Yb2+ ions is ≈ 0.6.

Three concentrations of YbF 3 -doped BaF 2 crystals were grown using the conventional Bridgman method. Transparent colorless crystals were obtained in graphite crucible in vacuum (∼10−1 Pa) using a shaped graphite furnace. The crystals have been cooled to room temperature using an established procedure. In order to study the etch pits morphology and the dislocations density the etching method have been used. This method consists in immersing the cleaved sample in 2N HCl at 60°C for 2 minutes. Small etch pits are developed at the emergence points of the dislocations. The etch pits have triangular or hexagonal shapes, depending on the dopant concentration. The dislocations density depends on the dopant concentration.

YbF3-doped and 1 mol% PbF2 co-doped CaF2 crystals were grown by vertical Bridgman method using a shaped graphite furnace in vacuum. Dielectric spectra and optical absorption spectra were measured in order to study the influence of Pb2+ ions on the charge compensating defects formation. The influence of Pb2+ ions on the various charge compensating defects depend on the YbF3 concentration. For low YbF3 concentrations the influence is stronger than for concentrations higher than 1 mol% YbF3. Dielectric relaxation in double doped (Yb,Pb):CaF2 crystals in order the study the charge compensating defects formation was not reported before.

Using a radiative and conductive heat transfer model for shaped semi-transparent crystals, the influence of various growth conditions on the stress field, namely the von Mises stress, has been studied in sapphire crystals obtained by the EFG method.

The effect of some thermal and geometrical-size configurations on the interface shape and on the temperature distribution in semi-transparent fluoride-type crystals grown by the vertical Bridgman method are studied by finite-element analysis.

The influence of the die geometry and of the thermal conductivity of materials on the solid-liquid interface shape in Edge-defined Film-fed Growth has been studied. Der Einflus der Geometrie des Formgebers und der thermischen Leitfahigkeit des Materials auf die Form der Flussig-fest Grenze bei der Zuchtung nach der E.F.G.-Methode wird analysiert.

CaF2 crystals doped with various concentrations of PbF2 (0.4, 0.5, 1 and 2 mol%) were grown in vacuum, in a shaped graphite furnace using the vertical Bridgman method. The optical absorption spectra reveal the four characteristic UV absorption bands (labeled A, B, C and D) of the Pb2+ ions. As the PbF2 concentration increases, the structure of the bands become clearly visible, that is characteristic for the ns 2 ions in various hosts. High intensity emission bands in the near UV spectral region have been observed. The dependence on the Pb2+ concentration of the optical absorption and emission of the Pb2+:CaF2 crystals were not reported before.

Various concentrations YbF3 -doped CaF2 and Pb2+ - codoped crystals were grown using the conventional Bridgman method. Transparent colorless crystals were obtained in graphite crucible in vacuum (∼ 10−1 Pa) using a shaped graphite furnace. The crystals have been cooled to room temperature using an established procedure. In order to study the etch pits morphology and the dislocations density we used the chemical etching method. This method consists in immersing the cleaved sample in 4NHCl at 60°C for 5 minutes. Small pits are developed at the emergence points of the dislocations. The etch pits have hexagonal shapes for pure CaF2 crystal and triangular sahpes for doped crystals. The dislocations density depends also on the dopant or on the codopant concentration.

Shaped sapphire single crystals almost always contain a typical defect: small bubbles also known as “micro-voids”. The goal of this paper is to review the existing literature and give a survey of our recent work on this specific defect. As a conclusion to our review we propose a mechanism for bubble formation and its occurrence and distribution in shaped sapphire. The bubbles contain CO which originates as a result of complex reactions between the molten alumina, Mo crucible and graphite pieces. The CO dissolves in the molten alumina and is probably present as a saturated solution whatever the growth conditions. Then the dissolved gas is rejected at the solid–liquid interface and bubble nucleation occurs where the concentration of CO is highest. Therefore, the location of the bubbles in the crystal is governed by the hydrodynamics in the meniscus, which include Marangoni convection. The bubble is finally incorporated in the crystal when it reaches a critical size which depends only on the growth rate. When a central capillary channel is used, the bubbles are located in a layer close to the surface of the crystal from where they can be easily removed by polishing. Also, all the literature agrees on the fact that interface destabilization at high velocity results in the presence of bubbles throughout the entire crystal and should be avoided.

The CaF2:0.83 mol% ErF3 crystal was grown using the vertical Bridgman method. The Judd‐Ofelt intensity parameters Ω2, Ω4, and Ω6 for f−f transitions of Er3+ ions were determined from the optical absorption spectrum. The predicted radiative lifetimes are compared to those of Er3+ transitions reported in various hosts. Apart from the green and red emission, reported by other researchers, an UV (314 nm) emission band was observed, not reported before.

Various concentrations YbF3 ‐doped CaF2 and Li‐ (Na)‐ codoped crystals were grown using the conventional Bridgman method. The effect of Na+ and Li+ ions on the dielectric spectra was studied. The activation energy and the reciprocal frequency factor have been calculated in order to characterize the observed relaxations. An anomaly of e1 behavior (not reported before) has been observed which is assigned with a phase transition of the order‐disorder type.