CaF2 Single Crystals Research Articles

Optical studies in gamma irradiated Mg doped CaF2 single crystals

Pure and Mg doped CaF2 single crystals grown by the Bridgman method were irradiated with gamma rays (γ-rays) for doses ranging from 97Gy to 9.72KGy. The pristine samples showed minimal absorption indicating the purity of the samples. The γ-irradiated pure CaF2 crystals showed prominent and strong absorption with a peak at ~374nm besides three weak ones at ~456, 523 and 623nm. However γ-rayed Mg doped crystals showed a prominent absorption with a strong peak at ~370nm and a broad one at ~530nm. The absorption indicated the generation of F and F-aggregate centers in the irradiated crystals. The photoluminescence (PL) emission spectrum of both pure and Mg doped crystals showed prominent emission at ~390nm when they were excited at ~250nm. Also, when the samples were excited at 323 and 363nm strong emissions were observed at ~430 and 422nm respectively. The optical absorption and PL intensities were found to increase with increase in dose.

Crystal growth and scintillation properties of Nd:CaF2

Nd3+ doped CaF2 single crystal scintillator has been investigated. We tried to grow 1%, 5%, 10%, 20%, 30% and 40% Nd3+ doped CaF2 single crystals by the simple melt-solidifying method. Powder X-ray diffraction (XRD) patterns were measured to identify the phase of all the samples. The XRD patterns of all the samples were similar to CaF2. Those samples are compared in terms of their X-ray-excited radioluminescence spectra, transmittance, α-ray-excited decay time and light yield. When the X-ray is used for excitation, luminescence is observed in the VUV region. Transmittance of the crystals is more than 70% at wavelengths longer than about 180nm. In the decay kinetics, the fast components of the samples are distributed in less than 25ns time range and the slow components of sample are distributed in more than 90ns. These decay times became shorter with increasing Nd3+ concentration. They are related to the Nd3+ 5d-4f VUV emission. The light yields of samples are distributed in 5–2500 photon/5.5MeV α-ray and decrease with increasing Nd3+ concentration.

ChemInform Abstract: Thermodynamic Properties of Pd-Y and Pd-Gd Intermetallic Phases.

The Gibbs free energies, enthalpies, and entropies of formation were determined for seven Pd–Y and for six Pd–Gd intermetallic compounds from electromotive force measurements on fluorine concentration cells in the temperature range from 973 to 1073 K using CaF2 single crystals as solid electrolytes. The compound formation in both systems is characterized by extremely exothermic mixing effects. The results are compared with calorimetric data and with the theoretical predictions of the Miedema model. The rise of the activities of Y and Gd over the phase fields of Pd3Y and Pd3Gd is described by a statistical model of disorder. The extraordinary stability of the alloys is attributed to a transfer of valence electrons from the electropositive component to the electron gas of the alloy.

Desorption of atomic particles and surface modification of fluorides under electron irradiation

Low-energy electron stimulated desorption (ESD) and surface modifications of CaF2 and BaF2 single crystals were experimentally studied at different surface temperatures. A technique based on the dependence of the surface composition on the electron irradiation dose was applied to determining the ESD cross section. The surface composition was measured by Auger electron spectroscopy (AES). Surface topography was examined by atomic force microscopy (AFM). The ESD mechanisms for fluorides were discussed.

Birefringence simulations for annealed ingots of the -and - growth calcium fluoride single crystals with consideration of creep deformation

We developed an analysis system for simulating birefringence of an annealed ingot of CaF2 single crystal caused by the residual stress after annealing process. The analysis system comprises the heat conduction analysis that provides the temperature distribution during the ingot annealing, the stress analysis to calculate the residual stress after ingot annealing, and the birefringence analysis of an annealed ingot induced by the residual stress. The finite element method was applied to the heat conduction analysis and the stress analysis. In these analyses, the temperature dependence of material properties and the crystal anisotropy were taken into account. In the residual stress calculation, we considered the time-dependent nonlinear deformation behavior of a material called creep. In the birefringence analysis, the distributions of optical path difference were calculated by using average stress method with consideration of the crystal anisotropy. We can perform the birefringence analysis of an ingot of CaF2 single crystal with any growth direction, using this analysis system, and we performed the analyses of the crystals with the <001> and <111> growth directions. From these analyses, we obtained reasonable results of optical path difference in comparison with the experimental results.

Potential energy - induced nanostructuring of insulator surfaces by impact of slow, very highly charged ions

We have recently shown that the impact of individual slow highly charged ions is able to induce permanent nano-sized hillocks on the surface of a CaF2 single crystal. The experimentally observed threshold of the projectile ion potential energy necessary for hillock formation could be linked to a solid-liquid phase transition (nano-melting). In this contribution we report on similar nano-sized surface modifications as a result of the potential energy of impacting highly charged ions for other surfaces.

Optical absorption in CaF2 nanoceramics

The optical characteristics of two Russian-made CaF2 ceramic samples are compared to those of single-crystal CaF2. The results indicate that the ceramic possesses high optical quality and a small absorption coefficient (∼10-3 cm-1 at a wavelength of 1.07 μm) and is suitable as a material for optical components. Experimental evidence is presented for spatial modulation of the thermally induced depolarisation in the ceramic.

Study of Chromium Activity in the Cr-Fe-N System by Galvanic Cell Method

In the present work, the Cr-Fe-N alloys with different compositions were synthesized by nitriding the Cr-Fe powder mixtures in the purified nitrogen gas (101,325 Pa) at 1473 K for 2 weeks. The phase relationships in the synthesized alloys and the alloys equilibrated at 1173 K were carried out by X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. The nitrogen content in the alloys equilibrated in the nitrogen gas (101,325 Pa) at 1173 K was analyzed using the inert-gas fusion thermal conductivity (IGFTC) method. The thermodynamic activities of Cr in Fe-Cr-N alloys were measured in the temperature range 973 to 1123 K using the solid-state galvanic cell technique with CaF2 single crystal as the solid electrolyte. Based on the measured EMF values, the chromium activities in the alloys were calculated with respect to pure Cr with bcc structure as the standard state. The effect of nitrogen on Cr activities in the Cr-Fe-N system was examined by comparing the experimental results of the Cr activities in the Cr-Fe and Cr-Fe-C systems.

Quantification of dislocation structures at high resolution by atomic force microscopy of dislocation etch pits

Atomic force microscopy of dislocation etch pit structures is a convenient means of characterising the dislocation structure in etchable materials at high resolution for dislocation spacing extending down to 25 nm . This is demonstrated for single crystals of CaF2. The local deformation zone generated around nanoindents at ambient temperature and the low-angle boundaries generated in the bulk during uniaxial compression at elevated temperatures are presented as examples.

Luminescence‐detected EPR of oxygen‐fluorine vacancy complexes in CaF2

AbstractIn several CaF2 single crystals grown by the Bridgman method and doped with CaO or in addition with SrF2 or NaF a luminescence band between 470 nm and 600 nm could be excited at 212 nm, its peak wavelength depending on the doping. With photo‐luminescence (PL) detected EPR five spin triplet centres were identified. Their axial fine structure constants D varied from 87 mT to 690 mT whereby the most intense spectra had the smallest D value. Theoretical calculations of the fine structure tensors and superhyperfine interactions show that the most intense and probable triplet centre consists of a pair of an OF– on a F– site next to a nearest neighbour F– vacancy which, compared to the well‐known F centre, is occupied by an excited electron in a shallow orbital and coupled to the 2p hole at the oxygen to form a S = 1 excited state. This centre can be excited optically at low temperature to a metastable new configuration where the F centre jumps into a 〈111〉 position opposite the OF–. The other triplet centres have probably also an O––F centre nearest neighbour configuration but have in addition a cation impurity or vacancy nearby. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Thermodynamic properties of calcium titanates: CaTiO3, Ca4Ti3O10, and Ca3Ti2O7

The chemical potentials of CaO in two-phase fields (TiO2 + CaTiO3), (CaTiO3 + Ca4Ti3O10), and (Ca4Ti3O10 + Ca3Ti2O7) of the pseudo-binary system (CaO + TiO2) have been measured in the temperature range (900 to 1250) K, relative to pure CaO as the reference state, using solid-state galvanic cells incorporating single crystal CaF2 as the solid electrolyte. The cells were operated under pure oxygen at ambient pressure. The standard Gibbs free energies of formation of calcium titanates, CaTiO3, Ca4Ti3O10, and Ca3Ti2O7, from their component binary oxides were derived from the reversible e.m.f.s. The results can be summarised by the following equations: CaO(solid) + TiO2(solid) → CaTiO3(solid), ΔG° ± 85/(J · mol−1) = −80,140 − 6.302(T/K); 4CaO(solid) + 3TiO2(solid) → Ca4Ti3O10(solid), ΔG° ± 275/(J · mol−1) = −243,473 − 25.758(T/K); 3CaO(solid) + 2TiO2(solid) → Ca3Ti2O7(solid), ΔG° ± 185/(J · mol−1) = −164,217 − 16.838(T/K). The reference state for solid TiO2 is the rutile form. The results of this study are in good agreement with thermodynamic data for CaTiO3 reported in the literature. For Ca4Ti3O10 Gibbs free energy of formation obtained in this study differs significantly from that reported by Taylor and Schmalzried at T = 873 K. For Ca3Ti2O7 experimental measurements are not available in the literature for direct comparison with the results obtained in this study. Nevertheless, the standard entropy for Ca3Ti2O7 at T = 298.15 K estimated from the results of this study using the Neumann–Koop rule is in fair agreement with the value obtained from low-temperature heat capacity measurements.

A study of the transport of thermal acoustic phonons in CaF2 single crystals and ceramics within the subterahertz frequency range

A study of the transport of thermal acoustic phonons in CaF2 single crystals and ceramics within the subterahertz frequency range

Laser spectroscopy of Eu3+ cubic centers in the CaF2 bulk single crystal

The optical transitions 5D0, 1 → 7FJ (J = 0, 1, ..., 6) of Eu3+ cubic centers in the CaF2 single crystal are investigated using combined excitation and emission spectroscopy at different time delays after the excitation pulse. The energies of the Stark sublevels of the 7FJ ground states are determined.

Evolution of dislocation structure and deformation resistance in creep exemplified on single crystals of CaF 2

Single crystals of CaF 2 oriented along 〈 111 〉 for glide on three equally stressed slip systems were used to study the evolution of the dislocation structure during deformation at temperatures from 873 to 1315 K. Structure quantification was done by the etch pit method using atomic force microscopy for high resolution. Evolution begins with free dislocations forming a cellular structure. The average spacings of free dislocations and of cell boundaries are generally close to their stress-dependent steady state values. The subgrain structure develops more slowly with strain. As it superimposes on the free dislocations, the crystals work harden and the creep rate decreases. The rates of change of the characteristic spacings of subgrain and cell boundaries, of free dislocations and of dislocations in subgrain boundaries are formulated as functions of stress, strain and the empirical steady state values of spacings. Combining the laws of structure evolution with those of deformation kinetics in the framework of the composite model, the creep behavior is modeled in a large range of homologous temperatures in consistence with the general picture of plasticity of crystalline materials.

Thermodynamics of CVD Synthesis of Multiwalled Carbon Nanotubes: A Case Study

Galvanic cells with CaF2 single crystal as electrolyte were utilized to determine the thermodynamic quantities of the solid-state process: delta NdNi5 (mu m) + MWCNT = {MWCNT-[NdNi5(nm)](delta)} where multiwalled carbon nanotubes (MWCNT) react with NdNi5 which is the catalyst of the chemical vapor deposition (CVD) synthesis of MWCNTs through the decomposition of a light hydrocarbon. It was shown elsewhere that the nucleation and growth of a MWCNT occurs by the nanodusting of the micrometer size powder of the unsupported catalyst. The composite {MWCNT-[NdNi5(nm)](delta)} is a nanoparticle of catalyst bonded to one of the ends of the MWCNT. The thermodynamic quantities of this process combined with those available for graphite to MWCNTs transformation and for decomposition of a light hydrocarbon, CH4, for instance, provide to obtain a complete picture of the thermodynamics of the CVD synthesis: CH4(g)+delta NdNi5(mu m) = {MWCNT-[NdNi5(nm)](delta)} + 2H(2)(g). At the standard state, the synthesis becomes feasible at T> 800 K. Between 300 and 1300 K, Delta H-r(T)theta and Delta S-r(T)theta are both positive, changing from 81 to 20 kJ mol(-1) and from 87 to 58 J K-1 mol(-1), respectively. To derive the thermodynamic quantities of formation of {MWCNT-[NdNi5(nm)](delta)}, the formation of NdNi5 was also studied by making use of a suitable CaF2 electrolyte cell. Between 600 and 1000 K, the formation of {MWCNT-[NdNi5(nm)](delta)} is always favored while Delta H-f(T)theta and Delta S-f(T)theta are positive but scarcely dependent on T (Delta H-f(T)theta approximate to 7 kJ mol(-1) Delta S-f(T)theta approximate to 12 J K-1 mol(-1)). A microscopic modeling of the process leading to the formation of {MWCNT-[NdNi5(nm)](delta) also been presented and discussed.

Deformation created by Berkovich and Vickers indenters and its influence on surface morphology of indentations for LiF and CaF2 single crystals

The study of the deformation zone around the indentations made by Berkovich and Vickers methods on LiF and CaF2 single crystals revealed several discrepancies in surface morphology. One of them is the presence of well-pronounced crack formation for Berkovich indentations and a total lack of that for the Vickers ones made at the same load on CaF2 crystals. The investigation of fine surface relief of indentations using atomic force microscopy revealed a faceted relief of the surface of Berkovich indentations as distinct from the smooth surface of the Vickers ones for both LiF and CaF2 crystals.Taking into account the peculiarities of each of the used methods, it was concluded that the geometry of indenters here has a major influence. The calculations showed that the trihedral Berkovich indenter with a smaller apical semi-angle against the tetrahedral Vickers indenter with a bigger semi-angle, on the one hand, and the smaller tip radius of the Berkovich indenter in comparison with the tip radius of the Vickers indenter, on the other hand, are the two factors which cause larger deformation of material under the Berkovich indenter as compared with the Vickers one. This fact leads to specific peculiarities of the deformation process and, as a result, to the appearance of specific surface morphology of indentations.

Refinement of thermodynamic data on GaN

Although GaN is one of the important electronic materials of this decade, thermodynamic data for this compound are not known with sufficient reliability. The limited information available is not internally consistent. Measured in this study are high-temperature heat capacities using a differential scanning calorimeter and Gibbs energies of formation employing a solid-state electrochemical technique. The solid-state cell was based on single-crystal CaF2 as the electrolyte and Ca3N2 as the auxiliary electrode to convert the nitrogen chemical potential established by the equilibrium between Ga and GaN into an equivalent fluorine potential. The heat capacity of GaN at ambient pressure can be represented by the equation: CoP / J mol−1 K−1 = 74.424 − 0.00106T + (46720/T2) − (685.9/T0.5), in the temperature range from 350 to 1075 K. The standard Gibbs energy of formation of GaN in the range from 875 to 1125 K can be expressed as: ΔfGo/ J mol−1 (±465) = −128,749 + 115.029 T. This corresponds to a decomposition temperature of 1119 ± 4 K for GaN in pure nitrogen at standard pressure. On the basis of these new measurements and a critical assessment of information that is available in the literature, a refined set of data for GaN in the temperature range from 298.15 to 1400 K is presented.

Comparative spectroscopic and laser properties of Yb3+-doped CaF2, SrF2 and BaF2 single crystals

We present the spectroscopic properties and room-temperature cw tunable laser operation of Yb3+-doped CaF2, SrF2 and BaF2 single crystals grown and studied in the same conditions. Emission cross sections, lifetimes, laser thresholds, laser slope efficiencies and laser wavelength tuning ranges are compared. It appears that Yb3+-doped BaF2 might be more promising for diode-pumped high power laser operation.

Emission Spectroscopy of Eu-Doped CaF2 under Static and Dynamic High Pressures

The time-resolved emission spectra of the CaF2(Eu) crystal subjected to impact-induced shock compression in the inelastic region have been measured up to 31.6 GPa and the effect of shock and release waves on the Eu emission spectrum has been recorded. The Eu2+ spectrum centered at ∼425 nm shifts toward the longer wavelength with a shock pressure of up to ∼15 GPa, but no further shift has been observed at higher pressures. Emission spectra under static pressure have also been measured up to 10.2 GPa and compared with those under shock compression. A blue Eu2+ emission at ∼425 nm and a sharp orange Eu3+ emission at ∼590 nm are observed under static pressure. A large pressure shift of ∼2.2 nm/GPa is observed for the blue emission at pressures of up to ∼10 GPa, but the orange emission spectrum hardly moves against pressure. The Hugoniot determined for the CaF2(Eu) single crystal is Us=5.15+1.18Up.

Determination of the standard Gibbs free energies of formation of barium zirconates in the temperature range (1073 to 1273) K

The standard molar Gibbs free energies of formation of BaZrO 3, and Ba 2ZrO 4 have been determined by solid-state galvanic cells involving a single crystal CaF 2 as a solid-state electrolyte. The e.m.f. measurements were performed in the temperature range (1073 to 1273) K. The average values of the standard molar enthalpy and the standard molar entropy of formation in the temperature range covered by the e.m.f. measurements have been also estimated. The results have been compared with those obtained by other authors.