Theoretical calculation for the high pressure properties of alkali fluorides

Abstract

The overlap repulsive potentials of ion pairs (Li ⁺?F^?, Na⁺?F^? and K⁺?F^?) are calculated by means of generalized Heitler-London method and the numerical results are fitted in the Born-Mayer form, i.e. Dexp (—αR), Then the parameter D for each pair potential is modified to satisfy the crystal equilibrium condition at the experimentally known lattice constant. The redetermined repulsive potentials are applied to calculate the cohesive energies, bulk moduli, equations of state and NaC1–to-CsC1 structural phase transition pressures of LiF, NaF and KF crystals. The results obtained are compared with some other theoretical values and the available experimental data, and good agreement is reached.     

Oscillator strength of F2 − colour centres in LiF crystal

The absorption cross-section integral and the oscillator strength of the 960 nm absorption band due to F₂ ^– colour centres in LiF crystal is determined at room temperature from the absorption cross-section spectrum of the F₂ ^– centres obtained from saturable absorption studies. A comparison with previous results is made.     

Positron studies of hydrogen-defect interactions in proton irradiated molybdenum

Molybdenum single crystals are irradiated at 20 K with 6 MeV protons. The radiation damage and lattice defect annealing is studied by positron lifetime spectroscopy in the temperature range from 15 to 720 K. Loss of vacancies due to recombination with mobile interstitials is observed at 40 K (Stage I) in agreement with resistivity measurements. This is the first time Stage I is observed by positrons below 77 K. The implanted hydrogen decorates the vacancies around 100 K, which is consistent with a hydrogen migration energy in molybdenum:E _M ^H = 0.3–0.4 eV. Clustering of spatially correlated vacancies takes place in a wide temperature region below the usual vacancy clustering stage (Stage III). Stage III is observed at rather low temperatures (400–480 K) due to the very high vacancy concentration. Hydrogen bound to vacancies and vacancy clusters is released above 540 K, which puts an upper limit to the hydrogen binding energy:E _B ^H 1.4 eV. The present work emphasizes the advantage of employing a vacancy sensitive technique to study hydrogen in metals, where its intrinsic solubility is low. In such metals (as molybdenum) both the effective solubility and the effective mobility of hydrogen are strongly influenced by the presence of vacancies.     

Color centres in crystalline Lif films

Abstract

Policrystalline LiF thin films were produced on amorphous substrates at different temperatures. Preliminary optical measurements on F₂ and F₃ ⁺ aggregate color centres produced by electron irradiation were performed.     

Equation of state and thermal expansivity of LiF and NaF

The high-pressure and high-temperature behaviors of LiF and NaF have been studied up to 37 GPa and 1000 K. No phase transformations have been observed for LiF up to the maximum pressure reached. The B1 to B2 transition of NaF at room temperature was observed at ～28 GPa, this transition pressure decreases with temperature. Unit-cell volumes of LiF and NaF B1 phase measured at various pressures and temperatures were fitted using a P–V–T Birch–Murnaghan equation of state. For LiF, the determined parameters are: α₀ = 1.05 (3)×10^?4 K^?1, dK/dT = ?0.025 (2) GPa/K, V ₀ = 65.7 (1) Å³, K ₀ = 73 (2) GPa, and K′ = 3.9 (2). For NaF, α₀ = 1.34 (4)×10^?4 K^?1, dK/dT = ?0.020 (1) GPa/K, V ₀ = 100.2 (2) Å³, K ₀ = 46 (1) GPa, and K′ = 4.5 (1).     

Self-trapped exciton in alkali fluorides: ODMR study

The optical detection technique is used to observe magnetic resonance in the triplet state ³B_1u of the self-trapped exciton in LiF, NaF, KF. The strong anisotropy of the hyperfine interaction in the x?y plane is related to the more localized character of the electronic wavefunction. The magnetic dipolar contribution to the fine structure can be described by a Mollwo-Ivey type law in alkali fluorides and chlorides.     

Defect parameters for strontium fluoride

The ionic conductivity of dilute Sr₁?xYb_xF₂+x solid solutions has been studied as a function of temperature. For the migration enthalpy of fluoride interstitials the value 0.946 eV is obtained, and the value 0.56 eV for the dissociation enthalpy of neutral associates (Yb_SrF_i)^x. The latter value is concordant with the theoretical binding energy for this type of associate. A re-analysis of the conductivity data for SrF₂ leads to the value 2.70 eV for intrinsic defect formation, contrary to reported values which range from 2.2 to 2.3 eV.     

Self-consistent analysis of absorption and emission data forF A centres in alkali halides

Summary A theory which explains self-consistently the emission and the polarization rotation of transmitted light has been developed forF _A centres in crystals with cubic symmetry. Azimuthal dependences ofF _A luminescence in alkali halides are very informative for determining the electron-phonon interaction as well as the parameters related to the structural and optical quality for coulour centre lasers crystals. The theoretical results are applied to the interpretation of the luminescence properties ofF _A (Li) centres in KCl, RbCl and KF.     

Hybrid density functional study of optically active Er3+ centers in GaN

Understanding the luminescence of GaN doped with erbium (Er) requires a detailed knowledge of the interaction between the rare‐earth dopant and the nitride host, including intrinsic defects and other impurities that may be present in the host material. We address this problem through a first‐principles hybrid density functional study of the structure, energetics, and transition levels of the Er impurity and its complexes with N and Ga vacancies, substitutional C and O impurities, and H interstitials in wurtzite GaN. We find that, in the interior of the material, Er_Ga is the dominant Er³⁺ center with a formation energy of 1.55 eV, Er_Ga–V_N possesses a deep donor level at 0.61 eV which can assist in the transfer of energy to the 4f ‐electron core. Multiple optically active Er³⁺ centers are possible in Er‐doped GaN. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Photoionization processes in F aggregate centres of LiF crystal

Extensive studies have been carried out on the optical conversion of F and F-aggregate colour centres produced in lithium fluoride single crystal on gamma irradiation. Using 308 nm XeCl laser it has been shown that significantly large population build-up of F ₃ ⁺ centre and reduction in the population of undesirable F₂ centres can be achieved in gamma irradiated crystal at room temperature due to multistep photoionization processes. These and other investigations have provided a scheme for possible laser action based on F ₃ ⁺ colour centres in LiF crystal at room temperature.     

Lattice defects of V3Si single crystals studied by positron annihilation

The sensitivity of positrons to point defects created by the irradiation of V₃Si with neutrons is demonstrated. We found no indication of thermal vacancies by thermal equilibrium measurement up to 1273 K which indicates that the monovacancy formation enthalpy for V₃Si isH _1V ^F ≧(1.84±0.14) eV. Investigations within the range of homogeneity for excess vanadium suppot the idea that substitutional defects are the dominating defect type, whereas for excess silicon a direct confirmation of existing structural vacancies as the dominating defect type is given.     

Ultraviolet photoemission study of LiF

Photoemission from evaporated films of LiF were measured at photon energies of 10-27 eV. The photoelectron spectra exhibit features that can be identified as density-of-states structures in the valence and conduction bands of LiF. Regions of high density of states can be seen at ca. 3.3 and 7.8 eV above the vacuum level. The valence-band spectrum shows a doublet structure similar to the calculated density of states for the F^?2p band of LiF. The base width of this structure is found to be 4.6 ± 0.3 eV. The photoelectron spectra for photon energies > 15 eV indicate that the highest occupied states of the F^?2p band are located at 11.8 ± 0.3 eV below the vacuum level. The photoelectron spectra in the exciton region, however, show photo-emission from higher occupied states.     

Room-temperature deposition of transparent conductive Al-doped ZnO thin films using low energy ion bombardment

Al-doped zinc oxide (AZO) films are prepared on quartz substrates by dual-ion-beam sputtering deposition at room temperature (∼25°C). An assisting argon ion beam (ion energy E _i =0–300 eV) directly bombards the substrate surface to modify the properties of AZO films. The effects of assisted-ion beam energy on the characteristics of AZO films were investigated in terms of X-ray diffraction, atomic force microscopy, Raman spectra, Hall measurement and optical transmittance. With increasing assisting-ion beam bombardment, AZO films have a strong improved crystalline quality and increased radiation damage such as oxygen vacancies and zinc interstitials. The lowest resistivity of 4.9×10⁻³Ω cm and highest transmittance of above 85% in the visible region were obtained under the assisting-ion beam energy 200 eV. It was found that the bandgap of AZO films increased from 3.37 to 3.59 eV when the assisting-ion beam energy increased from 0 to 300 eV.     

Study of cation tracer diffusion in NaF

Cation tracer diffusion coefficients were measured in pure NaF crystals in the intrinsic ionic conductivity range (876–970 °C). The results can be rationalized satisfactorily in terms of contributions to the observed Na tracer diffusivities arising from both free vacancies and neutral vacancy pairs, the latter contribution amounting to about 53 per cent of the total Na diffusion at the highest measuring temperature. The best-fit defect parameters derived in an earlier conductivity study [21] from this laboratory on similar NaF crystals give for the free vacancy contribution D_v^*(Na) = 4·25 exp (?2·21 eV/kT) cm²s^?1. A combination of these D_v^*(Na) values with the present diffusion data yields for the vacancy-pair contribution D_p^*(Na) = 1·15 × 10⁸exp (?4·04 eV/kT) cm²s^?1. Comparison of the present findings with published values of the anion tracer diffusion coefficient in NaF showed that D_p^* (F) is 2·3 to 4·4 times larger than D_p^*(Na) over the temperature range of our observations, the difference between the two contributions increasing with decreasing temperature. When approximate account is taken of the temperature-dependence of the two pair correlation factors, this last result indicates that the anion jumps into the vacancy pair occur with a higher frequency, and increasingly so at lower temperatures, than do those involving the cations.     

UV absorption edge shift by doping alkali fluorides in fluoroaluminate glass

The effect on transmittance and reflectance in the vacuum ultraviolet (VUV) region by doping alkali fluorides (LiF, NaF and KF) has been investigated in a ternary fluoroaluminate (18BaF₂-37CaF₂-45AlF₃) glass. The absorption edge of the glass obeys the Urbach rule and was shifted monotonically towards higher energies by increasing the concentration of each alkali fluoride. The VUV reflection peak at 11.3 eV was not sensitive to the change of the concentration of dopants. The magnitude of the edge shift was 10-20 times larger than that expected from the additive law on the magnitude of absorption coefficients of the glass and alkali fluorides. An excitonic interaction similar to that observed in mixed crystals of alkali halides is suggested from the monotonic shift toward the absorption edges of the dopants. The weak sensitivity of the reflection peak upon doping supports that the excitonic state lies predominantly around the edge energies.     

Aggregate color center formation processes in lithium fluoride crystals after irradiation

Lithium fluoride crystals were irradiated by different doses of gamma photons at a temperature of 77 K. We measured the aggregation kinetics for the color centers with different annealing temperatures above the temperature of anion vacancy mobility. We show that the lifetimes of the vacancies decrease while the lifetimes of the F₂⁺ F_2^{+} centers increase as the irradiation dose increases. We explain these types of dependences based on the aggregation processes for color centers in the post-radiation period. We determine the time constants and energies (analogous to activation energies in the Arrhenius equation) for the various processes involving rise and fall in the concentration of aggregate color centers. Based on the experimental data obtained, we have established the processes forming F ₂ and F₃⁺ F_3^{+} centers in the post-radiation period. The F ₂ centers are formed when vacancies ν_a add to F₁^- F_1^{-} centers. Vacancies arising during irradiation of the crystal participate in their creation in the first fast stage. In the long final stage, vacancies are used which appear in the post-radiation period on occurrence of the reaction F₂⁺ F_2^{+} + H → ν_a + fluoride ion at the lattice site, where H is an interstitial fluorine atom. The F₃⁺ F_3^{+} centers are formed both by merging F₂⁺ F_2^{+} and F ₁ centers and as a result of addition of vacancies to F ₂ centers. In this case, vacancies are used that are generated not only during irradiation of the crystal but also in the post-radiation period. The rise in the concentration of F₃⁺ F_3^{+} centers occurs faster than the rise in the concentration of F ₂ centers.     

Thermoelectric power measurements in polycrystalline AgI: Cd system

Thermoelectric power of polycrystalline AgI: Cd system was measured as a function of temperature from 50 to 180°C. The heat of transport $\text{Q}{}^1$, intrinsic vacancy concentration c_o and ratio of interstitial to vacancy mobilities in the β phase were deduced under the assumption of no association. The heat of transport $\text{Q}{}^1$ was also temperature dependent in β-AgI: 0.27 eV at 80°C, 0.21 eV at 100°C and 0.17 eV at 120°C. It was estimated that the formation energy of the defect pair h_F was 0.66 ± 0.06 eV and the activation energies for motion of vacancies and interstitials were 0.55 ± 0.03 and 0.38 ± 0.03 eV, respectively. These approximately agree with data reported up to date in single and poly- crystal β-AgI. The heat of transport of vacancies was approximately equal to the activation energy of vacancy migration.     

Off-axis configuration of FA(II) centres in alkali halides

Abstract

The experimental values of the off-axis angle of F_A(II) dipoles in KCl:Li⁺, RbCl:Li⁺ and KF:Li⁺ are in good agreement with those obtained from theoretical estimates of the off-centre displacements of the Li⁺ ion calculated with the shell model potentials. On this basis, one can quantitatively predict the off-axis behaviour of F_A(II) centres from the calculated off-centre positions of small impurity ions in alkali halides.     

Electrical and optical spectroscopy on ZnO:Co thin films

Magnetic oxide semiconductors, for example the highly transparent and intrinsically n-type conducting zinc oxide doped with the 3d transition metal Co (ZnO:Co), are promising for the emerging field of spintronics [1]. We investigated n-conducting ZnO:Co thin films with a Co content of nominal 0.02, 0.20, or 2.00 at. %. The substitution of Co cations in the tetrahedral sites of wurtzite ZnO with Zn was confirmed at low temperature by the 1.877 eV photoluminescence between crystal field split d-levels of Co²⁺ (d⁷) ions. Based on theoretical studies, it is predicted that the formation of electron levels with zinc interstitials (I_Zn) or hole levels with zinc vacancies (V_Zn) is necessary to induce ferromagnetism, whereas the formation of electron levels with oxygen vacancies (V_O) is detrimental for ferromagnetism in ZnO:Co [2]. Cobalt generates a hole level in ZnO [3]. We investigated the generation of electron levels in n-conducting ZnO:Co in dependence on the Co content by means of deep level transient spectroscopy (DLTS). However, because of the ambiguous categorization of deep defects in n-conducting ZnO (V_O, I_Zn), an optimization of defect-related ferromagnetism in ZnO:Co is not possible at the moment. PACS 78.30.Fs; 91.60.Ed; 91.60.Mk     

Ionic conductivity study of CaF2: Nd single crystals

The electrical ionic conductivity of unirradiated and irradiated CaF₂: Nd crystals in the range of 60 to 800°C has been measured. The conductivity plot is basically divided into four parts, i.e., intrinsic and extrinsic unassociated, extrinsic associated, and extrinsic segregated regions. Activation energy (for unirradiated samples) in the extrinsic unassociated region is in the range of 0.69 to 1.20 eV depending on the doping concentration while for the intrinsic region, it is of the order of 1.89 eV. The conductivity in the extrinsic unassociated region increases with increase of Nd content in the sample. Also, the conductivity in the extrinsic region forγ-irradiated sample is higher than that for unirradiated one. In the intrinsic region, however, the conductivity is independent of dopant concentration orγ-irradiation. From these results it is surmised thatF ⁻ interstitials are the charge carriers in this region for CaF₂: Nd³⁺ system.