Negative Vacancies Research Articles

Extended Hückel molecular orbital calculation of defects in LiH cluster crystals

AbstractAn LCAO‐matrix method (extended Hückel MO) is applied for the calculation of the intrinsic and defect properties of LiH crystals. The ionic clusters of Li+ and H− involving 8, 27, and 64 ions are considered as well as the atomic clusters of Li0 and H0. Defect properties of negative and positive ion vacancies, interstitials H− (H) and H0(H), and substitutional H0 at the anion and cation sites (H and H) are discussed from the result of the calculation taking the experimental results into account.

Thermoluminescence of KBr: Tl excited by ultraviolet radiation and X-rays

Abstract Thermal glow curves of KBr: Tl specimens, prepared by crystallization from aqueous solutions, are examined after their exposure either to ultraviolet radiation or x-rays. Specimens excited by ultraviolet radiation exhibit a glow peak at 370 K and a broad and pronounced peak around 450 K. Increase in thallium concentration reduces the strength of the former peak but favours the growth of the latter. A strong glow peak develops at 410 K when the specimens are subjected to prolonged x-irradiation. These specimens also exhibit a prominent glow peak in the 450 K region at higher thallium concentration. The study of the growth of these two peaks with the duration of x-irradiation indicates that the intensity of 450 K peak reaches saturation in the early period of x-irradiation. On the other hand, the strength of 410 K peak attains saturation only after prolonged x-irradiation. It is tentatively proposed that the exposure to x-rays for longer duration causes the formation of negative ion vacancies near ...

Notizen:Phosphorescence Decay in Thallium-Doped Ammonium Chloride

Abstract The phosphorescence decay rates of thallium-doped ammonium chloride (NH4Cl:Tl) phosphors, prepared by crystallization from aqueous solution, have been studied at room temperature for near-ultraviolet emission. The effects of impurity concentration as well as thermal and/or mechanical treatment on the decay rates have been examined. Phosphorescence centres consisting of a Tl+ion and a nearby negative ion vacancy are suggested to be responsible for the observed luminescence decay. The changes in the decay characteristics after pretreatments are explained on the basis of the location of the centres in normal and distorted regions of the host lattice.

Relaxation about the vacancy in diamond

The relaxation about a vacancy in diamond results from modifications in the local chemical bonding. Further relaxation occurs to lower the electronic degeneracy through the Jahn-Teller effect. Both effects are considered by using a semi-empirical molecular-orbital cluster-model of the vacancy to calculate the relaxation of the lattice about it. An 1E ground state of the neutral vacancy is found with a Jahn-Teller energy of 460 meV coupled to tetragonal modes. The negative vacancy is found to have an 4A2 ground state. Both give rise to substantial, totally symmetric, inwards relaxation.

Effect of PLZT composition on its switching characteristics

Significant correlations were observed between the switching time of PLZT and their chemical composition. The switching time and the closely related relaxation time are analyzed in the light of molecular capacitances residing in the ferroelectric domains. Space charges in the ferroelectric domains are attributed to the creation of highly negative charged cation vacancies created by the substitution of La for Pb in the perovskite structure.

Low-temperature radiation damage in Mn-doped RbMgF3

Defects introduced by high energy electron irradiation at low temperatures have been studied in manganese doped RbMgF3. At these low temperatures negative-ion vacancies are not mobile and only the various radiation induced interstitials and X2- (VK) centres are mobile. It is found that at 15K where X2- centres are not mobile the F band (due to negative vacancies which have trapped electrons) increases almost linearly with radiation dose and optical bands due to X2- centres and impurities yield the only obvious absorption between 200 nm and 1000 nm. No sign of radiation defect perturbed Mn2+ optical transitions can be observed for the radiation doses investigated. However, when the irradiation is done at 77K both F centres and radiation defect-Mn2+ complexes are formed. Since vacancies are not mobile at 77K it is suspected that interstitial defects perturb the Mn2+ ions.

Molecular orbital theory of electronic transitions at the vacancy in diamond

Abstract In diamond, a series of optical absorption lines lying between 1·66 and 3·0 eV, which have been labelled GR1 to GR8, are commonly observed. These lines are fairly sharp, and the symmetry of some of the states from which they are derived have been deduced from piezo-spectroscopic studies. Furthermore, photoconductivity indicates that the absorption lines GR2 to GR8 are associated with hole production. A model for the absorption lines is proposed and investigated. This model assumes the transitions take place around a vacancy, the vacancy being represented by a defect molecule as originally suggested by Coulson and Kearsley (1957). The lines GR2 to GR8 arise from transitions from the neutral vacancy to states associated with the negative vacancy and a hole located in one of the nearby molecular orbitals. The relative intensities of the transitions are calculated. Using results of a cluster model, the electronic orbitals occupied by the hole are located at the top of the valence band and are found t...

On the nature of green strength of compacted metal powders

ing the valency s ta te of Fe in NiO and the defect s t ruc ture of such a highly al loyed solid solution Nil_ xFexO. However, the higher act ivat ion energy of 291 • 40 kJ mo1-1 for the alloy compared to 192 + 25 kJ tool -~ for pure Ni, shows that vacancy movement in the alloy sca le is energe t ica l ly more difficult than in the pure Ni sca le . Any t r iva len t i ron ions in the sca le would be posi t ively charged and so coulombically a t t r ac ted to the negative vacanc ies . This would make the motion of vacancies energe t ica l ly more difficult and so would suggest that some of the i ron in the sca le i s t r iva len t . It would, therefore , appear on this bas is that the higher oxidation ra te of the alloy is due to the vacancies gene ra t ed by the d isso lved t r iva lent i ron ions.

Enhancement of F-centre colouration during plastic deformation of alkali halides

Stimulated growth of F-centres was observed by irradiating alkali-halide single crystals during uniaxial deformation at controlled rates. Quantitative monitoring of the phenomena was performed for pure KCl at L.N.T. as well as in the range from R.T. to 106°C. Distinct forms for the kinetics of growth were obtained in the two regions. A correlation was also established between the growth of excess colouration and the evolution of stress, in the higher temperature range. The data on the kinetics, stress dependence and temperature dependence suggest that the production of excess F-centres arises from the radiolysis of vacancy-producing debris (possibly, dislocation dipoles) that are also effective agents of hardening. The process is limited by thermally activated annealing, with an activation energy appropriate for free-vacancy migrations. It is suggested that the limiting reaction is a recombination of the excess negative-ion vacancies with an antimorph defect, contained in a complementary type of dislocation debris.

Electrical conductivity and dielectric properties of NaCl, KCl, and KBr single crystals near their melting temperatures

The dielectric constant (K′) and loss (K″) of single crystals of NaCl, KCl, and KBr have been measured in the frequency region 104 to 107 Hz and in the temperature range 25 to 8 °C below their melting temperatures. The electrical conductivity of these crystals has also been measured at 104 Hz near their melting temperatures. The activation energy values for conduction in the temperature vicinity of the melting points of these solids are calculated to be 2.15, 2.35, and 2.55 eV for NaCl, KCl, and KBr, respectively; these values are larger than those reported in literature. Two points are emphasized: 1) the electrical conduction in these crystals near their melting temperatures appears to be mainly due to the generation and movement of negative ion vacancies and 2) these crystals do not show any dipole relaxation effects up to a frequency of 107 Hz.

Quantum yield for photoionization of F centers at room temperature

The transfer of charge following absorption of a light pulse by colored KCl at room temperature in an electric field was studied as a function of the field. At sufficiently high fields the charge should reach a saturation value from which the quantum yield for photoionization η can be obtained directly. Saturation is difficult to achieve experimentally in KCl; however, in 2 of 12 samples studied, saturation was clearly established and η was found to be 0.49±0.07 and 0.17±0.03. Taking η to lie between about 0.2 and 0.5, the trapping cross section for negative ion vacancies lies between 50% and 80% of the capture cross section at room temperature.

Dielectric properties of manganese-or sodium-doped CaF 2 single crystals

The dielectric constant ( K), loss (tan δ) and conductivity (σ) of single crystals of CaF 2:Mn (with 0.2, 1 and 5 mol % of manganese content) and CaF 2:Na (with 0.2 and 1 mol% sodium content) have been measured as a function of frequency in the region 10 2−10 7 Hz and temperature 30°C−450°C. K of all these samples at 30°C (room temperature) is independent of frequency. The dielectric constant of CaF 2 does not appreciably change with Mn or Na doping. All these samples are found to exhibit dipolar relaxation effects at room temperature. The dielectric constant as a function of temperature (above 30°C) exhibits two stages of increase: (i) a slow frequency-independent rise which is attributed to an increase in ionic polarization and (ii) a fast frequency-dependent rise at higher temperatures which is ascribed to space-charge polarization. The dipole in Na-doped samples (negative-ion vacancy attached to a substitutional Na + ion) is found to have an activation energy of 0.43 eV. The nature of the dipole in Mn-doped CaF 2 is not clear. log σ versus 1/ T plots for these solids show that the activation energy for conduction at the high-temperature region is found to decrease considerably with Mn or Na doping of CaF 2 (compared to the value in pure CaF 2). An attempt has been made to understand these results.

Formation and stability ofV−andVAlcenters in MgO

${V}^{\ensuremath{-}}$ and ${V}_{\mathrm{Al}}$ centers in MgO have been shown to possess identical electronic $g$ values at low temperatures and this paper reports on the distinction between the two centers. Although these two centers have almost identical EPR spectra, their electron-nuclear-double-resonance (ENDOR) spectra are clearly different. Optical-absorption measurements, supported by ENDOR and EPR, were used to study the formation and annihilation of these two centers. Trapped-hole centers have also been observed which are charge compensated by silicon and off-axis aluminum. The ${V}^{\ensuremath{-}}$ center cannot be produced in all MgO single crystals, and we attribute this to impurity dependence. The high cross section for ${V}^{\ensuremath{-}}$-center production by electron irradiation, \ensuremath{\sim} ${10}^{2}$-${10}^{3}$ b (as compared to only a few barns for its antimorph, the negative-ion vacancy) indicates that the predominant formation mechanism is that of ionization rather than a knock-on process. This assertion is confirmed by the relative ease with which ${V}^{\ensuremath{-}}$ centers are formed by low-energy x rays. Irradiations with photons possessing energies of 1.25 MeV down to a few keV reveal a lack of energy dependence for ${V}^{\ensuremath{-}}$ center production.

Ionic Conductivity of Rubidium Iodide Crystals

The ionic conductivity of pure and strontium-doped rubidium iodide crystals was measured as a function of temperature. Eight transport parameters were calculated from the conductivity results by a nonlinear regression computation. The values obtained for the transport parameters were: Schottky defect formation enthalpy hs = 2.1 eV, entropy ss = 0.5 × 10−3 eV/deg; positive-ion vacancy migration enthalpy Δh1 = 0.60 eV, entropy Δs1 = 0.14 × 10−3 eV/deg; negative-ion vacancy migration enthalpy Δh2 = 1.6 eV, entropy Δs2 = 0.13 × 10−3 eV/deg; association enthalpy of strontium ions and positive-ion vacancies χ = 0.58 eV, entropy η = 0.22 × 10−3 eV/deg. Rubidium carbonate was found to have a negligible solid solubility in rubidium iodide, so that conductivity measurements could not be made on anion-doped crystals. The negative-ion vacancy migration parameters are thus not as accurately determined as the other transport parameters.

Lattice disorder studies in low-temperature nitrogen-implanted silicon

Abstract The amount of lattice disorder and its depth distribution in silicon crystals implanted to a total dose of 1014 nitrogen ions/cm2 at 80 keV have been investigated as a function of implantation temperature (25-300°K) and of dose rate (5-500 na/cm2), using the helium-backscattering technique. Evidence has been found for a strong reverse-annealing stage at the lower temperatures (< 80°K), indicating the existence of mobile defects in silicon down to 25°K. The depth distribution and dose-rate dependcnce of damage resulting from this reverse annealing stage implies the existence of an ionization-induced mobile defect, probably the negative vacancy.

RadiationF-center production in LiF at and above room temperature

Abstract Radiation defect production in LiF has been measured at and above room temperature as a function of dose, dose rate, temperature, impurity content and pre-irradiation sample deformation. Emphasis has been placed on the growth rate of the 250 nm F band and the ratio α445/αF2 where α445is the absorption coefficient at the peak of the 445 nm F-aggregate band. Comparison of the behavior of LiF with that of KCl indicates that the dose rate and temperature dependences of LiF can be accounted for in terms of the mobility of the F+ center (negative ion vacancy containing no electron) in the same way as they are for KCl and other alkali halides. On the other hand, the effect of trace impurities on the defect production rate appears to be different in LiF than in KCl. For example, among several impurities only Mg produces a significant increase in the F-center production efficiency. Also, no rapidly saturating early stage coloration can be observed for any impurity.

New electron-excess color centers in KCl: Effect of O 2− and SO 2−4 ions

Two new absorption bands, along with the T bands previously reported by Ring and Grossweiner, have been observed in additively colored KCl. The bands are developed by bleaching F centers at high temperature in relatively pure KCl, but may be produced rapidly at room temperature in colored KCl crystals containing O 2−, or in sulfate-doped KCl which has been additively colored. It is proposed that the development of these bands is accelerated by the presence of negative ion vacancies introduced as charge compensation for the divalent anions, and that the new centers are aggregates containing one or more F centers with a vacancy pair. The segregation coefficient of K 2SO 4 in KCl, between crystal and melt, was found to be 1.0 × 10 −3.

Conversion and equilibrium between F‐ and F2‐centers in X‐irradiated KCl. I. Slow warm up experiments

AbstractMeasurements are reported in which the number of F2‐centers produced by warming crystals previously X‐irradiated at liquid nitrogen temperatures is studied as a function of temperature and crystal purity. These data are analysed in terms of a diffusion model in which the energy of migration and the initial F+‐center distribution are adjustable parameters. It is shown that the best fit is obtained with an energy of motion of negative ion vacancies (F+‐centers) of 0.66 eV and a vacancy distribution such that the F+‐centres tend to be within five nearest neighbor distances of the F‐centers.

The F centre in MgF2. I. EPR and ENDOR

The magnetic resonance absorption at g=2 in irradiated MgF2 is positively identified in this paper as arising from F-centres. EPR and ENDOR investigations have determined the hyperfine constants and tensor axis orientations for the first six fluorine shells. These agree in detail with the interactions expected from an electron trapped at a negative ion vacancy. The large nearest neighbour interaction (A/sub ///=382.83 MHz, Aperpendicular to =288.23 MHz) coupled with the smaller second and third neighbour interactions have been used to reconstruct in detail the resolved EPR spectra when H0 is parallel to the crystal symmetry axes. A semiempirical wavefunction obtained from the observed contact interactions for all six shells predicts that neighbour cation ENDOR transitions for the 10% abundant 25Mg nuclei should be observed near the first neighbour fluorine transitions.

Ionic Conductivity of Potassium Bromide Crystals

The electrical conductivity of single crystals of potassium bromide was measured as a function of temperature, and as a function of the concentration of calcium bromide or potassium carbonate impurity. The results were analyzed on the assumption that the observed conductivity was due entirely to the motion of separated anion and cation vacancies (Schottky defects), with Coulomb interactions between charged defects. Agreement between measured and calculated conductivities at all temperatures showed that this assumption was reasonable. The values obtained for transport parameters were: Schottky-defect formation enthalpy hs = 2.53 eV, entropy Ss = 10.3k; positive ion vacancy migration enthalpy Δh1 = 0.65 eV, entropy Δs1 = 1.89k; negative ion vacancy migration enthalpy Δh2 = 1.22 eV, entropy Δs2 = 7.30k; association of calcium ions and positive ion vacancies enthalpy x = 0.61 eV, entropy η = 2.23k, where k is Boltzmann's constant.