Farbzentren in Ammoniumhalogeniden

Color centers produced by X-rays in ammonium halides at various temperatures between 20°K and room temperature have been investigated by means of paramagnetic resonance and by optical methods. Two kinds of paramagnetic defects were found to be predominant, the self-trapped hole (V _K-center) and another electron deficiency center involving a NH₃ ⁺-radical. The electronic structure of theV _K-center is the same as in the alkali halides, except that the orientation of the molecular axis is along [100] instead of [110]. The kinetics of the thermally activated motion of theV _K-centers and of their recombination with electrons has been studied. The electronic structure of the second center was derived from the hyperfine spectrum of the paramagnetic resonance. The rotation of the NH₃ ⁺ ion and its connection with the order-disorder transition in NH₄Cl has been studied.     

Spin-lattice relaxation of the Vk-center in LiF

The angular and temperature dependences of the spin-lattice relaxation (SLR) rate of V_k-centers in LiF doped with Mg or Ag have been investigated. In the temperature interval 4.2–100 K the results can be fitted by the formula $\text{τ}{}^{\mathrm{?1}}\text{= A(θ)T + Be}{}^{\mathrm{<mtext>?\Delta </mtext><mtext>T</mtext>}}$ with A(0°) = 0.11 sec^?1K^?1, A(90°) = 1.3 sec^?1K^?1, B = 3 × 10⁵sec^?1 and Δ = (175 ± 15)K.A mechanism for the SLR is considered, assuming the modulation of the hyperfine interaction by phononinduced transitions between the ground and excited states of the resonant molecular vibrations of the V_k-center. This mechanism is found to explain the value, the temperature dependence and the isotropy of τ^?1 in the interval T = 20–100 K.The one-phonon SLR mechanisms of the V_k-center in the T < 10 K region are discussed.     

Coloration under two-phonon excitation of KCl crystals containing halogen impurities

Color centers induced in KCl:I and KCl:Br crystals by N₂ laser irradiation have been studied at 9 K. It is found that the absorption spectra observed in both crystals are similar to that obtained in undoped KCl X-rayed at low temperatures. Resulting color centers are of the intrinsic lattice involving the F- and the H-centers in host KCl.     

Muonium centers in the alkali halides

Muonium centers (Mu) in single crystals and powdered alkali halides have been studied using the high-timing-resolution transverse field μSR technique. Mu has been observed and its hyperfine parameter (HF) determined inevery alkali halide. For the rocksalt alkali halides, the HF parameter A_μ shows a systematic dependence on the host lattice constant. A comparison of the Mu HF parameter with hydrogen ESR data suggests that the Mu center is the muonic analogue of the interstitial hydrogenH _i ⁰ -center. The rate of Mu diffusion can be deduced from the motional narrowing of the nuclear hyperfine interaction. KBr shows two different Mu states, a low-temperatureMu ^I -state and a high-temperatureMu ^II -state.     

Pressure dependence of F-center hyperfine interactions

We report a study of the effects of intense hydrostatic stress (10 Kbar) on the ENDOR spectra of F-centers in several alkali halides. In LiF, KCl, and KBr, the first shell contact hyperfine interaction constant, a_I, increases with compression, while the second shell term, a_II, decreases. In LiCl, however, da_II/dP and da_I/dP are both less than zero. We explain our experimental results, including the anomaly cf LiCl, with a model involving (1) ionic motion in a non-constant spin-density gradient and (2) ion-motion-induced perturbation of the F-center envelope function.     

M-Zentrenbildung in röntgenbestrahltem KCl

The equilibrium betweenF- andM-center concentration in KCl after X-irradiation at temperatures between 77 °K and 298 °K has been studied. At low temperatures (<200 °K) onlyF-centers which are formed statistically as nearest neighbours give aM-center. At higher temperaturesF-centers within a certain sphere of interaction with a radiusr formM-centers. The temperature dependance of this radiusr has been evaluated. The equilibrium constantK depends on the temperature in the form K=K₀exp(-Q/kT). The activation energy was determined to be Q=(0.5±0.1) eV.     

Effects of pressure on magnetic properties of F-centers in rubidium halides

Room temperature magnetic resonance measurements have been made on additively colored RbCl and RbBr and on X-irradiated RbH at pressures up to 8 kbar. In RbCl and RbH pressure shifts of the F-center isotropic coupling constant a₁ were obtained from changes in the resolved EPR hyperfine structure. Pressure shifts of first and second shell hyperfine coupling constants of F-centers in RbBr were found from high pressure ENDOR spectra. Effects of the pressure induced polymorphic phase transformations in RbCl and RbBr are discussed. No evidence was found for a structure transformation in RbH. Room pressure ENDOR measurements on additively colored RbI were made at 77°K. The first and second shell hyperfine coupling constants are reported. All results accord well with the behavior of F-centers in other alkali halides.     

VK centers in RbCaF3: Lattice distortion in the tetragonal phase

Intrinsic V_K centers have been produced in single domain RbCaF₃ crystals by irradiation at 77 K. The twisting of the CaF₆ octahedra about the tetragonal axis as a result of the 196 K phase change is readily detected in the V_K center EPR spectra. Analysis of these spectra required dividing the six distinct V_K center sites on a given octahedron into two classes and determining the spin Hamiltonian parameters independently for each class.     

Neutral beam sputtering of positive ion clusters from alkali halides

Neutral argon atom beams of 15 keV energy have been used to sputter alkali halides and the ejected positive ions have been analysed in energy, mass and angular distribution.

The use of a neutral beam, rather than an ion beam, minimizes surface charge and the deflection of ejected ions by electrostatic interaction with a charged incident beam.

A cluster component of the form K₂Cl⁺, K₃Cl⁺ ₂ and higher members of the series is found for all alkali halides studied.     

The theory of diffusion-limited recombination of donor—acceptor pairs

The asymptotics of the quasi-steady kinetics of diffusion-limited recombination of donor-acceptor (DA) pairs has been developed. The theory is based on the probability of DA pair recombination according to the relation w₀ exp [-(r/r_B)], where w₀ is constant, r is the separation between defects and r_B is the half Bohr radius of the more diffuse wavefunction. The reaction rate equation (for the thermal activation mechanism) includes the reaction constant as follows: k = 4πr_B[E_a/kT + ln (w₀r^2_B/D₀) + 1.54 + 2 K₀ (α)], α = 2r₀(w₀/D)$\text{1}\text{2}\text{,}$ where E_a is the activation energy, and K₀, I₀ are the modified Bessel functions. The quasi-steady recombination radius is introduced. The theory developed has been compared with the zone one. The interpretation of the activation energy and frequency factor have been presented for the diffusion model. An extension of the theory developed by taking into account Coulomb interaction between defects has also been presented. The applicability of the theory to the recombination of V_k centers with electronics centers is alkali halides has been shown. It is assumed that the theory is applicable to other centers of hole nature (H, proton, low symmetry exciton i.e. V_k + electron).     

Modes of vibrations due to (U-center + additive cation impurities) in alkali halides

Local and resonant modes due to hydride ions in various alkali halides containing additive cation impurities have been computed by the Green function technique. Local vibrations due to U-centers in alkali halides have O_h symmetry. When one of the six nearest neighbour cations is replaced by an additive impurity, the site symmetry of the system is lowered from O_h to C_4v. The phonon Green functions matríx is analysed according to the irreducible representation of the point group symmetry pertaining to the substitutional impurity. We have considered the vibrations of the hydride ion and all its six nearest neighbours. Analytical expressions have been derived for various modes of vibrations. Using group theory the 21 × 21 matrix has been block diagonalized into various irreducible representations. The effect of mass changes and the changes in short-range force constants have been taken into account. The computed results of the localized modes have been compared with the available experimental results. Good agreement has been found. Theoretical results on resonant modes are also displayed, which will be of use in future experiments on these systems.     

Optical transitions of Ag− centers in alkali halides

Ag^? centers in alkali halides give rise to a strong absorption band in the 300 nm region (formerly called “B band”). Its resolved triplet structure in CsCl suggests that it corresponds to the C band of the isoelectronic In⁺ center. Two very weak bands are found in several alkali halides in the 400 nm region. These new bands are assigned to the A and B transitions of the In⁺-type centers. This is supported by the doublet structure in the A band, and by the temperature dependence of the oscillator strength of the B band. In KCl∶Ag^? the ratios of the oscillator strengths are found to bef _c /f _A =610 andf _c /f _B =3,400 at low temperatures. The energy parameters of Ag^? centers are computed and compared with those of others ²-type centers. The electron-lattice coupling parameters are estimated from the Jahn-Teller splitting of the C band in CsCl and of the A band in KC1. The temperature dependence of the lifetime of the visible fluorescence suggests that a metastable state is involved in the emission process after a C band excitation.     

Spin-lattice relaxation of the O2 − molecule-ion in the potassium halides

An experimental and theoretical study of the spin-lattice relaxation (SLR) of the O₂ ^? molecule-ion in the potassium halides has been made. The SLR rate was measured by several methods between 1.32°K and 36°K with a 3 cm. ESR spectrometer. The results indicate a one phonon process with a large anisotropy dominates forT<2°K.T ₁ is much longer for the magnetic field parallel to the molecular axis. Detailed theoretical calculations indicate a new SLR process, namely phonon-induced libration of the molecular axis, can probably explain the field parallel case. The Van Vleck SLR process can account for the field-perpendicular case.     

Dislocation spectroscopy of crystals

A new method of studying the energy characteristics of dislocations is proposed, which is based on the investigation of the interaction of moving dislocations with purposefully introduced electronic and hole centers. A study has been made of KCl, NaCl, KBr, LiF, and KI alkali halide crystals containing electronic F and hole V _K and Me ⁺⁺ (Cu⁺⁺, Ag⁺⁺, Tl⁺⁺, In⁺⁺) centers. Investigation of the temperature dependence of the dislocation interaction with the F centers permitted determination of the position of the dislocation-induced electronic band (DEB) in the band diagram of the crystal. In KCl, the DEB is separated by ≈2.2 eV from the conduction-band minimum. It is shown that dislocations transport holes from the centers lying below the dislocation-induced hole band (DHB) (X ⁺, In⁺⁺, Tl⁺⁺, V _K) to those above the DHB (the Cu⁺ and Ag⁺ centers). Such a process is temperature independent. The DHB position in the crystal band diagram has been determined; in KCl it is separated by ≈1.6 eV from the valence-band top. The effective radii of the dislocation interaction with the electronic F and hole X ⁺, V _K, and Tl⁺⁺ centers have been found. Fiz. Tverd. Tela (St. Petersburg) 41, 2139–2146 (December 1999)     

Modes of vibrations due to (U-center + additive anion impurities) in KC1—II

Defect modes due to U-centers in potassium chloride containing additive anion impurities have been computed by Green's functions techniques. Local vibrations due to U-centers in alkali halides have cubic symmetry, but when one of the twelve nearest neighbour anions is replaced by an additive impurity, the site symmetry of the system reduces from O_h to C_2v and gives rise to new vibrational modes. The phonon Green's functions have been analysed according to the irreducible representation of the point group symmetry, pertaining to the substitutional impurity. We have considered the vibrations of the U-center, additive anion impurity and their nearest neighbours i.e. 36-dimensional defect space. Using group theory, symmetry coordinates were constructed and the 36 x 36 dimensional matrix was block diagonalised into various irreducible representations. The computed local mode frequencies have been conpared with the experimental measurements. Reasonably good agreement is found between them. The computed results for resonant mode frequencies are also displayed.     

Löslichkeit,Diffusion und Ultrarotabsorption von Wassermolekülen in Alkalihalogenidkristallen

Following a former communication, further experiments of solubility, diffusion and infraredabsorption of water molecules in KCl-, KBr- and KJ-crystals are given. Substituted mono- and bivalent ions to the refined crystals strongly increase the solubility of water at lower temperatures but there is practically no increase at higher temperatures. This points to two mechanisms for solubility: An interstitial at lower temperatures and probably by vacancies at higher temperatures. — The penetration of water into crystals containing K₂O orF-centers produces a colorless layer by chemical reaction. From investigation of such layers we get diffusion constants for the diffusion of water molecules. In crystals containingF-centers we find an additionalKH- layer. — The chemical reactions in the solid state produce H^?-, OH^?- and O^??-centers of well known concentration. This allows determination of the oscillator strengths of the characteristical ultraviolett absorptions. — The absorption in the near infrared by crystals containing water shows especially at low temperatures sharp peaks from single H₂O-molecules and broad peaks from precipitated water. An interstitial lattice model is discussed for the single H₂O-molecules.     

The three reorientation processes of O-2 centres in unstressed alkali halide crystals

Measurements of the reorientation of O^-₂ centres in RbI and RbBr at low temperatures together with earlier measurements in different alkali halides suggest the following sequence of relaxation mechanisms in an unstressed srystal: Below about 3°K a one phonon tunneling process, then an activated tunneling process through the first librational state, and above 10°K a classical jumping process over the potential well.     

On the change of polarization of positive muons in alkali halides

Positive muons implanted in nonconducting solids form with high probability hydrogenlike muonium atoms (µ ⁺ e ^–) with properties similar to those ofU ₂-centers. The influence of superhyperfine interactions with neighbor nuclei on the evolution of the polarization of the muon is investigated theoretically. The resulting muon polarization in longitudinal magnetic fields is calculated for muonicU ₂-centers in some alkali halides.     

Elektronen-Kern-Doppelresonanz-Untersuchung vonU 2-Zentren in Kaliumchlorid

U ₂-centers in alkali halides are neutral hydrogen atoms in interstitial lattice sites, as has been shown by EPR measurements. The hyperfine interactions with the proton and with the four nearest halogen nuclei are resolved in the EPR spectrum. In order to resolve hyperfine interactions with further nuclei of the surrounding lattice ENDOR measurements have been performed onU ₂-centers in KCl at 77 °K. The analysis of the ENDOR spectra gave precise values for the hyperfine and quadrupole interaction constants of the nearest neighbour chlorine and potassium nuclei. The isotropic hyperfine constant of the chlorine neighbours is 24 times larger than that of the potassium neighbours although both nuclei are on equivalent first shell lattice positions. The hyperfine interactions of second shell potassium nuclei [(1/2, 1/2, 3/2)-position] show an unexpectedly large isotropic hyperfine constant. One expects a pure magnetic dipole-dipole interaction for the outer shell nuclei because of the concentrated hydrogen wave function. Two further chlorine shells could be approximately analysed. A theoretical estimate of the hyperfine and quadrupole interaction constants was made by orthogonalizing the 1s hydrogen wave function to the cores of the surrounding ions. If one takes into account the mutual overlap of neighbouring potassium and chlorine ions, one gets the right order of magnitude of the measured constants and a value of 10.4∶1 for the ratio of the isotropic hyperfine constants of the first shell chlorine and potassium nuclei. The relatively large isotropic constant of the second shell potassium nuclei can also be explained on this basis.     

Formation time of F centers in alkali halides

It is argued that in most alkali halides an appreciable fraction of the self-trapped excitons may undergo non-radiative transitions from vibrationally excited states of B_3u to the A_1g state during the relaxation of the self-trapped excitons created by ionizing radiation. Numerical calculations show that the non-radiative transition probabilities, from vibrational levels of B_3u near or above the crossing point of the A_1g and B_3u potential curves to the A_1g state, are consistent with the observed formation times of F centers in various alkali halides. The exceptional case of KI is also discussed.