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.     

Migration polarization study in alumina phosphate glasses by the ionic thermocurrent technique

Ionic thermostimulated currents (ITC) of several alumina phosphate glasses are measured and analysed in terms of distributed polarizations. The relaxation spectrum analysis yields the value of the activation energy for reorientation of complexes 0·73 eV for all subpolarizations distributed in frequency factor with the most probable value ～1·25×10¹² sec^?1. The ITC experimental results are confronted with the dc conductivity measurements within the framework of the used model of alkali ion migration in glass.     

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.     

Third-order elastic constants for 2:2 chalcide crystals possessing the sodium chloride structure

Third-order elastic constants of 45 chalcide crystals having the sodium chloride structure are reported using Born-Mayer potential model. We have considered repulsive interaction up to second nearest neighbours. The temperature coefficients of the third-order elastic constants have also been computed for these crystals. As is the case for NaCl-type alkali halides we find that C₁₁₁, C₁₁₂, C₁₆₆ are negative and C₁₄₄ are positive for 2:2 chalcide crystals possessing the NaCl-type structure. We have found that a₁₂₃, a₄₅₆ and a₁₄₄ are negative whereas a₁₁₂ and a₁₆₆ are positive, once again in agreement with the situation found for the alkali halides a₁₁₁ values are positive for alkali halides whereas they are both positive and negative depending upon the interionic distance for the chalcide crystals. We have found that the nature of the variation of C⁰_αβγ with interionic separation is similar for alkali halides and for the 2:2 chalcides having the NaCl-structure. We have also computed the values of the pressure derivatives of second-order elastic constants for MgO, CaO, and SrO which agree well with the experimental values indicating the satisfactory nature of our computed data for C_αβγ.     

Influence of the isotopic constitution of the O2 −-center in alkali halides on its reorientation time

The theory of simple phonon induced reorientation of an O₂ ^?-center in alkali halides is refined to include the effect of the symmetry of its wave function. It is shown that the diagonal terms of the strain matrix and the off-diagonal ones lead to different coupling mechanisms between phonons and the impurity. For a rotation byπ/2 of a homonuclear molecule, either one or the other of the coupling mechanisms becomes ineffective, depending upon the isotopic constitution of the molecule and its nuclear spin state. Expressions for the reorientation time are obtained.     

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.     

Semiempirical pseudopotential surfaces for chemical reactions: The (AB)+ X - dialkali halide systems

A generalization of the Roach-Child semiempirical pseudopotential calculation for K + NaCl to several analogous dialkali halide systems has been used to elucidate the chemical interactions governing the reaction dynamics. The Li + LiF ground-state potential surface, which exhibits a ～ 20 kcal/mole basin for isosceles Li₂F, is qualitatively similar to one obtained in a recent configurational interaction calculation. It is shown that regions of the Na₂Cl ground-state surface corresponding to Na₂ ⁺ interacting with Cl^- can be described in terms of an ion-pair Rittner potential model similar to that employed for the alkali halides. Chemical trends in the triangular complex well depths satisfactorily account for the experimentally observed transition between the collision complex mechanism (Rb + KCl) and the osculating complex model (Li + KBr) for the alkali-alkali halide exchange reactions at thermal energies. For collinear configurations with the alkalis on opposite ends, avoided intersections between the lowest two potential surfaces are characterized in terms of diabatic surfaces computed from truncated basis sets. Crossings of these surfaces account for the vibrational-electronic energy transfer between alkali atoms and vibrationally excited alkali halides. The ionic X ^- + A ₂ ⁺ potential surfaces are used to predict the product electronic excitation and partitioning of exoergicity in reactions of halogen atoms with alkali dimer molecules.     

Punktdefekte im kubischen elastischen Kontinuum

The determination of the displacement and strain fields of a point defect in a cubic crystal requires even in the framework of continuum elasticity theory numerical calculation. These fields of elastic dipoles are expanded in suitable vector and tensor fields. The coefficients of this expansion are calculated up to polynomials of 5th and 4th order in the direction cosines using the ratios of elastic constants as parameters. With this expansion the interaction of elastic dipoles in a cubic medium can be calculated. The results have been applied to the interaction of F-centres and of O₂ ^?-centres in alkali halides.     

Phonon induced tunneling of ions in solids

A quantum mechanical analysis is given of the change of position or orientation of an atom, ion or molecule in a crystal as it occurs e.g. in the processes of diffusion or hindered rotation. One-phonon processes, Raman processes and indirect processes are discussed. The limits of applicability of the classical rate theory are given. Specifically, the analysis is applied to the reorientation of the O₂- center in alkali halides under the influence of external mechanical stresses at low temperatures, which has been investigated experimentally byKänzig. The measured dependence of the reorientation time upon temperature and applied stress can be explained by one-phonon processes. Random internal strains in the crystal are shown to play an important part. As further application of the theory the proton motions in ice and in iron are elucidated. Finally, an estimate is given of the effect of direct processes involving imperfections on the thermal conductivity of alkali halides.     

Quantum diffusion of muonium in insulators

In this paper we review our recent experiments conducted at TRIUMF on muonium diffusion in alkali halides. First, the technique of longitudinal-field muonium spin relaxation (T ₁) due to nuclear hyperfine interaction, an indispensabletour de force for the present work. is described. It is demonstrated in KCl that the technique provides spectacular sensitivity for muonium diffusion as well as determining the average nuclear hyperfine coupling constant. The muonium hop rate shows a minimum (T ^*≃80 K) and steep increase with decreasing temperature. The result is compared with the current theory of quantum diffusion in non-metallic crystals. A few more sets of new data may be presented for other alkali halides. In addition, we show that muonium forms a delocalized state in NaCl as evidenced by a large change of the average nuclear hyperfine parameter. Related topics of local tunneling system may be briefly reviewed.     

The lifetime of the 2pπu → 1sσg transition of the F2+ center in KCl

The lifetime of the 2pπ_u → 1sσ_g transition for the F₂⁺ center in KCl has been measured (15.8 ± 0.7 ns at 8 K) and found to be constant over the temperature range (8–90 K) investigated. No contradiction is found with the Aegerter and Lüty quantum yield data. A forecast for the lifetime of the 2pσ_u → 1sσ_g transition is also made, within the framework of the H₂⁺ model for the F₂⁺ center in alkali halides.     

Paraelastisches Verhalten von S2 −-Zentren in Alkalihalogeniden

The reorientation of S₂ ^? molecule ions on anion sites in alkali halide crystals under mechanical stress has been investigated by means of electron spin resonance and optical measurements. The reorientation rate is given by an Arrhenius relation. Tunneling in the ground state of libration is unimportant contrary to the case of the O₂ ^? center. The stress-induced dichroism permitted identification and assignment of two optical absorption bands of the S₂ ^? center.     

The structures of the ammonium halides

A comparison with the alkali halides suggests that all the ammonium halides should occur in the NaCl centre-of-mass structure. Experimentally, at room temperature and atmospheric pressure, only NH₄I crystallizes in this structure, while NH₄F is found in the ZnO structure, and NH₄C1 and NH₄Br occur in the CsCl structure. We show that a distributed charge on the NH₄⁺ ion can explain these structures. Taking charges of + 0.2e on each of the five atoms in NH₄⁺, as suggested by other studies, we have recomputed the Madelung energy in the cases of interest. A full ionic theory including electrostatic, van der Waals and repulsive interactions then explains the centre-of-mass structures of all the four ammonium halides. The thermal and pressure transitions are also explained reasonably well. The calculated phase diagram of NH₄F compares well with experiment. Barring the poorly understood NH₄F(II) phase, which is beyond the scope of this work, the other features are in qualitative agreement. In particular, the theory correctly predicts a pressure transition at room temperature from the ZnO structure directly to the CsCl structure without an intermediate NaCl phase. A feature of our approach is that we do not need to invoke hydrogen bonding in NH₄F.     

ESR- und ENDOR-Spektren von F-Zentren in RbCl und RbBr

ESR- and ENDOR-spectra of F-centers in RbCl and RbBr have been measured, giving the hyperfine constants for 5 resp. 6 shells of neighbouring ions. The resolved structure with a characteristic distance of 36 G in the ESR-spectrum of RbCl is due to the interaction with the nearest Rb⁸⁵ nuclei. All results fit well with the properties of F-centers in other alkali halides.     

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.     

Cerium influence on the spin reorientation in Er2−xCexFe14B evidenced by Mössbauer and DSC techniques

Polycrystalline Er_2−xCe_xFe₁₄B (x=0.25, 0.5, 1.0) compounds have been studied by ⁵⁷Fe Mössbauer spectroscopy and by differential scanning calorimetry (DSC) in the temperature range 80–470 K. The spin reorientation phenomenon has been studied extensively by narrow step temperature scanning in the vicinity of the spin reorientation temperature. It was found that in the region of transition, each Mössbauer subspectrum splits into two Zeeman sextets, which are characterised by different hyperfine magnetic fields and quadrupole splittings. A consistent way of describing the Mössbauer spectra in the reorientation region, below and above, was proposed. The composition and temperature dependencies of hyperfine interaction parameters and subspectra contributions were derived from experimental spectra.The endothermic DSC peaks were observed for all studied compounds, which correspond to the transition from basal plane to axial easy magnetisation direction on increasing the temperature. The spin reorientation temperatures and the enthalpies of transitions were established from DSC data. The spin arrangement diagram was constructed and the spin reorientation temperatures obtained by the two methods were compared.     

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.     

Mechanisms of spin-lattice relaxation of vk and related centers

An experimental and theoretical investigation of the spin-lattice relaxation (SLR) mechanisms of V_K-centers in alkali and alkaline earth halides has been carried out. It has been shown that at low temperatures the main role in SLR is played by rapidly relaxing centers (RRC). In some cases, however, an intrinsic one-phonon SLR process due to modulation of both the isotropic and anisotropic hyperfine (HF) interaction seems to be important. At higher temperatures SLR can be explained by an anharmonic Raman process with the participation of the resonant molecular vibrational (RMV) mode of the V_K-center. In this SLR process, similarly to the low-temperature intrinsic process, the spin-phonon coupling is due to the modulation of the HF interaction. Deviations from the Debye phonon spectrum and the local anharmonicity of V_K-centers turn out to be important. The SLR of several other hole and interstitial type centers in alkali halides has also been investigated and their SLR mechanisms are discussed.     

On the possibility of a forecast for off-centre configuration of Ag+ and Cu+ in alkali halide crystals

Summary The simple cut criterion based on the accurate determination of the radii of the ions in alkali halides and previously introduced by the authors for forecasting off-centre configuration of Li⁺ and F⁻ has been extended to heavy ions (Ag⁺ and Cu⁺). It has been found that this criterion is valid for the Ag⁺ ion, whereas for Cu⁺ gives a less precise forecast because of the lack in knowledge of the effective partial charge on Cu⁺ ion. It has been evidenced that the critical value of the ratior ₊ ^* /r ₊ between impurity and host ion radius which allows off-centre configuration is dependent on the impurity ion mass. Work jointly supported by the Ministero della Pubblica Istruzione and by Consiglio Nazionale delle Ricerche, Gruppo Nazionale di Struttura della Materia.