The stark effect of localized electronic centres

Methods which have been developed and employed for the calculation of point defects in ionic crystals are described. These methods involve a detailed Hartree-Fock-like treatment by way of angular-momentum-dependent exchange potentials of the ions within an inner region ${\ensuremath{\tau}}_{1}$ centered about the defect. In the outer region ${\ensuremath{\tau}}_{2}$, composed of the rest of the crystal, the ions are treated in two different approximations, the most successful of which involves an effective-mass formalism. Polarization effects are included by expressions of the form resulting from the work of Toyozawa, Haken, and Schottky. Although the main purpose of the paper is to describe the methods, applications are made to the calculation of the electronic energy levels in the optical absorption of the $F$ center in KCl and NaCl. Good agreement with experiment is found only if polarization effects, which are important even in the ground state, are included. Additional applications will be given in subsequent papers.

The energy, the elastic dipole tensor and the entropy of point defects in ionic crystals are usually calculated by the Mott-Littleton approach, which treats a single defect in an infinite crystal. The authors suggest that there may be advantages, particularly for the dipole tensor and the entropy, in performing the calculations in periodically repeating geometry. They examine how this 'supercell' method can be used for the energy and the dipole tensor, paying attention to the problem that for charged defects the repeating unit carries a net charge, so that the Coulomb energy is divergent. They test the supercell method on a number of both charged and uncharged defects, and show that the results for the energy and dipole tensor are in close agreement with those obtained by the conventional approach.

Various mechanisms for the formation of point defects in ionic crystals are considered. Their probable importance as a function of the energy of the ionizing radiation and the temperature at which the irradiation takes place is discussed. Complex centers formed from aggregates of simple point defects are reviewed and their occurrence is considered in terms of the expected spatial distribution of the point defects. The effect of high-energy X-rays and protons on the ionic conductivity is dis- cussed in terms of a new interpretation of the conductivity curve as a function of temperature, and it is suggested that radiation-enhanced diffusion may be important in explaining the experimental data.

The subsurface electric potential in oriented single crystals of silver chloride has been directly mapped out by a technique based on the equilibrium distribution of a very small concentration of a charged radiotracer. The potential was found to vary with depth according to the Gouy-Chapman solution of Poisson's equation, with screening lengths in the range 10\char21{}20 nm. The surfaces were all negative, relative to the deep interior, by 0.10\char21{}0.30 V, depending on temperature and surface orientation. The present results on the subsurface potential difference can be employed to resolve the Frenkel-pair formation enthalpy and entropy, as determined from transport measurements, into the formation enthalpies and entropies of the individual point defects comprising the pair. For the silver chloride (110) surface, for example, ${G}_{v}$=0.74 eV and ${G}_{i}$=0.37 eV at 150 \ifmmode^\circ\else\textdegree\fi{}C; the best values obtained to date for ${H}_{v}$, ${H}_{i}$, ${S}_{v}$, and ${S}_{i}$, are 0.77 eV, 0.70 eV, 0.79k, and 9.0k, respectively.

The electrostatic potential which arises from a lattice array of point ions is computed in terms of a Taylor's series expansion for small distances from a lattice site. This expansion gives the change in electrostatic energy when an ion moves in the background of a perfect point ion lattice potential. The Taylor's series coefficients for terms up to fourth order in the ion displacement are evaluated for the NaCl and CaF2 lattice structures.

Any deviation from a perfect periodic lattice or structure in a crystal is an imperfection. There are three main types of imperfections, point, line and plane imperfections1, of which we shall deal only with the point defects in this chapter. The point imperfections encountered in ionic solids are: (i) chemical impurities (ii) vacant lattice sites and (iii) interstitial ions. Real crystals are always imperfect in some respect. In thermal equilibrium in an otherwise perfect crystal a certain number of lattice vacancies or interstitials are always present, because the entropy is increased by the presence of disorder in the structure and at a finite temperature, the equilibrium condition of a crystal happens to be a state of minimum free energy.

The authors present and describe a new embedded-cluster approach for calculating both the electronic and the spatial structure of point defects in ionic crystals with self-consistent incorporation of the polarisation effects into the eigenvalue spectrum and the total energy of a crystal. For the first time both the electronic structure and the ionic displacements within a large cluster containing a defect are obtained in the framework of the same calculating scheme. It permits one to describe correctly back-coupling between redistribution of the electron density in a defective region and polarisation of the rest of the crystal outside the cluster, which can play a vital role for the study of small polaron effects. Making use of a 45-atom cluster with 175 electrons and the semi-empirical INDO method, the authors have simulated the impurity-induced trapping of a hole in the (Li)0 centre in an MgO crystal. The calculations do not use a priori assumptions and do confirm that one-centre localisation of a hole in the ground state is energetically favourable. They show that polarisation energies can differ considerably for the ground and charge-transfer excited states of the defect. It is also demonstrated that the generally accepted model of the O- hole polaron is oversimplified: a considerable charge redistribution between Li+ and another O- along the (100) direction takes place.

Excitation of Plasmons and Interband Transitions by Electrons. By H. Raether. (Berlin, Heidelberg, New York: Springer-Verlag, 1980.) [Pp. 192+subject index.] Price DM 59, – –/US $32.50 hardback. Physics of Superionic Conductors. Edited by M. B. Salamon. (Berlin, Heidelberg, New York: Springer-Verlag, 1979.) [Pp. xii+248+subject index.] Price DM 59, – –/US $32.50 hardback. Electronic and Vibrational Properties of Point Defects in Ionic Crystals. By Y. Farces and M. P. Fontana. (Amsterdam: North Holland, 1979.) [Pp. xix+247+author and subject index.] Price Dfl. 100.00/US $48.75 hardback. Defects and Diffusion in Solids. An Introduction. By S. Mrowec. (Amsterdam, Oxford, New York: Elsevier, 1980.) [Pp. 464+subject index.] Price Dfl. 170.00/US $83.00 hardback. Fluctuation Phenomena. Edited by E. W. Montroll and J. L. Lebowitz. (Amsterdam: North-Holland, 1979.) [Pp. 340+author and subject index.] Price Dfl. 120.00/US $58.50 hardback. Crystal Growth: A Tutorial Approach. Edited by W. Bardsley, D. T. J. Hurle and J. B. Mullin. (Amsterdam: North-Holland, 1979.) [Pp. vi+401+subject index.] Price Dfl. 110.00/US $53.75 hardback. Liquid Crystals. By S. Chandrasekhar. (Cambridge University Press, 1980.) [Pp. x+336+subject index.] Price £8.95 paperback. Light Scattering in Solids. Edited by J. L. Birman, H. Z. Cummins and K. K. Rebane. (New York: Plenum Press, 1979.) [Pp. xxi+523+author and subject index.] Price US $55.00 hardback (camera-ready typescript). Electrostatics 1979. Edited by J. Lowell. (London, Bristol: The Institute of Physics, 1979.) [Pp. 349+author index.] Price £32.00 hardback. Auger Electron Spectroscopy Reference Manual. By G. E. Mcguire. (New York: Plenum Press, 1979.) [Pp. ix+129.] Price US $39.50 paperback. Laser Handbook, Volume 3. Edited by M. L. Stitch. (Amsterdam: North-Holland, 1979.) [Pp. ix+846+author and subject index.] Price Dfl. 250.00/ US $122.00 hardback.

Investigation of the electronic structure of point defects in ionic crystals by the cluster scattered wave method with the self-consistent calculation of the lattice distortion and long-range polarization

Methods described in a previous paper are applied to the calculation of the electronic structure of the $F$ center in KCl, KBr, KI, and NaCl. Structure of the ions is explicitly taken into account in a region close to the defect site, and an effective-mass method is used in the more distant regions. Dielectric polarization effects are included on the basis of the formulation due to Toyozawa, Haken, and Schottky, but results in best agreement with experiment are obtained by a procedure which deviates somewhat from that formulation, as might be expected also on purely theoretical grounds. It does not appear to be possible to obtain agreement with experiment even for absorption energies if polarization effects are neglected. The calculated energy differences of the $1s$, $2p$, $3p$, and $2s$ states in absorption agree very well with presently available experimental data. Agreement with experiment is fairly good for the $1s$, $2s$, and $2p$ energies in emission. The calculated optical transition matrix elements are very sensitive to the details of the model; those obtained from our best model and wave functions are compatible with experimental results. The results for the spin density at the nearest-neighbor ions are reasonably close to the experimental values.

A mechanism of interaction of dislocations with point defects is suggestedon the basis of a study of deformation-stimulated luminescence (DL)from irradiated KCl. DL is considered to be a result of destruction ofhole colour centres by dislocations. The centres destroyed by dislocationsare responsible for irradiation-induced strengthening of alkali halides.

The paper briefly reviews the development of the atomistic method of calculating the properties of defects in ionic crystals, as it derives from the early work of Mott and Littleton (1938). The physical principles and assumptions of the method are described, although the main emphasis is on the development of the technique. It is pointed out that appeal to the quasi-harmonic approximation has made it possible to extend the technique so that the characteristics of defects can now be successfully obtained as functions of temperature. The insights into atomic transport phenomena which these calculations provide are illustrated by three examples.

Metal halide perovskites are promising materials for future optoelectronic applications. One intriguing property, important for many applications, is the tunability of the band gap via compositional engineering. While experimental reports on changes in absorption or photoluminescence show rather good agreement for different compounds, the physical origins of these changes, namely the variations in valence and conduction band positions, are not well characterized. Here, we determine ionization energy and electron affinity values of all primary tin- and lead-based perovskites using photoelectron spectroscopy data, supported by first-principles calculations and a tight-binding analysis. We demonstrate energy level variations are primarily determined by the relative positions of the atomic energy levels of metal cations and halide anions and secondarily influenced by the cation-anion interaction strength. These results mark a significant step towards understanding the electronic structure of this material class and provides the basis for rational design rules regarding the energetics in perovskite optoelectronics.

The Knotek-Feibelman mechanism of desorption is analysed in the context of a microscopic dynamical theory that has been proposed recently for closed-band systems. To understand the physical contents of the general equations, the probabilityP of ionic desorption is derived analytically in several selected model situations as a function of the characteristic desorption timet o. Although typicallyt o is quite long compared to the inverse level widthW B −1, the results show very significant deviations from the limit of small velocities. This is due to the fact thatP is not primarly determined by the level width, but by the structures, like peaks and singularities, that are contained in the two-hole continuum. Such features, that are completely missed in the traditional approach that represents the continuum by a Lorentzian, are naturally enbodied in the present theory. They may originate from the position of the adatom energy level ɛ relative to the substrate band and/or by hole-hole correlation. When split-off localized states arise,P may become comparable with unity even in the limit of small velocities, giving raise to the Knoteck-Feibelman reneutralization bottleneck; however, this is not the case when the localized states are readsorbed by the continuum in the course of desorption. An explicit example of this behaviour is given. On the other hand, when ions desorb from ionic crystals with a large Madelung energy, the bond-breaking process may approach the sudden limit. A strongly nonadiabatic behaviour is then predicted, including a correlation-induced enhancement ofP by several orders of magnitude, even in bandlike situations. This rapid variation leads to suggestions for a new experiment that should enable us to see the effects of a fine tuning of the electronic parameters.

It is shown that simple band representations tables can be used to solve the following two problems: 1) to find the irreducible components of the space group representations induced by the irreducible representations of their site symmetry subgroups; 2) to find the irreducible components of the site symmetry group representations subduced by irreducible representations of a space group. The solution of both the problems requires also the induced representation table for the crystallographic point grousp which is given here. The space group D is considered as an example. The simple band representations table for this group is also given. The application of the band representations of space groups in the theory of second‐order phase transitions and in the theory of point defects in crystals is discussed.