Impurities In Alkali Halides Research Articles

Electronic structure and optical spectra of Au− ions in RbCl

Ab initio wavefunction based quantum chemical calculations on the electronic structure and optical spectra of Au− ions in RbCl crystals are reported in this work. ns2 impurities in alkali halides are prototypical phosphor materials and have been extensively studied. In particular, RbCl:Au− has been recently studied experimentally, but some points of the interpretation of the spectra are still controversial. Our calculations, that include all the major interactions present in the system, show very good agreement with the experimental findings. The calculations show that the levels belonging to the 5d96s26p electron configuration lie relatively low in energy, and extensive mixing with some of the 6s6p levels is found. Large mixing of singlet and triplet states via spin-orbit coupling is found for most of the calculated levels. Due to this, band C in the absorption spectrum is found to consist of two transitions, to the 2 T1u and 3 T1u electronic levels, described as configurational mixtures of 6s6p-1P1 and 5d96s26p levels. Transitions to 5d96s26p levels are found to be responsible for the D absorption bands of the material. On the other hand, emission band C′ is assigned to a transition from the 1 A2u (5d96s26p) electronic level to the ground level, a transition that is electric dipole forbidden. This level lies just below the levels responsible of the C absorption (around 200 cm−1). These findings can justify the complex behavior of this C′ emission.

Density functional theory study of superoxide ions as impurities in alkali halides.

The orientation of diatomic molecular impurities in crystals is a classic problem in physics, whose analysis started in the early 1930s with Pauling's pioneering studies and has extended to the present day. In the present work, we investigate the orientation of a superoxide ion (O2-), which is known to be oriented in the 1 1 0 direction when replacing a halide ion in alkali halide rock salt lattices. The unpaired electron of the superoxide, whose ground state is degenerate (2Πg), is oriented in the 0 0 1 direction for sodium halides while it is oriented in the 1 1[combining macron] 0 direction for potassium and rubidium halides. We performed density functional theory (DFT) calculations to describe the full adiabatic potential energy surface (APES) of this complex system for the first time with ab initio methods. We are focused on four alkali halide lattices, namely NaCl, NaBr, KCl, and KBr. We show that DFT, at the generalized gradient approximation (GGA) and meta-GGA levels, is able to reproduce all the experimental features for potassium halides. However, for sodium halides, although the DFT predicts the correct unpaired electron orientation, the forecasted APES energy minimum for the molecular orientation is found to be close to the 1 1 3/4 orientation, in contrast to the experimental 1 1 0 orientation. The difference in energy between the 1 1 3/4 and 1 1 0 orientation is less than 10 meV, which points out the subtleness of the considered problem. In addition to assessing the DFT accuracy and limitations to treat these systems, we also paid special attention to analyze the geometry distortions of the host lattice for the high symmetry orientations of the superoxide ion, i.e., 1 0 0, 1 1 0 and 1 1 1. In the case of the 1 1 0 molecular orientation, we find a strong dependence on the distance between the alkali ions in the 0 0 1 direction and the superoxide ion upon the unpaired electron orientation. This fact explains why the orientation of the unpaired electron is different in sodium vs. potassium halides. In the case of the 1 0 0 and 1 1 1 molecular orientations, we analyze the Jahn-Teller vibronic coupling to find an unusually large vibronic centrifugal term in the latter.

Quantum Mechanical Study of the Lattice Distortions Induced by Aliovalent Pb2+ Impurities in Alkali Halides

An embedded cluster analysis of the lattice distortions induced by a Pb 2+ substitutional impurity in NaI, RbI and CsF crystals is presented. Active clusters containing more than 100 atoms and embedded in a quantum representation of the crystalline environment are used. Charge compensation is achieved by introducing a cation vacancy in the host lattice. Both nearest-neighbor (NN) and next-nearest-neighbor (NNN) vacancy positions are explicitly considered, as well as the case where vacancy and impurity are well separated from each other. Only in the first two cases, the impurity adopts an off-center equilibrium position. Distortion trends as a function of the host lattice are identified and discussed. These trends are then employed to advance the expected distortions in similar crystals. The examples of Pb 2+ -doped RbF and KBr are found to behave according to the expectations. Finally, the relative stability of the NN and NNN configurations is discussed.

Quantum Mechanical Study of the Lattice Distortions Induced by Aliovalent Pb2+ Impurities in Alkali Halides

An embedded cluster analysis of the lattice distortions induced by a Pb2+ substitutional impurity in NaI, RbI and CsF crystals is presented. Active clusters containing more than 100 atoms and embedded in a quantum representation of the crystalline environment are used. Charge compensation is achieved by introducing a cation vacancy in the host lattice. Both nearest-neighbor (NN) and next-nearest-neighbor (NNN) vacancy positions are explicitly considered, as well as the case where vacancy and impurity are well separated from each other. Only in the first two cases, the impurity adopts an off-center equilibrium position. Distortion trends as a function of the host lattice are identified and discussed. These trends are then employed to advance the expected distortions in similar crystals. The examples of Pb2+-doped RbF and KBr are found to behave according to the expectations. Finally, the relative stability of the NN and NNN configurations is discussed.

On the lifetime of KCI: Ag + fluorescence

The fluorescence of Ag + in KCI was investigated in the range 20–300 K, in particular by performing accurate lifetime measurements. It was found that, among the models proposed in the literature, the model introduced by Pedrini gives the most reasonable explanation of the Ag + de-excitation mechanism: two excited states in thermal equilibrium are involved in the emission process, one of which is metastable and lying just below the emitting level. A discussion of the decay properties of Ag + and Cu + impurities in alkali halides is presented.

Electronic-Vibrational Energy Transfer between F-Centers and OH<sup>-</sup> Impurities in Alkali Halides

Electronic-Vibrational Energy Transfer between F-Centers and OH<sup>-</sup> Impurities in Alkali Halides

Self-trapped exciton luminescence induced by photoexcitation of the impurity ions in alkali halides

Highly UV excited KI : In + and KI : Sn 2+ crystals exhibit at low temperatures pronounced emission bands peaking near 3.3 eV along with the activator luminescence. The spectra and the lifetimes of the 3.3 eV emission are close to the well-known radiative characteristics of the triplet self-trapped excitons in pure crystals of KI. The observation of exciton luminescence in doped crystals of KI confirms that excitons are involved in the process of F-center formation caused by intense optical excitation of the impurities in alkali halides.

Neighbor configurations of Ag+ and Cu+ impurities in alkali halides.

A simple rescaling scheme, based on ionic radii, provides nearest-neighbor distances in alkali halide alloys and, more importantly, of ${\mathrm{Ag}}^{+}$ and ${\mathrm{Cu}}^{+}$ impurities in alkali halide hosts. The distances agree closely with quantum-mechanical calculations and indicate the occurrence of on-center or off-center positions of the impurities.

EXAFS study of relaxation of Cu+ impurities in alkali halides

EXAFS study of relaxation of Cu+ impurities in alkali halides

Vegard's rule and volumes of formation for impurity ions

Calculated volumes of formation for substitutional impurities in alkali halides are shown to be approximately consistent with the empirical rule that for miscible crystals the average volume per ion pair scales linearly between the two single-component extremes (Vegard's rule). The small deviations found from the rule are found to be in good agreement with available X-ray measurements.

Off-center substitutional ions in alkali-halide crystals

The potential wells of alkali and halogen ions as impurities in alkali halides are investigated by means of a deformation-dipole model with single-ion parameters. The calculations indicate that Li + is off center in seven and F − in six of the sixteen rocksalt-type crystals considered. In about half of the off-center systems the configuration has D 4 h symmetry if the impurity is kept at its lattice position. The minimum-energy configuration of the off-center system KCl:Li + is calculated in detail, and the influence of hydrostatic pressure is investigated. Our results are in qualitative agreement with experiment.

Picosecond time-resolved spectroscopic study of vibrational relaxation processes in alkali halides

The luminescence properties of various kinds of impurities in alkali halides are studied with 1 ps timing accuracy. The relation between measured vibrational relaxation times, vibronic spectral structure and impurity-lattice coupling strength is investigated.

Raman scattering of pure, singly- and doubly perturbed interstitial hydrogen atom centers in alkali halides

Abstract Hydrogen atoms occupying interstitial tetrahedral sites are produced by U.V. excitation of OH−impurities in alkali halides. A single narrow Raman line is associated with this U2 center and polarization studies confirm the A1→T2 nature of the vibrational transition. In alkali halides containing also heavier halogen ion impurities, the interstitial hydrogen can be trapped next to them and a singly perturbed U2 center of C3v symmetry as well as a doubly perturbed U2 center of C2v symmetry are produced. Their identification was based on a careful analysis of polarized Raman studies on both randomly oriented and preferentially oriented centers. RbBr:I∼:OH∼ is a typical example: the fundamental A1→T2 of the pure U2 center is at 330 cm−1; the singly perturbed U2 center yields a A1→A1 Raman line at 284 cm−1 and a A1→E transition at 345 cm−1. The doubly perturbed center exhibits a single line at 269 cm−1. A qualitative discussion of these results is attempted.

Quadratic Jahn-Teller Effect in the Line Shapes of the A-Emission and A-Absorption Bands in KBr: Tl+-Type Phosphors

The adiabatic potential energy surfaces (APES's) of Tl + -type impurities in alkali halides, \(^{3}T_{1u}(\varGamma_{4}^{-})\) and \(^{3}A_{1u}(\varGamma_{1}^{-})\) have been calculated taking account of the spin-orbit interaction and both the linear and quadratic Jahn-Teller effect with the α 1 g and ε g vibrational modes. It has been found that the 3 T 1 u APES's have one kind of minima or the two kinds of minima, depending on whether we assume the linear or both the linear and quadratic Jahn-Teller interactions. A trap level 3 A 1 u lies just below both the minima. The line shapes of the A -emission and A -absorption bands in KBr: Tl + are calculated by using the Franck-Condon and Condon approximations. It has been found that the theoretical line shape of the A -emission band has the asymmetric doublet structure and reproduces the experimental emission bands A T and A X . This shows that the large energy differnce between the A X and A T bands is caused by the quadratic Jahn-Teller terms involving Q 1 (α 1 g ).

Restoration of Fluorescence from the Lowest Singlet State in the Self-Trapped Exciton by Perturbation with Monovalent Cation Impurities in Alkali Halides

The electronic structure and intersystem crossing at excited states of the relaxed exciton, ( V k e ) A , perturbed by monovalent cation impurity in KCl and KBr have been studied by pulsed laser irradiation of samples populated with the lowest triplet ( V k e ) A . It is found that the σ-polarized luminescence, at 3.03 eV and 2.93 eV in KCI:Na and KBr:Na, respectively, that decays in less than 10 ns is produced upon the photoexcitation of the lowest triplet ( V k e ) A . The fast luminescence is ascribed to the fluorescence (referred to as 1 -luminescence) from the lowest singlet ( V k e ) A . The degree of polarization of the optical absorption due to the lowest triplet ( V k e ) A is determined and it is shown that the electron orbitals of ( V k e ) A are in the order of b 2 u , b 3 u , b 1 u and b * 3 u from lower energy. The S 1 luminescence is found to be produced most efficiently by the electron excitation from the a g orbital to the b * 3 u orbital.

Modes of vibrations due to single ion substitutional impurities in alkali halides

AbstractModes of vibrations due to single ion impurities in alkali halides are studied by Green's function technique. The Green functions computed on the basis of shell model parameters fitted to the neutron data are used in the computation. The modes of vibrations are computed by considering the vibrations of the defect and its six nearest neighbour ions, having Oh symmetry. Using group theory symmetry coordinates are constructed for the seven‐atom defect space and analytical expressions are derived for various modes of vibrations, in terms of Green's functions of the perfect lattice and the perturbation matrix as a result of the defect. Mass changes at the defect site as well as the changes in the short‐range interaction due to the presence of the defect are taken into account. The change ΔA in the short range interaction parameter A is fitted to the observed single ion frequencies in various alkali halides. A considerable softening of the nearest neighbour short‐range interaction varying from 8% in KBr:Cl to 64% in KI:Na is found.

Raman spectra of NO −2 in KI and of NO 2 in Ar at high pressure and at low temperatures

The effect of pressure on the Raman scattering of impurities in alkali halides and in rare gas solids is studied at 77 K and 5 K up to 8 kbar. The spectra show different matrix-, temperature- and pressure shifts. The low energy part (region of phonons and impurity-lattice modes: 20–300 cm −1) and the high energy part (region of pure molecular modes: 700–1500 cm −1) reveal some interesting aspects. Most of the data will be discussed under the light of anharmonic effects.

Electron trapping by alkali-ion impurities in alkali halides

Evidence is presented which indicates that certain transient infrared absorptions normally observed in alkali halide crystals are due to trapping of electrons by substitutional alkali-ion impurities.

Origin of the triple decay-time in the low-energy emission of KI:TI

Non radiative transitions occuring between different minima on the 3 T 1 u potential surface of Tl +-like impurities in alkali-halides, have been studied by the WKB method to explain the presence of three different decay-times in the low-energy emission of KI:Tl.

Thermal transport in MnF2. I. OH−tunneling states

The thermal-transport properties of Mn${\mathrm{F}}_{2}$ have been investigated using thermal-conductivity and heat-pulse measurements. The temperature dependence of the conductivity exhibits a resonance at 1.0 K. This has been shown previously to be due to resonant scattering of the phonons by O${\mathrm{H}}^{\ensuremath{-}}$ impurities. The attenuation of the heat pulses indicates that only the ${E}_{g}$ phonon mode is resonantly scattered. These results are correlated with previous measurements of the elastic constants of Mn${\mathrm{F}}_{2}$. The softening of the ${C}_{44}$ elastic constant at low temperatures can be accounted for by the interaction of the ${E}_{g}$ mode with the O${\mathrm{H}}^{\ensuremath{-}}$ impurities. Theoretical expressions are fitted to the thermal-conductivity, heat-pulse, and elastic-constant results. The level splitting of the O${\mathrm{H}}^{\ensuremath{-}}$ states is found to be 2.8 ${\mathrm{cm}}^{\ensuremath{-}1}$, and the elastic dipole moment is estimated to be 3.4 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}24}$ ${\mathrm{cm}}^{3}$. The properties of the O${\mathrm{H}}^{\ensuremath{-}}$ impurity states are shown to be similar to those of tunneling states of molecular impurities in alkali halides. A tunneling model for the O${\mathrm{H}}^{\ensuremath{-}}$ states in Mn${\mathrm{F}}_{2}$ is proposed, and shown to be consistent with the experimental results.