Centers In KCl Research Articles

Theoretical studies of the defect structures for the two Cr3+ centers in KCl

The spin Hamiltonian (SH) parameters (i.e. the zero-field splitting parameters (ZFSPs) and g factors) and local structures of the two Cr3+ centers I and II at room temperature in KCl single crystals are theoretically investigated from the perturbation calculations for a rhombically distorted octahedral 3d3 cluster. The impurity systems are attributed to the doped Cr(CN)63− groups into KCl replacing the host KCl65− ones, associated with two nearest neighbor potassium vacancies VK in [011] and [01¯1¯] axes in center I and one nearest neighbor VK along [01¯1] and another next-nearest neighbor VK along [100] axis in center II, respectively. In center I, the four coplanar and two axial ligands CN− undergo the shifts ∆R1 (≈0.0044nm) away from the VK and ∆R2′ (≈0.0144nm) away from the central ion along Z axis, respectively, because of the electrostatic interactions. In center II, the impurity Cr3+ is found to undergo the shift ∆RC (≈0.0063nm) towards the nearest neighbor VK along [01¯1] axis, while the two ligands in [001] and [01¯0] axes closest to the VK undergo the shifts ∆R1 (≈0.0081nm) away from the respective VK, and the ligand intervening in the VK and the central ion experiences the shift ∆R2 (≈0.0238nm) away from the VK along [100] axis. The charge-transfer (CT) contributions to g-shifts are found to be opposite in sign and more than half (characterized by the ratios |ΔgCT/ΔgCF|>50%) in magnitude compared with the CF ones for both centers. The local structures and the microscopic mechanisms of the relevant impurity and ligand shifts are discussed for the two centers.

Stability and Polaronic Motion of Self-Trapped Holes in Silver Halides: Insight through DFT+U Calculations

Polarons and their associated transport properties are a field of great current interest both in chemistry and physics. To further our understanding of these quasi-particles, we have carried out first-principles calculations of self-trapped holes (STHs) in the model compounds AgCl and AgBr, for which extensive experimental information exists. Our calculations confirm that the STH solely stabilizes in AgCl but with a binding energy of only 165 meV, an order of magnitude smaller than that found for the Vk center in KCl. Key contributions to this stabilization energy come from the local relaxation along breathing (a1g) and Jahn–Teller (eg) modes in the AgCl64– unit. To study the transfer of the STH among silver sites, we (i) use first-principles calculations to obtain the hopping barrier of the STH to first and second neighbors, involving eight distinct paths, using first-principles and (ii) construct a simple model, based on Slater–Koster parameters, that highlights the similarity of polaron transfer with m...

Temporal signal stability of KCl:Eu2+ storage phosphor dosimeters

Current KCl:Eu(2+) prototype dosimeters require a wait time of 12 h between irradiation and dosimetric readout. Although irradiating the dosimeters in the evening and reading on the following day works well in the clinical schedule, reducing the wait time to few hours is desirable. The purposes of this work are to determine the origin of the unstable charge-storage centers and to determine if these centers respond to optical or thermal excitation prior to dosimetric readout. Pellet-style KCl:Eu(2+) dosimeters were fabricated in-house for this study. A 6 MV photon beam was used to irradiate the dosimeters. After x ray irradiation, dosimeters were subjected to external excitation with near-infrared (NIR) light, ultraviolet (UV) light, or thermal treatment. Photostimulated luminescence (PSL) signal's temporal stability was subsequently measured at room temperature over a few hours using a laboratory PSL readout system. The dosimeters were also placed in a cryostat to measure the temperature dependence of the temporal stability down to 10 K. Strong F-band was present in the PSL stimulation spectrum, indicating that F-centers were the electron-storage centers in KCl:Eu(2+) where an electron was stored at a chlorine anion vacancy. Due to deep energy-depth (2.2 eV), F-centers were probably not responsible for the fast fading in the first a few hours post x ray irradiation. In addition, weak NIR bands were present. However, there was no change in PSL stabilization rate with intense NIR excitation, suggesting that the NIR bands played no role in the PSL fading. At temperatures lower than 77 K there was almost no signal fading with time. Noticeable PSL was observed for undoped KCl samples at room temperature, suggesting that Cl(2) (-) V(k) centers served as hole-storage centers for both undoped and doped KCl where a hole was trapped by a chlorine molecular ion. V(k) centers were stable at low temperature and became mobile at room temperature, probably causing the observed PSL fading with time. On the other hand, V(k) center could be stabilized by Eu(2+) activator or oxygen in the lattice, leading to the stable component in the PSL. A thermal process at elevated temperatures (60 °C or higher) was able to significantly accelerate the migration process resulting in a fast stabilization of PSL. However, this could not be accomplished using intense UV excitation. Thermal treatment enables KCl:Eu(2+) prototypes to be ready for readout in 1 h without the need of applying a large time-dependent correction factor. However, this cannot be achieved using optical preexcitation.

G-tensor and hyperfine splitting of , , defect centers in KCl from DFT calculations

We have studied the electron paramagnetic resonance (EPR) properties of a classical group of paramagnetic centers in solid KCl, the H center (a trapped species) and the analogous and centers. The calculations have been performed at the density functional theory level using pure and hybrid exchange-correlation functionals in combination with cluster models embedded in point charges. The attention has been focused on the reproduction of key EPR parameters such as the g- and the A-tensors. The A-tensor is satisfactorily reproduced for all three species. The g-tensor is well described in the case of the H center, while large deviations are found for the and trapped radicals. The different behavior is rationalized in terms of different symmetries of the singly occupied molecular orbitals, σ in and π in and .

On the incorporation of local correlation effects into ionic solids

Two distinct means of introducing local correlation effects are considered here. In the first we develop a one center pseudopotential method, and apply it to the lowest excitons of the F and U centers in KCl and the newly measured core (1s) excitonic spectra of NiO. While the results obtained at the lowest truncation level are in good agreement with experiment, convergence is poor as higher terms are added, thus precluding general use of the method. In the second method, the solid is replaced by a cluster with appropriate boundary conditions, and correlation effects are treated by the independent electron pair approximation (IEPA). Application to the lowest exciton of LiF produces an energy of 12.6 eV (to which correlation contributes 2.5 eV) in good agreement with the experimental value of 12.5 eV.

Second2s−2plevel crossing at theFcenter in KCl evidenced by frequency upconversion spectroscopy

Subpicosecond dynamics of resonant secondary emission from the $F$ center in KCl at 5 K is investigated by frequency upconversion method. Detected photon energy range is from 0.65 to 1.55 eV and covers almost all the stationary emission band. The decay time of the luminescence intensity is longer in the low-energy side of emission band than in the high-energy side. A series of transient luminescence spectra shows relatively high intensities in the low-energy side. These two features indicate the existence of the second $2s\text{\ensuremath{-}}2p$ level crossing near the relaxed excited state.

Magnetic field dependence of dielectric polarization echoes in KCl doped with Li

We have studied the effect of nuclear quadrupolar moments on the energy level structure of tunneling systems in KCl:Li. The resulting fine structure splitting leads to a quantum beating in echo decay measurements and to a non-monotonic dependence of the echo amplitude on magnetic fields, which was both observed in dielectric polarization echoes experiments. Since the microscopic nature of Li tunneling centres in KCl is well known, this system can be used to compare experimental results with calculations based on a detailed microscopic model. We present the experimental data and discuss the role of nuclear quadrupole moments of the KCl host material in the vicinity of tunneling Li ions on the observed effects.

Dynamics of nuclear wave packet in the excited state of KCl F centers

AbstractDynamics of nuclear wave packet at the F center in KCl at 5 K is investigated by frequency up‐conversion spectroscopy. The oscillation frequency of the nuclear wave packet is 6.0 THz, which lies in the LO phonon branch near the Γ point of the Brillouin zone. The luminescence intensities decay slower in the low energy side of the stationary luminescence spectrum than in the high energy side. The transient luminescence spectra show relatively high intensities in the low energy side. These two features indicate the existence of the second 2s ‐2p level crossing near the relaxed excited state. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Triplet‐triplet absorption transitions in self‐trapped exciton and F2 centers in alkali halides

AbstractThe results of comparative analysis of triplet‐triplet absorption spectra, optical binding energy, the lifetime of self‐trapped excitons (STE) and F2 centers in KCl, KBr, KI and NaCl are presented. The coincidence of triplet–triplet absorption spectra of F2 centers and of STE in the spectral interval of 1‐3 eV in one and the same crystal was discovered. It is true for each crystal under consideration in spite of that the calculated value of axial shift of STE Vk core varies in a wide range of 0.9–4.3 Å. It was revealed that the value of optical binding energy of the both STE and F2 center coincides in one and the same crystal. Consequently the depth of the well in which the electron of STE is trapped seems to be defined by the lattice energy only. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Luminescence of Pr 3+ doped K 2LaCl 5 microcrystals encapsulated in KCl host

Scanning electron microscopy and luminescence spectroscopy studies have shown the formation of Pr 3+ doped K 2LaCl 5 microcrystalline phase embedded in KCl host after annealing of KCl–LaCl 3 (0.5 mol.%)–PrCl 3 (0.05 mol.%) at 250 °C. The luminescent characteristics of KCl–LaCl 3 (0.5 mol.%)–PrCl 3 (0.05 mol.%) are found to be similar with those of K 2La x Pr 1− x Cl 5 bulk crystals. The luminescence of Pr 3+ ion is observed from KCl–LaCl 3 (0.5 mol.%)–PrCl 3 (0.05 mol.%) due to the radiative transitions within Pr 3+ 4f 2 configuration whereas Pr 3+ 4f5d → 4f 2 emission is not revealed. Excitonic mechanism of energy transfer to the Pr 3+ activator centers in KCl–LaCl 3 (0.5 mol.%)–PrCl 3 (0.05 mol.%) crystal is considered.

Line shapes of narrow optical bands: Infrared absorption byUcenters and heavier impurities in alkali halides

The shape of the bands for photon absorption and emission by the local constituents of a solid is governed mainly by processes involving many low-energy acoustic phonons. This applies not only to wide bands, such as those exhibited by F centers, but also to narrow ones, as those observed for infrared absorption by local vibration modes of U centers and heavier impurities. The line shapes are theoretically studied on a general basis to show they provide a nice example to illustrate the power of field theory and methods to reproduce experimental facts. To this aim, the phonon induced broadenings of infrared absorption lines by U centers in KCl and KBr, and by substitutional Ag+ in KI, were calculated to compare theoretical predictions with experiment. The agreement obtained between both is remarkable.

De-excitation process and hot luminescence in the F A(type I) center in KCl:Na

The de-excitation process of F A(type I) centers in KCl:Na has been investigated by measuring the hot luminescence spectrum from optically excited F A centers with time-resolved spectroscopy. The experimental results are analyzed by using a model that describes a time evolution of the phonon wave packet during the vibronic relaxation process from the Franck–Condon state to a relaxed excited state. From the analysis of the experimental data, information on the vibronic mixing between 2p and 2s states, whose magnitude varies during the relaxation process, and the adiabatic potential energy curves of 2s and 2p states are extracted. The present results are compared with the already known ones of the F A(type II) centers.

Vibronic relaxation processes of FA centers in KCl:Li

The vibronic relaxation process of FA centers in KCl:Li has been studied for the first time by measuring the hot luminescence spectrum from optically excited FA centers with time-resolved spectroscopy. It is shown that the FA center in KCl:Li has a relaxation process belonging to a Type I center relaxation behavior from its absorption at ∼670nm up to ∼840nm, although it is classified as a Type II center because of the large Stokes-shift of its emission band at ∼2.7μm. The relaxation after FA2 band excitation undergoes an anticrossing process due to the vibronic mixing effect of 2s and 2px,(y) states, similarly to that of F centers in KCl. For FA1 band excitation, the relaxation proceeds along the adiabatic potential of 2pz state, at least within the energy range investigated here. From the data obtained in the present study, the adiabatic potential energy curves of 2s and 2p states have been tentatively deduced.

Vibronic relaxation processes of F A centers in KCl:Li

The vibronic relaxation process of F A centers in KCl:Li has been studied for the first time by measuring the hot luminescence spectrum from optically excited F A centers with time-resolved spectroscopy. It is shown that the F A center in KCl:Li has a relaxation process belonging to a Type I center relaxation behavior from its absorption at ∼670 nm up to ∼840 nm , although it is classified as a Type II center because of the large Stokes-shift of its emission band at ∼2.7 μm . The relaxation after F A2 band excitation undergoes an anticrossing process due to the vibronic mixing effect of 2s and 2p x,( y) states, similarly to that of F centers in KCl. For F A1 band excitation, the relaxation proceeds along the adiabatic potential of 2p z state, at least within the energy range investigated here. From the data obtained in the present study, the adiabatic potential energy curves of 2s and 2p states have been tentatively deduced.

Anticrossing process in the vibronic relaxation ofFcenters in KCl

The deexcitation process of F centers in KCl has been investigated by means of hot luminescence spectra measured with time-resolved spectroscopy under one-photon and two-photon excitation. An analysis of the experimental results was performed by considering the vibronic mixing of $2s$ and $2p$ excited states. From this analysis, it was found that the deexcitation after one (two) -photon excitation does not proceed along the zero-order potential energy surface (PES) of the $2p(2s)$ state which crosses the PES of the $2s(2p)$ state, but proceeds along one of the PES's that are obtained after $2s\ensuremath{-}2p$ mixing interaction is included. By using the present results and other existing data on the F centers in KCl, a brief sketch of the potential energy curves is given.

Hyperfine interactions and lattice distortion of the F center in KCl, NaCl and LiCl crystals

Detailed embedded-cluster quantum-chemistry (ROHF, UHF and UMP2) calculations of hyperfine interactions for the F center in KCl, NaCl and LiCl are presented. Our values for the isotropic hyperfine interaction show a good general agreement with experiments. We show that a careful treatment of the lattice environment, local lattice distortion, spin polarization and correlation are important for this good agreement.

Study of silver atoms in anionic position in KCl

The local geometry and electronic distribution of silver atoms trapped in anionic centres (Ag a 0 ) of KCl has been studied by means of Density Functional Theory (DFT) and cluster models involving up to 123 ions. This leads to an important outwards relaxation of the metal-ligand distance of about 13.1% with respect to the pure lattice (R K m Cl =3.14 v Å), which plays an important role for understanding experimental EPR data. The higher stability and more localised nature of this centre, compared to Ag 0 at a cationic site (Ag c 0 ), is shown to be related to the energy difference between the Ag(5s) level and the conduction band. The charge distribution in the a 1g level for Ag a 0 and Ag c 0 are compared. These results allow one to understand the different hyperfine constants due to both centres in KCl.

Ab initio model potential embedded-cluster calculation of the geometric structure of Tl + monomer and dimer centers in KCl

When alkali halide crystals are doped with ns 2 impurities such as Tl +, dimer centers as well as monomer centers are formed. In this paper, we present the results of ab initio model potential (AIMP) embedded cluster calculations of the geometric structure of the centers formed in Tl +-doped KCl. Using the AIMP method, which is able to bring into an ab initio cluster calculation all the main interactions with a frozen lattice (classical Madelung and short-range Coulomb, quantum mechanical exchange and orthogonality) as well as the effects of polarization and relaxation of the lattice ions external to the cluster, we have optimized the geometry of clusters representing the monomer and the D 2 h and D 4 h dimer defects, in their electronic ground state. We have found that the main result is an expansion of the clusters with respect to the perfect lattice geometry, in line with the larger ionic radius of Tl + compared to K +. As it was observed in other point defects, the lattice relaxation effects result in an enhancement of the distortions calculated with a frozen lattice. Even though the structures of the present defects are important ingredients for the detailed understanding of ns 2 defects in halides, and in particular of their spectroscopy, no direct measurements of them exist. Furthermore, the measurements are expected to be very difficult to perform, which leads to the relevance of the present calculations. The reliability of the results lies in the systematically good performance that the present method observes in transition metal doped ionic crystals.

Transformation of the luminescent centres in KCl–In crystals under synchrotron radiation

In view of the opportunity to use alkali–halide crystals with indium impurity for the registration of ionising radiation we have studied the influence of a method of impurity doping and mechanical treatment of a crystal on its sensitivity to X-ray. It is shown that the doping of a crystal with In 3+ reduces the sensitivity of medium. Optical and X-ray spectral experiments have allowed us to conclude that this phenomenon is caused by a change in the impurity centre structure. It is proved experimentally that mechanical crushing results in the transformation of the luminescent centres. This does not allow the effective conversion of the centres under synchrotron radiation.

Excitonic and electron–hole mechanisms of the creation of Frenkel defect in alkali halides

Excitonic and electron–hole (e–h) mechanisms of stable F centre creation by VUV radiation in alkali halide crystals are discussed. In KCl at 4.2 K, the efficiency of stable F–H pair creation is especially high at the direct optical formation of triplet excitons with n=1. At 200–400 K, the creation processes of stable F centres in KCl are especially efficient at the formation of one-halide exciton in the Urbach tail of an exciton absorption. In KCl and KBr, the decay of a cation exciton (∼20 eV) causes the formation of two e–h pairs, while in NaCl a cation exciton (33.5 eV) decays into two e–h and an anion exciton. An elastic uniaxial stress of a crystal excited by VUV radiation decreases the mean free path of excitons before their self-trapping (KI) and increases the mean free path of hot holes before self-trapping (NaCl).