Centers In NaCl Research Articles

Atomistic insight into the formation dynamics of charged point defects: A classical molecular dynamics study ofF+-centers in NaCl

This paper provides an atomistic exploration of the lattice dissipation mechanisms accompanying the formation of charged point defects through a femtosecond resolved study of ${\mathrm{F}}^{+}$-center creation in NaCl. Our findings, following from a classical molecular dynamics based investigation of this model system, point to general range of properties that should be present in similar systems. Immediately after the creation of such a charged defect center, its excess energy is imparted amongst the highest energy optical modes with no clear preference based on their degree of localization. This energy is then dissipated through equilibration amongst a bath of lower energy phonon modes. The temporal behavior primarily follows exponential decay trends at all the temperatures and energies explored, with a small degree of competition between phonon population and depopulation amongst lower energy bath modes. Moreover, the dissipation timescale is found to be approximately the same amongst all phonon energies. A temperature-dependent analysis shows the expected decrease in phonon lifetimes with increasing temperature. This is accompanied by similarly more rapid dissipation of thermal energy around the defect center at lower temperatures when the phonon mean free path is increased. An intuitive phenomenological model based on Langevin dynamics is also provided to interpret the atomistically derived phonon decay characteristics in the temporal domain. More broadly, these results are expected to aid the design and experimental investigation of strongly correlated materials where charged defect centers can play an important role in technological applications.

Supercontinuum generation in the absence and in the presence of color centers in NaCl and KBr

We study supercontinuum generation with femtosecond mid-infrared laser pulses in NaCl and KBr crystals when pumped at the vicinity of their zero group velocity dispersion points. Almost three octave-spanning SC spectra (covering the 0.7–5.4 μm and 0.85–5.4 μm ranges in 5 mm-thick NaCl and KBr samples, respectively) were produced by filamentation of 70 fs, 3.6 μm input pulses in continuously translated samples. In the static setup, we show that rapid formation of persistent color centers (within a few seconds at 500 Hz repetition rate) inside these materials results in a marked decrease (by 40%) of the overall energy transmittance and in significant narrowing of SC spectra. We also demonstrate that the effect of color centers on the SC spectrum could be adequately simulated numerically using a simple phenomenological model which considers color centers as impurities with a certain energy bandgap.

Theoretical analysis of the kinetics of low-temperature defect recombination in alkali halide crystals

We analyzed carefully the experimental kinetics of the low-temperature diffusion-controlled F, H center recombination in a series of irradiated alkali halides and extracted the migration energies and pre-exponential parameters for the hole H centers. The migration energy for the complementary electronic F centers in NaCl was obtained from the colloid formation kinetics observed above room temperature. The obtained parameters were compared with data available from the literature.

Investigations of spin Hamiltonian parameters and local structures for three Cr(CN)63− centres in NaCl at three temperatures

The spin Hamiltonian parameters (g factors and zero-field splittings (ZFSs)) and local structures of three Cr(CN)63− centres in NaCl are studied at three temperatures based on the perturbation calculations. The signs and magnitudes of the ZFSs D and E are in accordance with those of average bond length discrepancy ΔR‾(=(1/6)∑δRi,i=1–6) and rhombic distortion parameter H(=-(Δφ‾)1/2(ΔR‾)3/D), relevant to average planar bond angle discrepancy Δφ‾(=(1/4)∑δφi,i=1-4) related to an ideal octahedron. The SHPs exhibit the smallest rhombic (or orthorhombic) distortions and the weakest covalency at 4.2K, and the largest rhombic distortions and strongest covalency at 77K with comparable D and E. The magnitude of g-shifts (=gi−gs) obey the tendency 77K<290K⩽4.2K, in consistent with that of covalency factor N. The CT contributions to g-shifts are opposite in sign and about 51–63% in magnitude compared with the CF ones. The microscopic mechanisms of the local structures (i.e., the impurity displacements ΔRX and ΔRY and ligand displacements ΔRP, ΔRP′ and ΔRP″) are analyzed for the various centres in view of the electrostatic interactions arising from the cation vacancies. Unlike conventional rhombic centres I and II for all three temperatures, the new orthorhombic centre III occurs only at low temperatures (4.2K and 77K) and indeed exhibits new structural properties, especially the largest ΔR‾ and H as well as D and E.

Investigation of the EPR g factors and defect structure for the Cu 2+– VNa center in the X-irradiated NaCl: Cu + crystal

The g factors of a tetragonally-compressed Cu 2+ center in NaCl: Cu + crystal X-irradiated at room temperature are calculated from the high-order perturbation formulas based on the two-mechanism model. In the model, the contribution to g factors from both crystal-field ( CF) and charge-transfer ( CT) mechanisms are included. The calculations are based on the defect model that the tetragonally-compressed Cu 2+center is assigned to the Cu 2+ ion (which is caused by Cu + ion (at the Na + site) irradiated by X-ray) associated with a nearest Na + ion vacancy V Na along C 4 axis due to charge compensation. From the calculations, the g factors g || and g ⊥ are explained and the defect structure (charactering by the displacement Δ Z of the Cl − ion intervening in Cu 2+ and V Na) of the Cu 2+ (or Cu 2+– V Na) center is obtained. The results are discussed.

Assessment of Periodic and Cluster-in-Vacuo Models for First Principles Calculation of EPR Parameters of Paramagnetic Defects in Crystals: Rh2+ Defects in NaCl as Case Study

In order to find a reliable and efficient calculation scheme for electron paramagnetic resonance (EPR) spectroscopic parameters for transition metal complexes in ionic solids from first principles, periodic and finite cluster-in-vacuo density functional theory (DFT) simulations are performed for g tensors, ligand hyperfine tensors (A), and quadrupole tensors (Q) for Rh(2+)-related centers in NaCl. EPR experiments on NaCl:Rh single crystals identified three Rh(2+) monomer centers, only differing in the number of charge compensating vacancies in their local environment, and one dimer center. Periodic and cluster calculations, both based on periodically optimized structures, are able to reproduce experimentally observed trends in the ligand A and Q tensors and render very satisfactory numerical agreement with experiment. Taking also computation time into account as a criterion, a full periodic approach emerges as most appropriate for these parameters.The g tensor calculations, on the other hand, prove to be insufficiently accurate for model assessment. The calculations also reveal parameters of the complexes which are not directly accessible through experiments, in particular related to their geometry.

Color centers in NaCl by hybrid functionals

We present in this work the electronic structure and transition energies (both thermodynamic and optical) of Cl vacancies in NaCl by hybrid density functionals. The underestimated transition energies by the semi-local functional inherited from the band gap problem are recovered by the PBE0 hybrid functional through the non-local exact exchange, whose amount is adjusted to reproduce the experimental band gap. The hybrid functional also gives a better account of the lattice relaxation for the defect systems arising from the reduced self-interaction. On the other hand, the quantitative agreement with experimental vertical transition energy cannot be achieved with hybrid functionals due to the inaccurate descriptions of the ionization energies of the localized defect and the positions of the band edges.

Investigations on the Spin Hamiltonian Parameters and the Local Structures for Various Rh2+ Centers in NaCl

The spin Hamiltonian parameters (the g factors, the hyperfine structure constants, and the superhyperfine parameters) and the local structures for various Rh2+ centers OI, OII, and RTAX in NaCl are theoretically investigated from the perturbation formulas of these parameters for a 4d7 ion in tetragonally and orthorhombically elongated octahedra. The related molecular orbital coefficients and the ligand unpaired spin densities are determined quantitatively from the cluster approach in a uniform way. The centers OI, OII (orthorhombic) or RTAX (tetragonal) are attributed to the substitutional Rh2+ on Na+ site, associated with two, one or none next nearest neighbour cation vacancies VNa along [100] (or [010]) axis, respectively. The ligand octahedra in the orthorhombic centers OI and OII are found to suffer the relative elongations ΔZ ≈0.071 and 0.068 °A along the [001] axis due to the Jahn-Teller effect, and the intervening ligand(s) in the VNa and the Rh2+ may undergo the inward displacements ΔX ≈ 0.001 and 0.011 A° towards Rh2+, respectively. As for the tetragonal center RTAX, the uncompensated [RhCl6]4− cluster is found to experience the relative elongation ΔZ ≈ 0.067 °A along the [001] axis of the Jahn-Teller nature. The calculated spin Hamiltonian parameters based on the above local structures show good agreement with the observed values for all the centers.

Theoretical study of the electron paramagnetic resonance parameters and local structure for the tetragonal Ir2+ centre in NaCl

The electron paramagnetic resonance (EPR) parameters (the g factors, hyperfine structure constants and the superhyperfine parameters) for the tetragonal Ir(2+) centre in NaCl are theoretically investigated from the perturbation formulas of these parameters for a 5d(7) ion in tetragonally elongated octahedra. This impurity centre is attributed to the substitutional [IrCl(6)](4-) cluster on host Na(+) site, associated with the 4% relative elongation along the C (4)-axis due to the Jahn-Teller effect. Despite the ionicity of host NaCl, the [IrCl(6)](4-) cluster still exhibits moderate covalency and then the ligand orbital and spin-orbit coupling contributions should be taken into account. In addition, the theoretical EPR parameters based on the Jahn-Teller elongation show good agreement with the observed values.

Investigation of the structure of impurity defects in alkali halide crystals by the method of fluorescence EXAFS

It has been shown by experiment that the structure of impurity centers in NaCl–Sr crystals depends on impurity concentration and storage time of crystals. At low concentration and in quenched crystals the impurity is in site of the lattice. At concentrations higher than 0.15 wt% Sr, the impurity forms SrCl 2 phase. For the crystals containing Cu + impurity, it has been shown that an increase in the impurity concentration leads to the formation of clusters with CuCl structure. At the same time the existence of isolated copper ions shifted from the sites of the lattice is obvious. This leads to asymmetry of the environment of copper, which is manifested both in the absorption spectra in the ultraviolet region and in the EXAFS spectra. Heating of the crystals leads to destruction of complex clusters and formation exclusively of centers containing isolated copper ions.

Heavy ion induced damage in NaCl and KCl crystals

NaCl and KCl single crystals were irradiated with swift heavy ions (Au, Pb, Bi and U) of energy between 560 and 2640MeV. The damage induced by the ions was studied using optical absorption spectroscopy, chemical etching and small-angle X-ray scattering. The damage in the tracks is dominated by the creation of simple color centers (F and V3 centers). Their concentration increases as a function of fluence, saturating when single tracks overlap. The F-center containing track zone extends up to several tens of nanometers. In addition to this large track halo, there exists also a much smaller region (1–2nm) close to the ion trajectory in which defect aggregates are formed. Indications for this track core are given by the preferential chemical etchability of ion tracks and also by the anisotropic pattern produced by small-angle X-ray scattering. This halo-core track structure is very similar as in LiF, however given by the higher mobility of the primary hole centers in NaCl and KCl, recombination with electrons is facilitated resulting in a lower efficiency for defect creation.

Structural, Magnetic and Optical Properties of Rh2☎ Centres in NaCl Lattices: An Explanation

A method for exploring Jahn—Teller systems in the framework of Density Functional Theory is described in the present work. Using such a method the properties of the Rh2☎ centre in NaCl with remote charge compensation are studied. The calculations support the existence of a strong Jahn—Teller effect and account reasonably for the main features of EPR and optical spectra. The onset of charge transfer excitations is calculated to be at about 3.7 eV a fact which concurs with available experimental data. The elongated geometry is found to be about 200cm−1 more stable than the compressed one in agreement with experiments.

Single Crystal Magnetic Resonance Study of a Rh2☎ Dimer Centre in NaCl at Q-Band

Electron nuclear double resonance spectroscopy at 34 GHz is applied to determine the signs of the principal values and the directions of the corresponding principal axes of the zero field splitting tensor for a Rh2☎ dimer centre in NaC1. The results are in good agreement with the hypotheses concerning the spin Hamiltonian parameters deduced from the X- and W-band EPR analysis and support the model proposed for this centre consisting of two Rh2☎ ions on cation positions and two Na☎ vacancies.

Puzzling absence of hyperfine interaction inthe D4hRhCl64− centre: role ofthe 4d–5s hybridization in Rh2+ centres

The amazing absence of hyperfine interaction in EPR spectra of the tetragonallyelongated RhCl64− centre in NaCl is explored through the present work. In additionto the core polarization contribution it is shown that there is anotherisotropic contribution from the small 5s admixture in the a1g*(≈ 3z2 − r2)level. These two contributions together with the anisotropic ones are shownto lead to a null hyperfine tensor for a 5s admixture of only ∼ 1.5%.Density functional calculations reveal that such an admixture is stronglysensitive to the tetragonality of the RhCl64− unit. Writing the axial(Rax) and equatorial(Req) Rh2+–Cl− distancesas Rax = R0 + 2aand Req = R0 − ait is shown that (i) the 5s admixture is proportional toa2 (fora≤25pm), a fact which can be accounted for through a simple model; (ii) relative variationsof Reqand Raxof only −0.4 and 0.8% lead to a hyperfine constant of 7 MHz. The presentresults are shown to shed some light on EPR data of the orthorhombicRhCl64− complex with two cis-equatorial Na+ vacancies in the fourth shell.It is concluded that the hyperfine interaction measured for this centrecan be explained in terms of the small strain induced by equatorialNa+ vacancies upon the RhCl64− unit. This idea can also explain the ∼ 2%decrement of the isotropic axial superhyperfine constant when compared with thatfor the tetragonal RhCl64− centre.

EPR spectrum and formation properties of a cubic Rh+ centre in NaCl

A cubic Rh+ centre in NaCl is studied with electron paramagnetic resonance (EPR) at Q-band (34 GHz). Its ΔMS = 1 EPR spectrum is shown to be broadened due to random distortions, lowering the cubic symmetry. At high microwave power, a sharp double quantum transition is observed, which exhibits a partially resolved hyperfine structure. From the formation conditions of this paramagnetic centre in solution and melt-grown NaCl crystals, its Rh2+ precursor centres are determined.

DIFFUSION OF Tl+ IONS THROUGH THE INTERFACE OF SOLID NaCl, KCl OR RbCl AND LIQUID TlCl

The diffusion phenomena of thallous ions ( Tl + ions) through solid-liquid interface of liquid Tl + ions diffusion source and sodium chloride (NaCl) , potassium chloride (KCl) or rubidium chloride (RbCl) single crystals are studied by optical method. The characterisic absorption peaks of Tl + center in NaCl, KCl or RbCl single crystals were used as the tracer for measurements and the diffusion constants are evaluated at various temperatures. The results show that the diffusion constant of solid-liquid interface is about 103 times larger than that of solid-solid interface of KCl and TlCl .

Q-band electron nuclear double-resonance detection of the cation vacancies near thecis−[RhCl6]4−⋅2Vaccomplex in NaCl

A low spin ${d}^{7}{\mathrm{Rh}}^{2+}$ center in NaCl with nearly axial symmetry is studied using Q-band electron nuclear double resonance. The analysis of the interaction of the unpaired electron with the first shell ${\mathrm{Cl}}^{\mathrm{\ensuremath{-}}}$ ions along the ${g}_{z}$ axis of the complex proves that the center has ${C}_{2v}$ symmetry, strongly supporting its identification as a $[{\mathrm{RhCl}}_{6}{]}^{4\ensuremath{-}}$ complex accompanied by two next-nearest-neighbor cation vacancies in a cis configuration. In addition, the twofold symmetry axis of this complex is identified as the g tensor axis corresponding to the largest principal g value. The magnetic resonance properties of this center can be explained by assuming that the central ${\mathrm{Rh}}^{2+}$ ion exhibits an off-center displacement toward the cation vacancies.

Theoretical insight into the Jahn–Teller system NaCl:Rh 2+

The RhCl 6 4− Jahn–Teller centre in NaCl has been explored through Density Functional calculations on clusters involving up to 39 atoms for different values of the Q θ (≈3 z 2− r 2) coordinate, where Q θ=( 12 /3)(R ax −R eq ), and R ax and R eq mean the axial and equatorial Rh 2+–Cl − distances. The equilibrium situation is found for Q θ=30.3 pm (R ax =270.7; R eq =244.5 pm) while another minimum is found at Q θ=−21.7 pm whose energy lies 213 cm −1 above. These facts and the different curvature at the two minima are all shown to arise from the anharmonicity of the E g mode. As a salient feature both the axial ( A s(ax)) and equatorial ( A s(eq)) isotropic superhyperfine constants are found to decrease when Q θ increases, A s(ax)/ A s(eq) being around 5 at equilibrium. This surprising result which is pointed out to be related to the 4d–5s hybridisation concurs with experimental findings. The crystal-field and charge transfer transitions calculated at the equilibrium geometry reasonably account for the experimental optical transitions of NaCl:Rh 2+.

Intracavity laser spectroscopy with NaCl:OH crystal colour-centre lasers

Lasers, based on the F2+ colour centres in NaCl:OH crystals and emitting in the range of 1.5 μm, were used to develop further the method of intracavity laser spectroscopy and to widen the spectral range of the method. Up-conversion was used in the detection stage. By way of example, the absorption spectra of water vapour, atmospheric air, carbon dioxide, and acetylene were recorded in the operating range of these lasers.

Reanalysis and identification of an Rh2+ dimer center in NaCl by combined application of 9.5 and 95 GHz EPR

A combined 9.5 (X-band) and 95 GHz (W-band) EPR study of an Rh2+ dimer center in Bridgman-grown rhodium-doped NaCl is presented. A close-to-tetragonal symmetry, similar to that of the monomer centers, with dominant symmetry axis along 〈001〉, the z-axis, is observed for this defect. The dimer and monomer spectra, which strongly overlap in the X-band spectra, could be well separated in the W-band measurements due to the higher g-resolution at higher magnetic fields. The anisotropy in the perpendicular plane, with 〈110〉 symmetry directions, is shown, from the combined X-and W-band spectra, to derive predominantly from a field-independent contribution, i.e. an orthorhombic spin–spin interaction, S1·D·S2 (S1 = S2 = 1/2), while the g matrices are near to axial, as in the case of the RhCl64−·2Vac monomer center (Vac = cation vacancy). This dimer center is shown to consist of a square arrangement of two substitutional Rh2+ ions and two next-nearest neighbor cation vacancies in an {001}-plane, with the Rh2+ ions on opposite corners at a distance of ca. 7.6 A, as estimated from the magnitude of the dipolar interaction. This work illustrates the ability of combined EPR experiments at widely different frequencies to discriminate field-independent from field-dependent terms in the spin-Hamiltonian.