Energy of migration of monovalent ions in NaCl: An experimental and theoretical study

Experimental diffusion measurements show that migration enthalpies of Cl^?, Br^? and I^? in NaCl are comparable, while that of F^? is considerably lower. Earlier studies had shown that migration enthalpies of Na⁺, K⁺, Rb⁺ and Cs⁺ in NaCl were similar. The polarised point ion model predicts migration energies of ions (by vacancy mechanism) to monotonically increase with ion size, contrary to experiment. Inversely, the shell model calculations rightly predict the variation of migration energies with ionic size. Thus, migration energies by vacancy mechanism do not vary significantly for ions larger than the host ions. However, in the case of the small ions, Li⁺ and F^?, the migration energies by vacancy mechanism are much lower and in good agreement with experiment for F^?.     

A theoretical study of F centres in BaClF and SrClF

We present point-ion calculations of the electronic structure of F centres in BaClF and SrClF. The relative positions of the singlet and doublet absorption bands of the F centres at F^? and Cl^? sites suggest a new assignment of the bands to the F centre sites. The F(F^?) has the singlet below the doublet, the F(Cl^?) the reverse order. Comparison of calculated spin resonance parameters with experiment supports this suggestion.     

Temperature variation of the Debye-Waller factors of metal and halide ions in CsCl-Br solid solutions between room temperature and 90°K by powder X-ray diffraction

Solid solutions of CsCl-Br in different molal concentrations were prepared and X-ray diffractograms taken. The integrated intensities of the Bragg peaks have been estimated and structure factors were obtained. The crystal lattice in each of these solid solution samples is made up of a random distribution of Cs⁺, Cl^? and Br^? ions depending upon the molal concentration. A pseudo-atom model has been proposed in which the Cs⁺ ions occupy (000) position and a pseudo-atom occupies (1/2 1/2 1/2) position. The presence of incoherence scattering in these solid solutions has been considered by applying the necessary correction to the atomic scattering factors. The integrated intensities have been analysed and the temperature variation of the Debye-Waller factors of the metal (Cs⁺) ion and the pseudo ion (CB^?) have been estimated.     

The Stretching Vibration of Hydroxyl Ions in LaGaO3

The presence of hydroxyl ions in LaGaO₃ single crystals has been detected by means of Fourier Transform Infrared (FTIR) absorption spectroscopy. A single narrow absorption line of the OH^? stretching vibration at about 3521 cm^?1 has been observed at T=9k in most of the as-grown crystals. The vibrational frequency decreased abruptly at the orthorhombic-rhombohedral structural phase transition temperature (T_c=418K). The parameters of the anharmonic potential of the OH^? stretching vibration have been determined from the isotopic replica and the overtone frequency and intensity, and found to be in agreement with those measured for hydroxyl ions in other oxide crystals.     

X-ray structure refinement and ionic conductivity of Na6.69Ca3.355(SO4)6Cl0.77F0.63

The structure of Na_6.69Ca_3.355(SO₄)₆Cl_0.77F_0.63, isostructural with fluorapatite, was determined by X-ray powder diffraction methods. The results of Rietveld refinement revealed a space group P6₃/m with lattice parameters of a?=?9.477 (2) Å, c?=?6.865 (5) Å. Final refinement led to R _F?=?1.83 % and R _B?=?7.64 %. The location of Na⁺ ions in the M (2) sites surrounding the channels was related particularly to the high polarizability of the Ca²⁺. The ionic conductivity over a wide range of temperature was investigated according to the complex impedance method. The highest overall conductivity values were found at σ _500 °C?=?1.03?×?10^?5?S?cm^?1 and E_a?=?0.70 eV.     

Anharmonicity in silicate crystals: Temperature dependence of Au type vibrational modes in ZrSiO4 and LiAlSi2O6

Infrared reflection spectra for the extraordinary ray of zircon and spondumene have been obtained in the frequency range from 270 to 1500 cm^?1 and from room temperature up to 1300°K. Using a combination of Kramers-Kronig analysis and a classical dispersion theory, the oscillator parameters of the A_2u modes in zircon and A_u modes in spodumene were determined. The temperature dependence of observed mode damping is analysed on the basis of quantum theoreticall results involving both cubic and quartic anharmonic contributions. The comparison of the present results with other data reported in previously studied silicate crystals, shows that the observed anharmonic behavior of internal and external modes is consistent with simple considerations based on the site vicinity of each ion.     

Electron paramagnetic resonance of the Eu2+ ion in SrCl2 at high pressure

This paper reports an EPR study of the effect of hydrostatic pressure (up to 10 kbar) and temperature (300, 77, and 4.2 K) on the spin Hamiltonian parameters of the Eu²⁺ ion in a SrCl₂ cubic crystal. It is found that the b ₄ parameter is related by a power law to the distance from the Cl^?1 ligand (b ₄ ～ R ^?13.5). The pressure and temperature are shown not to be equivalent thermodynamic parameters. Lattice vibrations contribute noticeably to the initial S-ion splitting.     

Theoretical calculation of the entropy change associated with substitutional anion defects in KCl

The entropy change S_d accompanying the substitution of a Cl^? ion in KCl by a F^?, Br^? or I^? ion has been calculated by a method that involves the whole phonon spectrum of the perfect crystal. The results are: $\text{S}{}_{\mathrm{d}}\text{k}\text{= -0.909}$ for KCl:F^?, 1.897 for KCl:Br^? and 4.055 for KCl:I^?. These figures are compared with the results obtained by approximate methods that have been used previously.     

Structure of negative impurity ions in liquid helium

The experimental mobilities of negative halogen (Cl^?, F^?, and I^?) and metal (Ba^? and Ga^?) impurity ions in superfluid ⁴He are close to each other and much lower than the mobilities of not only He⁺ ions but also electron bubbles. It has been shown that the formation of multiatomic complexes (clusters or bubbles) around ions is responsible for this low mobility. Although the mobilities are similar, the structures of the resulting complexes are qualitatively different in the cases of halogens and metals: solid clusters, which are similar to a well-studied cluster at the He⁺ ion, are formed near halogen ions, which exhibit high electron affinities, whereas metal ions are localized in bubbles, which are similar to electron bubbles. The temperature and pressure dependence of the mobility of these complexes is qualitatively different. Experiments in this area, most likely, performed with a wider variety of negative ions, would enhance the understanding of the structure of charged complexes in liquid helium.     

ENDOR and transferred hyperfine interaction of impurity rare-earth ions with nearest diamagnetic ions in crystals

The tetragonal Er³⁺ ion associated with the interstitial F^? ion along the [100] axis in CaF₂ is studied using ENDOR. The parameters of the transferred hyperfine interaction and of the nuclear Zeeman interaction of the surrounding fluorine ions are determined. Anomalously large values of the pseudo-nuclear Zeeman effect on the F^? nuclei are found. The theoretical analysis of these parameters is carried out in a frame of operator techniques in the theory of transferred hyperfine interactions. A number of useful relations for practical calculations of the values of the local field at ligand nuclei are reported.     

An Analysis of the Trigonal Center Structure of Yb3+ Ion in CsCaF3

The crystal field parameters determined from interpretation of optical spectra are used to analyze distortions of a crystal lattice in the vicinity of an impurity ion and vacancy at a Cs⁺ site compensating the excess positive charge in the trigonal centers of Yb³⁺ ions in CsCaF₃ crystal. Interactions of the impurity ion with the nearest neighbors (an octahedron of F^? ions) and the next nearest neighbors (a cube of Cs⁺ ions) are considered within the superposition model. It is established that, at formation of the trigonal center, three F^? ions of the nearest octahedron, placed symmetrically along the threefold axis on the side of the vacancy, move away from the impurity ion a little and significantly deviate from this axis. The second triangle of F^? ions, on the contrary, comes nearer to the impurity ion and nestles on the axis of the center a little. The three Cs⁺ ions, the second neighbors on the side of the vacancy, slightly come nearer to Yb³⁺ ion and considerably nestle on the center axis. The second triangle of Cs⁺ ions, from the opposite side of vacancy, also comes nearer to the paramagnetic ion and also nestles on the center axis a little. The Cs⁺ ion, lying on the center axis, comes considerably nearer to the impurity ion.     

High-field EPR Study of the Effect of Chloride on Mn2+ Ions in Frozen Aqueous Solutions

The effect of chloride concentration on Mn²⁺ (S = 5/2, I = 5/2) ions in frozen aqueous solutions is studied by high-field high-frequency electron paramagnetic resonance (HFEPR). The usually six sharp lines characteristic of Mn²⁺ ions, arising from the m _s  = ?1/2 → 1/2 transition, is modified by the addition of Cl^? anions and the six resonances become much broader and more complex. This new feature likely arises from the ligation of one Cl^? anion to a hydrated Mn²⁺ ion forming a [Mn(H₂O)₅Cl]^? complex. This complex increases linearly with Cl^? concentration with an association constant of K _{a, apparent} = 61 M^?1. The structure of the putative chloride complex was studied using density functional theory calculations and the expected zero-field interaction of such a manganese center was calculated using the superposition model. The predicted values were similar to those determined from the simulation of the spectrum of the m _s  = ?5/3 → ?3/2 transition of the chloride complex. This effect of Cl^? anions occurs at biologically relevant concentration and can be used to probe the Mn²⁺ ions in cellular and protein environments.     

Local structure of bivalent copper centers in SrF2 crystals: An ENDOR spectroscopic study

Structural distortions of the SrF₂ crystal lattice near the bivalent copper impurity Jahn-Teller center are investigated by the ENDOR method (v=9.3 GHz, T=4.2 K). The approximate directions and the magnitudes of displacements of a Cu²⁺ impurity ion and its surrounding F^? ions are determined with respect to one of the anionic networks in the crystal matrix. The tensor components for the ligand hyperfine interaction (LHFI) with fluorine ions separated from the impurity by a distance R≤6.2 Å are obtained from the angular dependences of the location of the ENDOR resonance lines. It is found that the parameters of magnetic interactions between the impurity and these ligands contain the contributions determined by the covalence of bonds in the impurity complex and the polarization of electron shells of the ligands.     

Analysis of the structure of tetragonal oxygen centers associated with Yb<Superscript>3+</Superscript> ions in the KMgF<Subscript>3</Subscript> crystal

The parameters of the crystal field of the tetragonal oxygen center associated with a Yb³⁺ ion in the KMgF₃ crystal found previously in a study of optical and ESR spectra are applied to analyze lattice distortions in the vicinity of the impurity ion and the O^2? ion compensating for the excess positive charge. Within the superposition model, it was ascertained that the Yb³⁺ ion and the neighboring ions of fluorine and oxygen on the axis of the center shift significantly along the direction from the O^2? ion to the Yb³⁺ ion during the formation of the tetragonal oxygen center. As this takes place, the distances of both (fluorine and oxygen) ions from the impurity ion increase. The four F^? ions of the nearest octahedral neighborhood of Yb³⁺ that are arranged symmetrically in the plane perpendicular to the axis of the center slightly recede from the axis.     

Theoretical study of trigonal Fe center in SrCl2:Fesystem

The presence of Fe³⁺ centers with trigonal symmetry in chlorinated SrCl₂ crystal is an interesting phenomenon. By diagonalizing the complete energy matrices for a d⁵ configuration ion in a trigonal ligand-field and simulating the EPR low-symmetry parameters D and (a−F) simultaneously, the local lattice structure around trigonal Fe³⁺ center in SrCl₂:Fe³⁺ system has been studied. It is shown that Nistor et al.'s viewpoint about replacement is right, but the Cl⁻ ion along 〈111〉 axis around the Fe³⁺ center is replaced not by an O²⁻ ion but by some negative ion with effective charge to be less than that of Cl⁻ ion. Our results indicate that when the ratio of the effective charge of the negative ion to that of Cl⁻ ion is 0.8 as well as the distortion angle of the upper triangle is Δθ=−4.682°, the EPR parameters D and (a−F) can be explained satisfactorily.     

Electron nuclear double resonance of the cubic Dy3+ center in the KZnF3 single crystal

ENDOR measurements on the¹⁹F^? nuclei in the first four shells of KZnF₃ containing Dy³⁺ ions in the cubic site are reported. The values and signs of the hyperfine and transferred hyperfine interaction parameters are determined. The local deformation of the crystal lattice in the vicinity of the impurity ion is estimated. The theoretical analysis of the THFI parameters for the first coordination shell of the F^? ions has been carried out. For the Dy³⁺ ion the influence of spin polarization of the closed 5s and 5p shells is considered for the first time. Spin polarization is shown to play a significant role in the mechanisms of rare-earth ion-ligand coupling.     

Anionische Silberzentren („B-Zentren“) in Alkalihalogeniden

It is proposed that negatively charged silver ions on anion sites are responsible for theB bands found in silver doped alkalihalides. Experimental investigations confirming this model of theB center are presented. Optical absorption and emission ofB centers were measured in seven alkalihalides in the temperature range from 450 to 4 °K. Configuration coordinate diagrams were obtained for KCl, KBr, and KI. A chemical method was used to determine the charge of theB center. Thermal or optical excitation causes theB centers to dissociate into neutral silver atoms andF centers. The Ag⁰ centers are bound to interstitial positions at low temperature. At high temperature they collect together forming colloidal centers. TetragonalB _A centers were formed during optical bleaching of theB band in mixed crystals of the type KCl+ΔNaCl. Their absorption and emission spectra, optical orientation, and polarized emission were investigated. These properties are shown to be similar to those ofF _A centers.B _A fluorescence polarized almost completely in the [100] direction was observed. A Jahn-Teller splitting of theB absorption band was resolved in RbCl at low temperature. The splitting confirms the configuration5s² for the Ag^? ion. The results are compared with those for the isoelectronic centers In⁺ and Sn⁺⁺. Thin films of alkalihalides containing small amounts of silver, copper, or thallium were condensed simultaneously with alkali vapor. New bands in the UV region were found, possibly due to Cu^? and Tl^? centers.     

Resonance fluorescence of gaseous chlorine excited with a single longitudinal mode argon ion laser

Resonance fluorescence spectra of gaseous chlorine, excited with a high power single longitudinal mode argon ion laser which was tuned through the lasing profiles of the 4880-, 4965-, 5017-, and 5145-Å lines, were studied. Five progressions of fluorescence lines were found and assigned as the fluorescence of ³⁵Cl³⁵Cl and ³⁷Cl³⁷Cl isotopic molecules resonantly excited from v = 0 and 1 levels of the X¹Σ state to the B³Π state. The fluorescence line positions agreed with those of the theoretical calculation within 0.2 cm^?1. The fluorescence intensities were very weak due to the extremely small transition probability between the B³Π and the X¹Σ states.     

Propagation and Diagnostic of Ion Acoustic Waves in a Low Pressure Argon Discharge

In a dc argon discharge column of pressure 5 × 10^?4 torr, ion acoustic waves are studied experimentally. Plasma parameters obtained from the phase velocity of waves are compared with the Langmuir probe measurements. The wave diagnostic method yields the electron temperature lower than the probe method by a factor of 3…?5, if the atomic argon ion is assumed as a dominant ion species. The axial profiles of ion drift velocity show the presence of ions flowing toward the anode along the potential drop in flont of it. Also it is found that the surface condition of the cathode sensitively affects the propagation of ion acoustic waves through the changes in discharge parameters.     

Crystal structure,spectroscopy and magnetism of binuclear complexes of mercury and lanthanides; the Ln(NCS)(HMPA)4(μ-SCN)2HgCl(SCN) type

A series of lanthanide compounds of type Ln(NCS)(HMPA)₄,(μ-SCN)₂HgCl(SCN) (Ln = Pr, Nd, Eu) were synthesized and grown in the form of single crystals.

The crystal structure of the neodymium complex was determined by X-ray diffraction. Its space group is Cc, with the following unit cell parameters; a = 17.338(3) Å, b = 15.795(3) Å, c = 21.828(4) Å, β = 107.65(3)°. The structure has an unexpected architecture in which one Cl^? ion, four SCN^? ions, and four oxygen atoms of HMPA groups are engaged in the metal ion coordination.

The binuclear complex is composed of two types of subunits; seven coordinated Nd (III) and four coordinated Hg (II). The results obtained were compared with the earlier published data on the crystal structures of polynuclear complexes with ions of the IIa group (Zn or Cd). Luminescence, excitation of luminescence and absorption spectra of lanthanide (Pr, Nd, Eu) single crystals, as well as vibrational IR and Raman spectra at 293, 77 and 4K, were recorded. Non-trivial results of reabsorption of the d-level of Pr(III) emission by ³H₄ → ³ P_J, ₁D² transitions were observed with simultaneous detection of emission from the ³P₀ level after excitation in the UV region. The experimental oscillator strengths of the transitions were determined from the absorption spectra and parametrized in terms of the Judd-Ofelt intensity parameters Ω_λ (λ = 2, 4, 6).

Satisfactory results for the calculation with low errors of estimation of the parameters were obtained for a crystal of the Nd-Hg compound, which reproduced the intensities of the electronic transitions well. Positive values of Ω_λ were evaluated for Pr(III) after including the ³H₄ → ³F₂ hypersensitive transition (obeying selection rules δJ = 2, δL = 2) in the calculations.

Based on the above results, the radiative rate constant can be determined. Strong vibronic components were found in the low temperature spectra for both types of ligands involved in metal ion coordination. The vibronic transitions are mainly associated with modes of groups directly coordinated to the metal ions. Electron-phonon coupling including the resonant vibronic effect was analysed based on IR and Raman data.

Magnetic susceptibility measurements were carried out down to 1.7 K. Correlation of the spectra and magnetic properties with details of the structure of the title compound was studied.