Analysis of a qualitative criterion for forecasting off-centre displacement of impurity ions in ionic crystals

Summary With the aid of the Sanderson model for nonmolecular structure a qualitative criterion is derived for forecasting off-centre configurations of monovalent impurity ions (Li⁺, F⁻, Cl⁻, Na⁺, Ag⁺, Cu⁺) in alkali halide crystals. The same criterion is checked for Mn²⁺ impurity ions in some oxides. We also present a critical review of the criteria introduced in the past, as well as of the experimental results up to now known in the literature. Due to the relevance of its scientific content, this paper has been given priority by the Journal Direction. Work jointly supported by the Ministero della Pubblica Istruzione and by Consiglio Nazionale delle Ricerche, Gruppo Nazionale di Struttura della Materia.     

A simple criterion for forecasting off-centre configuration of monovalent impurity ions in alkali halide crystals

Summary With the aid of the Sanderson model for nonmolecular structures a simple criterion has been derived for forecasting off-centre configuration of monovalent impurity ions in alkali halide crystals based on the effect that the Sanderson partial charges on atoms have on their radii. Work jointly supported by Ministero della Pubblica Istruzione and by Consiglio Nazionale delle Ricerche, Gruppo Nazionale di Struttura della Materia.     

Photochromism in Cu-and Ag-doped Bi<Subscript>12</Subscript>SiO<Subscript>20</Subscript> crystals

The equilibrium and photoinduced absorption spectra of copper-and silver-doped Bi₁₂SiO₂₀ crystals are studied. It is demonstrated that the impurity absorption is due to Ag²⁺, Ag⁺, Cu³⁺, Cu²⁺, and Cu⁺ ions occupying almost octahedral Bi³ positions. A mechanism of photochromism is suggested, involving changes in the charge states of copper and silver impurity ions according to schemes Cu²⁺-e → Cu³⁺ and Ag⁺-e → Ag²⁺.     

Spectroscopy of silver complexes in solutions of inorganic compounds

The absorption, photoluminescence, and photoexcitation spectra of a number of inorganic solid solutions with a silver ion impurity have been examined. The influence of the temperature on the spectral characteristics of haloid and oxygen-containing solutions activated with Ag⁺ ions has been investigated. The temperature dependences of the luminescence quantum yield of solid solutions with Ag⁺ impurity in the temperature range 77-150 K have been studied. It is shown that the spectra under observation are conditioned by electron transition between energetic levels of Ag⁺ ion which are deformed because of the interaction with environment.     

Off-axis configuration of FA(II) centres in alkali halides

Abstract

The experimental values of the off-axis angle of F_A(II) dipoles in KCl:Li⁺, RbCl:Li⁺ and KF:Li⁺ are in good agreement with those obtained from theoretical estimates of the off-centre displacements of the Li⁺ ion calculated with the shell model potentials. On this basis, one can quantitatively predict the off-axis behaviour of F_A(II) centres from the calculated off-centre positions of small impurity ions in alkali halides.     

Calculation of exciton-neutral and exciton-charged impurities elastic scattering cross-sections in Cu2O

The scattering cross-sections for exciton-neutral and exciton-charged impurities have been calculated for Cu₂O doped with Ag⁺ ions. The cross-section for exciton-neutral impurity is in good agreement with that obtained previously by the authors from the experimental n = 1 line width in the yellow series of this material.     

A model for the acceptor state in II–VI compounds

The binding energy and the spatial distribution of an acceptor in an ionic semiconductor are calculated on the Anderson model with three parameters. The result is applied to the case of Li⁺, Na⁺, Cu⁺ and Ag⁺ in II–VI semiconductors. The parameters are evaluated by the dielectric theory of a chemical bond. It is shown that Li⁺ in ZnO, CdS, and CdSe forms an acceptor state with a larger amplitude at nearest-neighbor anion sites than at an impurity site; while Cu⁺ and Ag⁺ in Zn and Cd compounds have the opposite properties. The Na⁺ ion forms an acceptor state more localized densely at the impurity site in Zn compounds and vice versa in Cd compounds.     

Physical and Spectral Investigations of Cu<Superscript>2+</Superscript>-Doped Alkali Zinc Borate Glasses

Physical and spectral studies on 20ZnO + xLi₂O + (30-x)Na₂O + 50B₂O₃ (5 ≤ x ≥ 25) doped with 0.1 mol% of paramagnetic CuO impurity are carried out. Powder X-ray diffraction patterns of the glass samples confirm the amorphous nature. The physical parameters of all the glasses were also evaluated with respect to the composition. The electron paramagnetic resonance spectra of all these glasses exhibit resonance signals that are characteristic of Cu²⁺ ions. The optical absorption spectra also confirm the Cu²⁺ ion in tetragonally elongated octahedral site. Various crystal field, spin-Hamiltonian and bonding parameters are evaluated. It is observed that the mixed alkali effect is significant.     

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.     

XAFS study of copper and silver nanoparticles in glazes of medieval middle-east lustreware (10th–13th century)

It has recently been shown that lustre decoration of medieval and Renaissance pottery consists of silver and copper nanoparticles dispersed in the glassy matrix of the ceramic glaze. Here the findings of an X-ray absorption fine structure (XAFS) study on lustred glazes of shards belonging to 10th and 13rd century pottery from the National Museum of Iran are reported. Absorption spectra in the visible range have been also measured in order to investigate the relations between colour and glaze composition. Gold colour is mainly due to Ag nanoparticles, though Ag⁺, Cu⁺ and Cu²⁺ ions can be also dispersed within the glassy matrix, with different ratios. Red colour is mainly due to Cu nanoparticles, although some Ag nanoparticles, Ag⁺ and Cu⁺ ions can be present. The achievement of metallic Cu and the absence of Cu²⁺ indicate a higher reduction of copper in red lustre. These findings are in substantial agreement with previous results on Italian Renaissance pottery. In spite of the large heterogeneity of cases, the presence of copper and silver ions in the glaze confirms that lustre formation is mediated by a copper- and silver-alkali ion exchange, followed by nucleation and growth of metal nanoparticles.     

Electron paramagnetic resonance and thermoluminescence study of Ag ions in Li2B4O7 crystals

Electron paramagnetic resonance (EPR) is used to investigate the effects of ionizing radiation on Ag-doped lithium tetraborate (Li₂B₄O₇) crystals. Two similar, yet distinct, trapped-hole centers (Ag²⁺ ions substituting for Li⁺ ions) are produced by 60 kV x rays. One Ag²⁺ ion, labeled Center A, has no nearby defects and the other Ag²⁺ ion, labeled Center B, has a neighboring impurity which is most likely a Ag⁺ ion substituting for a Li⁺ ion. The production and thermal decay properties of the two Ag²⁺ ions are described and their g matrices and ¹⁰⁷Ag and ¹⁰⁹Ag hyperfine matrices are obtained from the EPR angular dependences. The principal values of the g matrices are similar for the two centers, but the hyperfine principal values differ significantly (Center B has smaller values than Center A). There are also differences in the directions of the principal axes for the two centers. Together, these results imply (1) that the unpaired spin is less localized for Center B and (2) that the ground-state positions of the neighboring oxygen ions are different for Centers A and B. This explains why the peaks of the Ag²⁺ charge-transfer photoluminescence bands associated with Centers A and B occur at different wavelengths (502 and 725 nm, respectively). An isochronal pulsed thermal anneal shows that these radiation-induced Ag²⁺ ions serve as the recombination site for the intense thermoluminescence peak observed near 152 °C.     

The effect of thiourea as a complexing agent on the separation of metallic ions through cation exchange membranes by Donnan dialysis

Donnan dialysis has been investigated in this work in the order to exhibit the influence of the composition of solution on the performance of commercial cation exchange membrane toward metallic ions. Transference fluxes have been determined either as experimental values or calculated according to theoretical models. A good agreement between theoretical and experimental flows has been obtained principally in the case of CMV membrane which presents a lower thickness. It has been demonstrated that addition of thiourea causes an increase of separation properties of membrane. The separation is mainly due to the complex formation between Ag⁺ and Cu⁺ ions and thiourea. Monovalent copper ion is obtained after reduction of Cu²⁺ and oxidation of thiourea and formation of bisulfide of formamidine which constitute a barrier to monovalent complex transference.     

Photoinduced dissociation reactions of silver fluoride cluster ions

Photoinduced dissociation in the ultraviolet region has been investigated for Ag _nF _n-1 ⁺ cluster ions. Photodissociation spectrum of Ag₂F⁺ in the energy of 3.8–5.6 eV exhibits several sharp bands corresponding to the transition to electronically excited states. In this dissociation, only the Ag₂ ⁺ ion was observed as a fragment ion. Theoretical calculation indicates that the parent Ag₂F⁺ ion has a linear Ag-F-Ag equilibrium geometries in the ground and excited states. Since conformational changes by excitation of bending vibration are necessary for the fragmentation of an F atom, this indicates that production of Ag₂ ⁺ from Ag₂F⁺ is a result of internal conversion and following conformational changes.     

Off-centre effects in the triplet relaxed excited state of impurity ns2 ions in alkali halides

Abstract

An off-centre displacement of an excited ns² ion from a crystal lattice site is studied for Ga⁺, In⁺ and TI⁺ centres in alkali halides at 0.09-5 K by the time-resolved polarization spectroscopy method. A theory is developed, which considers the off-centre effects as a result of the mixing of the impurity ³nsnp and ³npnp excited states by T_1u vibrations.     

Fast Cu+ ion conducting phosphate iodide-glasses

The glass-forming region in the ternary system CuI-Cu₂O-P₂O₅, from which Cu²⁺ ion formation has been excluded, is described and compared with that in the corresponding Ag⁺-based system. The glasses are characterized by T_g values some 50°C higher than in the silver systems. Frequency independent (dc) conductivities along the join CuI+CuPO₃ have been determined over a wide range, and found to be characterized by both higher activation energies and pre-exponential constants than the silv?r glasses. The combination leads to higher conductivities in the Cu⁺-based glasses in the composition range 20–30 mole% CuI at room temperature and above.     

Electrical transport and crystallization in Cu+ ion substituted AgI–Ag2O–V2O5 glassy superionic system

A new glassy solid electrolyte system Cu_xAg_1 − xI–Ag₂O–V₂O₅ has been synthesized. The structural, thermal and electrical properties of the samples have been investigated. The glassy nature of the samples is confirmed by X-Ray diffraction and Differential Scanning Calorimetry studies. The electrical conductivity of these samples increases with CuI content and approaches a maximum value of ∼ 10⁻² Ω^− 1 cm^− 1 for x = 0.35 at room temperature. Ionic mobility measurements suggest that enhancement in the conductivity with Cu⁺ ion substitution may be attributed to increase in the mobility of Ag⁺ ions. The electrical conductivity versus temperature cycles carried out at well-controlled heating rate above T_g and T_c reveal interesting thermal properties. For lower CuI content samples conductivity exhibits anomalous rise above T_g and subsequent fall at T_c. It is also found that CuI addition into AgI–Ag₂O–V₂O₅ matrix reduces the extent of crystallization.     

Depth profiles of metal ions implanted in dielectrics at low energies

The depth profiles of Cu⁺, Ag⁺, and Au⁺ ions implanted into amorphous dielectric SiO₂, Al₂O₃, and soda-lime silicate glass (SLSG) are simulated by the DYNA program. The algorithm follows projectile-ion-substrate-atom pair collisions giving rise to a dynamic variation in the phase composition in the surface layer of the irradiated material and takes into account surface sputtering. Ion implantation up to doses of ≤10¹⁶ ion/cm² at low ion energies of 30, 60, and 100 keV is considered. The measured dynamic variation of the depth profiles of implanted ions as a function of the dose is compared with the standard statistical distribution calculated by the TRIM algorithm.     

Evaluating the electric property of different crystal faces and enhancing the Raman scattering of Cu2O microcrystal by depositing Ag on the surface

The surface electric property of Cu₂O microcrystal affects the interaction of facets with substance in the aqueous solution, and hence plays a key role in determining the photocatalytic activity. In this paper, the capability of Cu₂O microcrystals with exclusive {111}, {110} or both lattice surfaces in reducing Ag⁺ to Ag⁰ were investigated. Ag particles selectively deposited on {111} surfaces of Cu₂O, while not on {110} surfaces. The different behaviors of the two surfaces are mainly attributed to their different electric properties: negatively-charged {111} surfaces absorb Ag⁺ ions while positively-charged {110} surfaces repel them. Raman scattering of Cu₂O {111} surfaces was enhanced by the photo-deposition of Ag particles.     

Ab-initio electronic structure studies of mobility paths in fast-ion conductors—I: Results for MI4−3 clusters

We report results of minimum-basis Pseudopotential Hartree-Fock studies of MI₄^?3 clusters (M = Na⁺, K⁺, Ag⁺, and Cu⁺), and of HgI₄^?2. The calculations are designed to characterize local-site effects on mobility paths in solid state electrolytes. We observe qualitatively correct behavior, with Ag⁺ predicted to be the most mobile ion. Quadrupolar polarizability of the metal ion, which is produced by s-d mixing, lowers the energy of trigonal transition state, thus accounting for the observation that quadrupole polarizable species are ideal mobile ions in close-packed halide frameworks. Mulliken populations show that there is considerable local covalency, so that electrostatic potential studies must be done very carefully. Expansion of the I₄ tetrahedron lowers the barrier energy.     

The characterisation of model Cu/Ru(001) bimetallic catalysts by static SIMS with XPS and TPD

The deposition of Cu onto Ru(001) at 540 and 1080 K has been studied by SSIMS supplemented by XPS and TPD. The SSIMS mass spectrum of the Cu/Ru surfaces showed the presence of Cu⁺ and Ru⁺ ions as well as the cluster ions RuCu⁺, Ru⁺₂, Cu⁺₂ and lower intensities of Cu⁺₃, RuCu⁺₂ and Ru₂Cu⁺. Copper coverage was independently monitored by XPS and TPD. It was shown that Cu coverage in the sub-monolayer region was directly proportional to $\text{RuCu}{}^{\mathrm{+}}\text{Ru}{}^{\mathrm{+}}$. Secondary ion emission was strongly influenced by work function changes consequent on Cu adsorption.Studies of the variation of the elemental and cluster ion intensities as a function of Cu coverage, at the two temperatures, has provided detailed information on the dispersion of Cu in the sub-monolayer region, and on the physical form of the ad-layer above a monolayer coverage.