Neutron scattering in Ag+ and Na+ β-aluminas

Na⁺ and Ag⁺ β-alumina for various stoichiometries and for various hydration states have been studied by neutron scattering using incident energy of 0.8 meV with a resolution of 20 μeV in the 200–325° C temperature range. From the weak quasielastic signal (QES) observed for the non- stoichiometric (NS) Ag⁺ β-alumina a translational diffusion constant D_t has been estimated at about 10^-5 cm² s^-1 at 200°C. No such QES has been observed for stoichiometric Ag⁺ and Na⁺ compounds. The dehydration of Na⁺ β-alumina has been evidenced by the disappearance of the broad QES above 300°C; correlatively the low frequency inelastic spectrum of Na⁺ of β-alumina is changed.     

Temperature dependence of anomalous and ordinary Raman scattering in β-AgI

We report careful measurements of the temperature dependence of Raman scattering in β-AgI, with particular emphasis on the region 12°K to 150°K. Our main result is that the anomalous features in the Raman spectra of β-AgI are definitely first order. Furthermore from the temperature dependence of the anomalous scattering intensity we infer the possibility of a first partial disordering in the Ag⁺ sublattice at about 50°K.     

ß″-and ion-rich ß-alumina: Comparison of vibrational spectra and conductivity parameters

Single crystals of ß″-alumina containing Na⁺, K⁺, Ag⁺ and Tl⁺ ions are prepared and studied by Raman spectroscopy between 2 and 1000 cm^?1 in the 20–400 K temperature range. Far-infrared measurements are performed between 10 and 250 cm^?1. The Raman bands assigned to the spinel block vibrational modes are broad (Δv = 20 cm^?1); this reflects a high degree of disorder and can be related to stabilizing Mg²⁺ ions randomly distributed. In-plane cation vibrations are identified below 150 cm^?1 in infrared and Raman spectra. Potential barriers associated with this type of motion are discussed. The temperature dependence of the relevant low-frequency Raman spectra for K⁺ß″-alumina may be interpreted in terms of a small proportion of very mobile K⁺ ions. Tl⁺ spectra are discussed in terms of clusters. Finally, a comparison with literature results shows that the ß″ phase can be differentiated from t he ion-rich ß-alumina phase of the same composition.     

129I-Mössbauer study of diffusion effects in the superionic conductor Ag3SI

The superionic conductor Ag₃SI was studied by¹²⁹I-Mössbauer spectroscopy in the rhombohedral γ-phase and in the cubic β-phase at temperatures between 4.2 K and 180 K. In the low-temperature γ-phase, one observes well below the γ-β phase transition at 157 K a motional-narrowing-like decrease of the electric-quadrupole interaction, roughly linear with temperature, which points to a diffusional motion of the Ag⁺ ions already in the γ-phase. Diffusional effects are also reflected by anomalous decrease of thef-factor in the same temperature region, which originates from a restricted short-range diffusional displacement of the iodide ions induced by the hopping Ag⁺ ions.     

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.     

Infrared,Raman and neutron scattering studies of β- and β″-alumina: a static and dynamical structure analysis

Infrared and Raman data on β-aluminates with different compositions (x)=0.0, 0.25, 0.66) containing Na⁺, Ag⁺, K⁺, Tl⁺, H₃O⁺, NH⁺₄ ions are reviewed. β-gallates and β″-aluminates are also considered. Assignments of the cation modes are discussed in terms of oscillations of single cations and of normal modes of clusters (pairs and triangles formed by the conducting cations). The assignment of the spinel block spectra is proposed in the light of recent normal mode calculation and comparison of different experimental results in the literature. Potential barriers and activation energies obtained from these techniques are compared with calculation results and with data obtained from different sources. Inelastic and recent quasi elastic neutron scattering results are reviewed and compared with those deduced from the vibrational study.     

Ultrasound-assisted microwave preparation of Ag-doped CdS nanoparticles

Ag-doped CdS nanoparticles were synthesized by an ultrasound-assisted microwave synthesis method. The X-ray diffraction patterns reveal a structural evolution from cubic to hexagonal with increasing molar ratios of Ag⁺/Cd²⁺ from 0% to 5%. It shows that the Ag-doped hexagonal CdS nanoparticles are polycrystal. The X-ray photoelectron spectroscopy of the CdS nanoparticles doping with 5% Ag⁺ shows that the doped Ag in CdS is metallic. Simultaneously, the characteristic Raman peaks of the CdS nanoparticles enhance with increasing Ag⁺ concentrations. The photocatalytic activity of different Ag-doped samples show a reasonable change due to different ratios of Ag which doped into CdS.     

The influence of low-temperature silver-ion exchange on the spectral-luminescent properties of fluorophosphate glasses doped with PbSe

Changes in the absorption and luminescence spectra of fluorophosphate glasses doped with PbSe caused by low-temperature Ag⁺–Na⁺ ion exchange are considered. It is found that the silver distribution gradient in a near-surface layer about 16 μm thick leads to two different processes of interaction between metal and semiconductor nanoparticles. PbSe molecular clusters and quantum dots more efficiently grow in deep layers with a low silver concentration. The near-surface glass layers with a high silver concentration exhibit formation of Ag metal nanoparticles, on the surface of which interaction with PbSe molecular clusters leads to the formation of Ag–Se–Pb bonds, which transform into Ag₂Se layers in the process of heat treatment. The appearance of the new phase is confirmed by X-ray diffraction.     

Ab Initio Calculations and Raman and SERS Spectral Analyses of Amphetamine Species

Abstract

For the first time, the differences between the spectra of amphetamine and amphetamine-H⁺ and between different conformers are thoroughly studied by ab initio model calculations, and Raman and surface-enhanced Raman spectroscopy (SERS) spectra are measured for different species of amphetamine. The spectra of amphetamine and amphetamine-H⁺ samples were obtained and assigned according to a comparison of the experimental spectra and the ab initio MO calculations, performed using the Gaussian 03W program (Gaussian, Inc., Pittsburgh, PA). The analyses were based on complete geometry minimization of the conformational energy of the S-plus-amphetamine molecule and the S-plus-amphetamine-H⁺ ion. The harmonic frequency calculations provide information about the characteristic features of the Raman spectra and the nature of the bonding in the molecule. It is concluded that vibrational bands from salt anions with internal bonds (sulfates, hydrogen phosphates, etc.) need to be taken into account when employing these spectra for identification purposes. These results also show how Raman spectroscopy can assist the forensic community in drug profiling studies. Furthermore, because their spectra are different, discrimination between the free and protonated forms of amphetamine salts can be observed. Here, we provide evidence for this difference and show experimentally how it has been overseen.     

The influence of temperature on surface enhanced Raman scattering in the electrochemical environment: Evidence for adatoms

The surface enhanced Raman signal from the Cl-surface stretch vibration on Ag in aqueous KCl is irreversibly quenched by thermal dissociation of Ag-adatom-Cl-surface complexes. In a KF electrolyte, where a low concentration of Ag⁺ ions is present, temperature changes alter the steady state adatom concentration and give rise to reversible changes in the inelastic continuum background Raman signal.     

The diffusion of silver ions in AgBr/Agi sheet crystals

Abstract

The results of the tracer diffusion of Ag⁺ by chemical microsectioning technique in AgBr/I sheet crystals up to the solubility limit in the temperature range 110–240°C are presented. The monotonic increase in the diffusivity and decrease of the activation enthalpy for diffusion with the increase of the iodide component in AgBr result from the decrease of the formation energy for Frenkel defects with the increase of elastic strain introduced by the oversized iodide ions.     

Relation entre structure et conductivite ionique basse temperature de Ag3PO4

The ionic conductivity of Ag₃PO₄ versus the temperature was determined. The cristal structure was reinvestigated at different temperatures (170 K, 290 K and 570 K) showing a normal increase of the unit cell with the temperature but a contraction of PO₄ tetraedra. This leads to Ag-0 bonds somewhat looser and too more space for the Ag⁺ cation to move.     

Raman scattering study of crystal dynamics and order-disorder transitions in alkali hydrosulfides

Raman spectra have been measured for NaSH, CsSH and RbSH over a range of temperatures from 85 to 400°K. The results show that the order-disorder phase transitions in these solids are associated with significant changes in the details of their crystal dynamics. In addition the Raman spectra provide clear evidence for the existence of previously unobserved low-temperature phases in each compound, which apparently involve total ordering of the SH^? ions.     

PAC study of ionic motion in silver compound superionic conductors

Ionic motion in superionic conductors, Ag₂S, Ag₂Se and Ag₃SI was investigated by γ-γ PAC on¹¹¹Cd. Diffusion constant measurements showed that probe ions migrate almost as fast as Ag⁺ ions above 500K in Ag₂S and Ag₂Se and above 700K in Ag₃SI. Multivalent impurities were found to be unstable in AgI and Ag₂Te. The correlation time of ionic motion was deduced from the observed relaxation rate together with the diffusion constants. The correlation time and its activation energy increase in order of Ag₂S, Ag₂Se and Ag₃SI.The flight distance of Ag⁺ ions remains almost constant in the measured temperature range.     

Involvement of weak C?H···X hydrogen bonds in metal‐to‐semiconductor regime change in one‐dimensional organic conductors (o‐DMTTF)2X (X = Cl,Br, and I): combined IR and Raman studies

We report on the infrared (IR) and Raman studies of the three isostructural quasi‐one‐dimensional cation radical salts of 3,4‐dimethyl‐tetrathiafulvalene (o‐DMTTF)₂X (X = Cl, Br, and I), which all exhibit metallic properties at room temperature and undergo transitions to a semiconducting state in two steps: a soft metal‐to‐semiconductor regime change in the temperature region T_ρ = 5–200 K and then a sharp phase transition at about T_MI = 50 K. Polarized IR reflectance spectra (700–16 000 cm⁻¹) and Raman spectra (50–3500 cm⁻¹, excitation λ = 632.8 nm) of single crystals were measured as a function of temperature (T = 5–300 K) to assess the eventual formation of a charge‐ordered state below 50 K. Additionally, the temperature dependence of the IR absorption spectra of powdered crystals in KBr discs was also studied. The Raman spectra and especially the bands related to the CC stretching vibration of o‐DMTTF provide unambiguous evidence of uniform charge distribution on o‐DMTTF down to the lowest temperatures, without any modification below 50 K. However, the temperature dependence of Raman spectra indicates a regime change below about 200 K. Temperature dependence of both electronic dispersion and vibrational features observed in the IR spectra also clearly confirms the regime change below about 200 K and shows the involvement of C H···X hydrogen bonds in the electronic localization; some spectral changes can be also related with the phase transition at 50 K. Additionally, using density functional theory methods, the normal vibrational modes of the neutral o‐DMTTF⁰ and cationic o‐DMTTF⁺ species, as well as their theoretical IR and Raman spectra, were calculated. The theoretical data were compared with the experimental IR and Raman spectra of neutral o‐DMTTF molecule. Copyright © 2011 John Wiley & Sons, Ltd.     

First order phase transition of Li3 ThF7 at 281 K: A comparative study between EPR and Raman scattering

Abstract

Electron Paramagnetic Resonance (EPR) and Raman Spectroscopy were used to study the low temperature phase transition of pulled Li₃ThF₇ single crystals, occurring at 281 K. In both cases, the room temperature spectra were very broad, owing to the statistical disorder and high ionic mobility of the lithium ions, which occupy only 3/4 of their structural sites. The results are compatible with a first order ferroelastic transition from the room temperature orthorhombic D_2h ²² phase to a monoclinic C_2h ^(x) one. The symmetry rules are well respected assuming a model with four lithium ions in average per chemical formula. The EPR spectra show also the appearing of additional ferroelastic domains.     

Temperature and size dependence of the optical response of small Ag clusters

The absorption spectra of small Ag ⁺ n clusters are calculated at finite vibrational temperature by using a microscopic tight-binding RPA method. We consider free clusters with sizes between n = 3 and n =13 and take into account explicitly the degrees of freedom corresponding to the 4 d-electrons. We analyze the optical absorption as a function of the cluster size. We show that the contribution of the d-electrons has an important influence on the size dependence of the energy of the Mie plasmon. We also perform ensemble averages to obtain the absorption spectra for different vibrational temperatures. We obtain relatively good agreement with experiment for a temperature . The dynamics of the 4 d-electrons, which shows in small clusters an incipient delocalized character for n >7, yields an important contribution to the absorption spectrum already for n =13. We find that the strength of this contribution can be controlled by varying the vibrational temperature. Received: 4 January 1999 / Received in final form: 12 May 1999     

Infrared spectra and stability of CO and H2O sorption over Ag-exchanged ZSM-5 zeolite: DFT study

The infrared spectra and stability of CO and H₂O sorption over Ag-exchanged ZSM-5 zeolite were investigated by using density function theory (DFT). The changes of NBO charge show that the electron transfers from CO molecule to the Ag⁺ cation to form an σ-bond, and it accompanies by the back donation of d-electrons from Ag⁺ cation to the CO (π*) orbital as one and two CO molecules are adsorbed on Ag-ZSM-5. The free energy changes ΔG, −5.55 kcal/mol and 6.52 kcal/mol for one and two CO molecules, illustrate that the Ag⁺(CO)₂ complex is unstable at the room temperature. The vibration frequency of C-O stretching of one CO molecule bonded to Ag⁺ ion at 2211 cm⁻¹ is in good agreement with the experimental results. The calculated C-O symmetric and antisymmetric stretching frequencies in the Ag⁺(CO)₂ complex shift to 2231 cm⁻¹ and 2205 cm⁻¹ when the second CO molecule is adsorbed. The calculated C-O stretching frequency in CO-Ag-ZSM-5-H₂O complex shifts to 2199 cm⁻¹, the symmetric and antisymmetric O-H stretching frequencies are 3390 cm⁻¹ and 3869 cm⁻¹, respectively. The Gibbs free energy change (ΔGH₂O) is −6.58 kcal/mol as a H₂O molecule is adsorbed on CO-Ag-ZSM-5 complex at 298 K. The results show that CO-Ag-ZSM-5-H₂O complex is more stable at room temperature.     

Vibrational study of H3OUO2PO4 · 3 H2O (HUP) and related compounds. phase transitions and conductivity mechanisms: part I,KUO2PO4 · 3 H2O (KUP)

Infrared and Raman spectra of polycrystalline KUO₂PO₄ · 3 H₂O (KUP) and its isotopic derivatives KUO₂P¹⁸O₄ · 3 H₂O and KUO₂PO₄ · 3 D₂O have been investigated in the 4000-10-cm^?1 range at different temperatures. An assignment of the bands in terms of UO₂, PO₄ and H₂O vibrations has been proposed. Combined differential scanning calorimetry and spectroscopic data show two diffuse phase transitions near 130 and 230 K. Comparison of the vibrational spectra of phase I at 300 K and phase IV at 100 K indicates that ordering of the water molecules with subsequent ordering of PO₄ tetrahedra on a site with lower symmetry appears to be the main mechanism responsible for the phase transformation. All the six O-H distances of water molecules in phase IV are found to be crystallographically nonequivalent. Conducting ion frequencies and the corresponding force constants have been determined for the analogous compounds MUP with M = K⁺, Na⁺, Ag⁺, NH⁺₄, Tl⁺ and H₃O⁺ and compared with other properties of these ionic conductors. Conductivity mechanisms in these materials are discussed.     

Temperature‐dependent Raman scattering of KDP:Mn (0.9% weight of Mn) crystal

Low temperature polarized Raman scattering measurements of KDP:Mn (0.9% weight of Mn) were performed at temperatures ranging from 14 to 300 K, over the spectral range 50–1250 cm⁻¹. In the present results we can see that the spectra of undoped and doped samples at room temperature are very different. Doped samples maintain the KDP structure as tetragonal, with the same factor group D_2d but with a different class of the space group, different from the original 12. The results show that the crystal undergoes a phase transition at temperature between 115 and 97 K, which is much lower than the phase transition temperature of undoped KDP that occurs at 122 K, where the crystal changes from the para‐electric to the ferroelectric phase. Further, at very low temperature (14 K) we can see that the spectra of KDP:Mn (0.9% weight of Mn) present a behavior very different from the behavior presented by the spectra of KDP doped with low Mn concentration. Copyright © 2010 John Wiley & Sons, Ltd.