E. I. Kamitsos

Infrared reflectance spectra of lithium borate glasses

Abstract The infrared reflectance spectra of lithium borate glasses (0–73 mol% Li 2 O) have been measured over a broad and continuous spectral range (30–4000 cm −1 ) in an attempt to study systematically their structure. The reflectance data have been analyzed by Kramers-Kronig inversion to obtain the optical and dielectric properties of the materials. The study of the deconvoluted mid-infrared spectra has revealed a glass network built up of various boron-oxygen groups, with structure and concentration strongly dependent on the Li 2 O content. Of particular interest was the far-infrared asymmetric absorption profile, which could be well simulated by two broad bands attributed to vibrations of lithium cations in two distinct network environments. The presence of these two Li + -host site distributions was found to be independent of the details of the borate network structure and has been taken to imply a more general characteristics of ionic conducting oxide glasses.

Borate glass structure by Raman and infrared spectroscopies

Abstract Raman and infrared spectroscopies have been employed to probe the continuous evolution of borate glass structures as a function of the nature and concentration of alkali oxide modifier. At relatively low alkali contents, the glasses contain covalent networks consisting of interconnected units bearing BO − 4 tetrahedra. Further addition of alkali oxide causes the progressive depolymerization of the network as a result of the formation of non-bridging oxygen atoms. Eventually, complete network disruption into small and highly charged borate units results in “ionic” glasses. The nature of the alkali cation determines the structure of these glasses by affecting several high- temperature isomerization or disproportionation equilibria.

Infrared reflectance investigation of alkali diborate glasses

Alkali diborate glasses have been investigated by infrared reflectance spectroscopy to demonstrate the potential of this technique for a quantitative description of the glass structure. Kramers-Kronig transformation was performed on the reflectance data to obtain the optical and dielectric properties of glasses over a broad and continuous spectral range (30–4000 cm−1). Analysis of the mind infrared spectra shows that the normalized absorption area attributed to boron-oxygen tetrahedra decreases upon increasing alkali cation ionic radius, and scales linearly with the fraction of four-coordinated boron atoms determined recently from NMR spectra. The far infrared asymmetric absorption profiles were deconvoluted into two broad bands and attributed to vibrations of alkali cations in two distinct network environments. Cation effective charges were calculated from the integrated absorption of the bands due to cation vibrations, and were found to increase with alkali ionic radius.

Raman study of the mechanism of electrical switching in Cu TCNQ films

Abstract The mechanism of electrical switching in Cu/Cu TNCQ films has been studied in situ by Raman spectroscopy. The initial film is Cu+TCNQ-., while neutral TCNQ was found after electrical switching in amounts increasing with time of application of the electric field.

Vibrational study of the role of trivalent ions in sodium trisilicate glass

Abstract The structure of glasses in the system Na 2 O · x R 2 O 3 · (3 − 2 x )SiO 2 (RAl, B) has been studied by employing Raman and infrared reflectance spectroscopies. These particular glass compositions were chosen for elucidating the role of aluminum and boron substitution on glass structure and properties while keeping the Na/(Si + R) ratio fixed. In the presence of aluminum, the equilibrium 2Q 3 ⇔ Q 4 + Q 2 shifts to the left. The role of boron was found to be the opposite, since it induces the destruction of the Q 3 species and the creation of the highly charged Q 2 and Q 1 species, as well as the fully polymerized Q 4 silicate units. The combined far-infrared and XPS results on the same glasses suggest that the average charge density of sites populated by sodium ions is smaller in aluminosilicate than in borosilicate glasses.

The devitrification of lithium metaborate: polymorphism and glass formation

Abstract The ambient pressure devitrification of glassy lithium metaborate has been studied by Raman and infrared spectroscopies. Of the three different polymorphs obtained, two are identified as α- and γ-LiBO2, while a third one, β′-LiBO2, is thought to consist of alternating borate triangles and tetrahedra respecting the metaborate stoichiometry (BO2-. The network of glassy LiBO2 prepared by melt quenching consists mainly of local arrangements similar to those encountered in all three polymorphs in accordance with the ideas of Krogh-Moe. It is argued that the glass structure exhibits the memory of structural transformations occuring in the vicinity of Tg, and may be described by models implying the existence of structural ‘granularity’.

Optically induced transformations of metal TCNQ materials

Abstract Photoinduced transformations of CuTCNQ and AgTCNQ films, and several metal TCNQ salts with visible light have been observed and studied by Raman spectroscopy. Both partial and complete transformations, leading to metal and TCNQ in various material forms, are affected, and implications for photoswitching and light writing discussed.

X-ray diffraction and infrared investigation of RBa2Cu3O7 and R0.5Pr0.5Ba2Cu3O7 compounds (RY and lanthanides)

Abstract RBa2Cu3O7 and [R0.5Pr0.5]Ba2Cu3O7 compounds with RY and lanthanides have been prepared and studied at ambient temperature by means of XRD and infrared-reflectance spectroscopy. In Pr free compounds, increasing the Shannon radius of R, rR, leaves the puckering of the Cu(2)−O(2, 3) sheet unaffected. In the Pr containing compounds, the PrO coordination polyhedron is typical of R+3 ions in eight-fold coordination and argues against differentiating Pr in terms of valence or hybridization. Instead, a repositioning of Ba within the cuprate block is observed, leading to the flattening of the sheet and the anomalously high frequency of the Ba and Cu(2)−O(2, 3) B1u modes. In RPr compounds the c-axis and the interplanar distances related to the position of Ba and O(2, 3) are found to exhibit a sigmoidal dependence on rR. It is argued that this sigmoidal dependence is related to the two-mode behavior exhibited by several phonon modes of these compounds.

Vibrational investigation of lithium metaborate-metaaluminate glasses and crystals

Abstract Lithium metaborate-metaaluminate glasses xLiAlO2·(1−x)LiBO2, (0≤ x ≤0.42), and crystals (Li3B2AlO6, Li2BAlO4) have been prepared and studied by infrared and Raman spectroscopies. The two crystalline compounds have similar networks made of AlO4− and BO2O− units (O, O: bridging and terminal oxygen atoms, respectively) in arrangements which maximize the number of BOAl linkages. Both compounds decompose at high temperatures into α-LiBO2 and γ-LiAlO2. Increasing x in glass induces the isomerization of BO4− tetrahedra to BO2O− triangles, while aluminum forms AlO4− units. This structural scheme is sufficient to account for the minimum in the glass transition temperature observed for x ≈ 0.25. There is no vibrational evidence for the presence of AlO5 or AlO6 polyhedra in glass, although their presence in small amounts cannot be excluded. Glasses and crystals of the same stoichiometry (x = 0.33) are differentiated by an excess of kinetically arrested BOB and AlOAl linkages in the former.

Alkali sites in glass

In this paper we study the nature of sites occupied by alkali metal ions in oxide and organic glasses by probing the localised metal-site vibrations which are active in the far-infrared. Analysis of the far-infrared profiles of the glasses considered in this investigation indicates the presence of one or two distributions of cation hosting sites. It is argued that the energetics of these sites are determined by both the bonding requirements of the metal cation and the versatility of the anionic glass network to fulfil these requirements. This is demonstrated by comparing earlier data on alkali borates and more recent results on alkali germanates and ionic organic glasses, as well as by studying the effect of the method of glass formation (fast melt quenching, slow cooling, sol–gel) on the far-infrared absorption envelope. The alkali metal sites inferred from far-infrared have been compared to those probed by ultraviolet spectroscopy using Tl+ and Pb2+ ions electrolysed into borate glasses. The results suggest that mobile alkali ions are mainly those occupying sites of high basicity.