A neutron and x-ray diffraction investigation of intermediate range structure in silver borate glasses

The ion diffusion mechanism in ion conducting glasses has in many reports been suggested to be linked to the structure. However, in most cases the structures of the glasses are not very well known. We have investigated the structure of the glass system (AgI)_x (Ag₂O-2B₂O₃)_1−x, with x=0, 0.6 by neutron and x-ray diffraction experiments. AgI doping produces an intense peak in the neutron data at an anomalously low Q-value (0.8 Å⁻¹) that indicates increased intermediate range ordering. Only a weak peak is observed at a similar Q-value in the x-ray structure factor. By comparing the different neutron and x-ray weighting factors the origin of the anomalous peak is unambiguously attributed to correlations within the B-O network. The experimental results have been used to model the glasses by Reverse Monte Carlo simulations to obtain more detailed views of the structures. Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, 11 – 18 Sept. 1994     

Spectroscopic and optical properties of Nd doped fluorine containing alkali and alkaline earth zinc-aluminophosphate optical glasses

Nd³⁺ doped fluorine containing zinc-aluminophosphate glasses have been prepared with alkali and alkaline earth content to understand the effect of network modifiers on radiative process. The physical and optical properties of these glasses have been evaluated. The Judd-Ofelt model for the intensity analysis of induced electric dipole transitions has been applied to the measured oscillator strengths of the absorption bands to determine the three phenomenological intensity parameters Ω₂, Ω₄ and Ω₆ for each glass. Using these parameters, transition probability (A), total transition probability (A_T), branching ratios (β_R) radiative life times (τ_R) and integrated cross-section (σ_a) for the stimulated emission have been theoretically calculated for certain excited Nd³⁺ fluorescent levels. From the obtained results the conclusion is made about the possibility of using these glasses as laser material.     

Structure determination of Ls-threonine by neutron diffraction

The structure of the aminoacid, L_s-threonine [NH ₃ ⁺ CH(CHOHCH₃)COO^?], space groupP2₁2₁2₁,a=13.630(5),b=7.753(1),c=5.162(2) Å,z=4, has been determined from neutron diffraction data using direct methods. The intensities of 1148 neutron Bragg reflections were measured from a single crystal. The structural parameters were refined by the method of least squares using anisotropic temperature factors. The finalR(F ²) is 0.068. The structure was also refined from the x-ray data of Shoemakeret al (1950J. Am. Chem. Soc. 72 2328); there is good agreement between the two sets of heavy atom parameters. The parameters of hydrogen atoms are of course more precisely determined in our neutron study. The molecular conformation and the hydrogen bonding scheme are discussed. Weighted average values of bond distances and angles from 14 aminoacid structures with ionized carboxylic groups studied by neutron diffraction at Brookheven and Trombay are also presented.     

Structural studies of liquid alcohols by neutron diffraction

Neutron diffraction measurements have been made on methyl alcohol at room temperature for an incident wavelength of 0·94 Å. Cross sections have been obtained for CD₃OD, CD₃OH, and mixtures of these compounds. These data are subtracted to obtain the separated structure factors for intermolecular H₀H₀, the hydroxyl components, and the non-hydroxyl components. The Fourier transformations of the structure factors show components of both intra- and intermolecular distribution functions. Width parameters obtained from model fits are too large for thermal vibrations and are interpreted as geometrical broadening due to the stretching of bonds, variations in bond angles, and rotation of the methyl group. Differences in the real space distribution function between hydrogen and deuterium are noted.     

Dielectric properties of alkali halide and alkaline earth oxide crystals

The deviations of ionic charge and compressibility from their nominal values in alkali halides and alkaline earth oxide crystals have been discussed and explained in terms of a theory based on shell model and exchange charge interactions originally developed by Dick and Overhauser. The exchange charge polarization parameters have been evaluated in the present study using the appropriate values of overlap integrals. It has been concluded that the second neighbour short range forces have significnant magnitudes in alkaline earth oxides.     

Structure determination of AgPO3 and (AgPO3)0.5(AgI)0.5 glasses by neutron diffraction and small angle neutron scattering

Neutron diffraction and small angle neutron scattering (SANS) were performed on AgPO₃ and (AgPO₃)_0.5(AgI)_0.5 glasses. AgPO₃ glass is made up of long chains of PO₄ tetrahedra joined together by Ag atoms. When silver iodide is added, the radial distribution function shows a large peak at 2.83 Å, due to AgI interactions. AgI does not modify the network forming unit. The existence of small clusters is confirmed by analysing the coordination number of AgI pairs obtained by subtracting the experimental structure function of the AgPO₃ glass from that of the corresponding AgI-doped glasses. A rough estimation of their size is given by SANS experiments. Not all the AgI pairs are involved in AgI cluster units. The compatibility of the results obtained with recent structural investigations by non diffractometric techniques is examined.     

Structure of liquid SiO2: a measurement by high-energy x-ray diffraction

The x-ray structure factor for liquid SiO2 has been measured by laser heating of an aerodynamically levitated droplet. The main structural changes of the melt compared to the room temperature glass are associated with an increase in the size of the SiO4 tetrahedra, indicating a small reduction in the average Si-O-Si bond torsion angle and an expansion of the network between 5 and 9 A. Strong directional bonds with little high temperature broadening and a high degree of intermediate range order are found to persist in the liquid state.     

Photoabsorption of alkali and alkaline earth elements calculated by the Scaled Thomas Fermi method

Zeitschrift für Physik A Hadrons and nuclei - Photoionization cross sections have been calculated by means of the Scaled-Thomas-Fermi method for the alkali metals and the alkaline earths....     

Structural and electrical properties of iron molybdenum phosphate glasses

Summary  Iron molybdenum phosphate glasses [xMoO₃ · (0.6 -x)P₂O₅ · 0.4Li₂O] :yFe₂O₃ with 0 ≤x ≤ 0.6 andy = 0.03 (mol%) prepared in ambient atmosphere using the melt quenching technique were studied by using DC electrical conductivity,⁵⁷Fe M?ssbauer and infrared spectroscopies. The DC conductivity depends on the MoO₃ concentrationx. It was observed that, with increasingx, the ratio Fe²⁺ /(Fe³⁺ + Fe²⁺) and the DC conductivity increase. Infrared spectroscopy and X-ray powder diffraction indicate that a Li₂ MoO₄ crystalline phase is present for high MoO₃ content samples (x = 0.5, 0.6). This work was partly sponsored by FINEP, CNPq (Brazilian agencies) and UECE (Universidade Estadual do Cearà).     

Structure and correlation in liquid alloys by X-ray and neutron diffraction

A method for obtaining information about correlation in a liquid or amorphous alloy is presented. The explicit structure in a binary could in principle be determined from three independent diffraction patterns. It is relatively much simpler to measure and analyse the x-ray and neutron intensity patterns and with these alone obtain considerable correlation information, even if more than two atom types are present, by calculating the ratio of the two experimental radial distributions. For the binary the relevance of the ratio derives from the fact that each distribution is a sum of the radial distributions of the three types of atom-pairs, each term being weighted by the appropriate product of scattering amplitudes. The structure of the sample can be analysed by comparing the ratio for any interatomic distance to the values it would have for pure A-A, pure B-B, pure A-B, and random pairing. In a true liquid the results are semiquantitative. In an alloy with well-defined short-range order, the correlation of these close pairings can be clearly determined. Results are shown for liquid NaK and vitreous silica. For any alloy the experimental results would serve to test structural models.     

Temperature dependence of the x-ray and neutron diffraction from a metallic glass

The amorphous alloy Be_0.4Ti_0.5Zr_0.1 has one of the largest negative temperature coefficients of resistivity known in a metallic glass and thus presents a demanding test case for any theory that would explain this phenomenon. By measuring the temperature dependence of the x-ray and neutron diffraction profiles we have attempted to test the extended Ziman theory for the resistivity of amorphous metals. The observed change in the structure factor is large enough to explain the observed resistivity data in terms of this theory.     

Electronic polarizabilities and sizes of ions in alkaline earth halides and alkali chalcogenides

An analysis of the electronic polarizabilities and sizes of ions in the crystals of alkaline earth halides and alkali chalcogenides has been performed using a relation between polarizability and ionic radius. The electronic polarizabilities and sizes of ions are calculated using the free state data reported by Pauling. The quantities obtained in the present study are found to vary from crystal and crystal, thus showing the deviations from the additivity rule. The results are discussed and compared with those obtained by other investigators.