K.J. Volin

The structure of vitreous and liquid GeSe2 : a neutron diffraction study

Abstract High-quality neutron diffraction data are presented for GeSe 2 glass and liquid. In GeSe 2 glass at 10 K there is direct, clearly resolved evidence for edge-sharing of tetrahedra. C Ge (Ge) for dge-sharing tetrahedra measured in the glass (0.14), approaches that of the crystal (0.17). The Ge(Se 4 ) tetrahedron is the unit of short-range order in the glass, as it is in the crystal. The angle between neighboring corner-sharing tetrahedra in the glass is similar to that in the crystal. The tetrahedral building block persists in the liquid state at 1084 K, but it is broken up. The anomalous temperature dependence of the first sharp diffraction peak (FSDP) observed in the glass persists in the liquid. Experimentally, the height of the FSDP in the 1084 K liquid is 10% less than in the 10 K glass. The positions and the half-widths are identical. GeSe 2 retains considerable intermediate-range order in the liquid state. A comparison with molecular dynamics computer simulations is also presented.

Structure of vitreous AgGeSe

Pulsed neutron diffraction has been used to study the short- and intermediate-range order of vitreous g-Ag 4 Ge 3 Se 9 . By utilizing the techniques of isotopic substitution and temperature variation, it has been possible to estimate four of the six partial pair-distribution functions for the first coordination shells. Measurements at T = 10 K extending out to wavevectors Q ≈ 34 A −1 provide accurate results for the bond lengths and coordination numbers. Comparison is also made with results derived from a variant of the maximum entropy method. Ge is fourfold-coordinated by Se at an average interatomic distance of 2.37 ± 0.005 A which is consistent with the GeSe 4/2 tetrahedron also being a structural unit in the Ag-doped Ge Se glass. Ag is predominantly bonded to Se at an average distance of 2.68±0.01 A and is threefold-coordinated. This low coordination might be a key factor in understanding the fast ion motion in this system. The temperature dependence of the structure factor, S ( Q ), and the radial distribution functions were investigated. The Ge Se and Se Se correlations solely reflect increased thermal vibrations of the atoms, whereas the Ag correlations behave quite differently with increasing temperature up to 170°C. The Ag correlations change rapidly and become liquid-like at elevated temperatures reflecting the diffusive motion of Ag. The first sharp diffraction peak at 1.04 A −1 in S ( Q ) exhibits the anomalous temperature dependence typical of chalcogenide glasses in that the peak becomes narrower and increases in amplitude with increasing temperature (∼ 2% from room temperature to 170°C).

Intermediate-range order in binary chalcogenide glasses: The first sharp diffraction peak

The structure factor of SiS/sub 2/, SiSe/sub 2/, GeS/sub 2/, and GeSe/sub 2/ glasses, and of GeSe/sub 2/ liquid, have been measured using time-of-flight, pulsed-neutron diffraction. An ubiquitous first sharp diffraction peak (FSDP) appears at /approximately/1 /angstrom//sup /minus/1/ and is a signature of intermediate-range order. In GeSe/sub 2/, the FSDP of the liquid at 1084 K and of the glass at 10 K have the same halfwidth and position in reciprocal space. The origin of the FSDP in these systems is discussed in terms of correlations arising in large ring structures. It is shown that the intensities of the FSDPs scale with coherent scattering from A-A and A-X correlations in the rings (A = Si or Ge; X = S or Se). Departures from simple scaling indicate a change in the ring sizes contributing to the FSDP. 11 refs., 2 figs., 2 tabs.

Preparation and characterization of SixSe1−x glasses and determination of the equilibrium phase diagram

Glasses in the Si/sub x/Se/sub 1-x/ 0.0 < x < 0.40 alloy system have been prepared and characterized by bulk glass density and glass transition temperature measurements. Systematic changes in molar volume vs. concentration indicate the onset of major structural changes at xapprox. =0.22 and x=0.33. The first determination of the equilibrium phase diagram for the Se-rich region of the Si-Se alloy system is presented.

A new method for the preparation of fast-conducting, reactive glass systems

Abstract A relatively simple apparatus has been designed for synthesizing glasses in a controlled environment at temperatures up to 1050°C. The glasses are cast in disc form - a geometry suited for experimental measurement without further sample alteration. This arrangement has enabled us to extend the region of glass formation in the binary system Na 2 S:GeS 2 and to prepare a glass with as high as 70 mole % of Na 2 S. This glass shows a conductivity of 1.0×10 −3 Ω −1 cm −1 (100°C)-higher than the best sodium conducting glasses currently known. Using the new method, a completely new Na 2 S:B 2 S 3 glass system has been prepared. The electrical conductivity of these samples has been measured as a function of frequency in the range 1 Hz to 7×10 5 Hz and in the temperature range of −5°C to +150°C. Replacement of the oxygen atom by a sulphur atom (comparison with oxide glasses at the same composition) is found to improve the ionic conductivity.

Neutron scattering function of vitreous and molten zinc chloride

The dynamics of vitreous and molten zinc chloride have been studied with inelastic neutron scattering at the Intense Pulsed Neutron Source at Argonne National Laboratory. The results are analyzed in terms of the scattering function S(Q, E) and the effective vibrational density of states G(E). The vibrational spectra of both glass and liquid are dominated by broad features centred at 15 and 35 meV which are identified with the F2 modes of ZnCl42- tetrahedra. The other two normal modes are not observed because of inadequate resolution and because of broadening and overlap resulting from coupling between tetrahedra. The energy widths of the scattering are lower than those predicted for independent particles over the entire Q range of the measurement, indicating strong binding of the ions to the network structure in both the glass and liquid.

Combustion calorimetry of rhombohedral sulfur in fluorine: a question of impurities. The standard molar enthalpies of formation of SF6(g) and SO2−4(aq) at the temperature 298.15 K

A specially prepared sample of rhombohedral sulfur (USBM-P1b) used in several earlier thermodynamic investigations has been shown to contain mass fraction ≈ 310 · 10−6 of hitherto unsuspected impurities. Among the studies that may possibly be affected by this discovery is a determination by one of us of the energy of combustion of sulfur in fluorine from which was deduced the standard molar enthalpy of formation of sulfur hexafluoride ΔfHom(SF6, g, 298.15 K) (J. Chem. Thermodynamics 1985, 17, 349). Very recently, much purer sulfur, containing mass fraction

The energy difference between the crystalline and vitreous forms of germanium diselenide as determined by combustion calorimetry in fluorine. The Ge-Se bond energy

Standard specific energies of combustion in fluorine of high-purity crystalline and vitreous GeSe2 have been determined by isoperibol calorimetry at 298.15 K. The derived standard molar enthalpies of formation at 298.15 K are: ΔfHm0, (GeSe2, cr, monoclinic) = − (103.7 ± 3.1) kJ·mol−1 and ΔrHm0 (GeSe2, vit) = − (91.6 ± 3.2) kJ.mol−1. The Ge-Se bond energy in crystalline GeSe2 is (237.7 ± 1.1) kJ·mol−1 and in vitreous GeSe2 it is (234.7 ± 1.2) kJ·mol−1, where the bond energy is defined as 0.25 ΔrHm0 for the reaction: GeSe2 (solid) = Ge(g) + 2Se(g) at 298.15 K (ΔrHm0 denotes the standard molar enthalpy of reaction). The standard enthalpy (also energy) of the transition: GeSe2(vit) = GeSe2(cr), is −(12.1±4.2) kJ·mol−1 at 298.15 K.

Standard molar enthalpies of formation and transition at the temperature 298.15 K and other thermodynamic properties of the crystalline and vitreous forms of arsenic sesquiselenide (As2Se3). Dissociation enthalpies of AsSe bonds

Abstract Fluorine-combustion calorimetry was used to determine the standard molar enthalpies of formation (at the temperature 298.15 K and with standard pressure po = 101.325 kPa) of the crystalline and vitreous forms of arsenic sesquiselenide. The following results were obtained: ΔfHmo(As2Se3, cr) = −(86.1±4.1) kJ·mol−1 and ΔfHmo(As2Se3, vit) = −(58.1±4.2) kJ·mol−1. The enthalpy of the transition from the vitreous to the crystalline forms of As2Se3, −(28.0±3.9) kJ·mol−1 at 298.15 K, is consistent with two of several published values for the enthalpy of fusion of the crystalline sesquiselenide. The present results have been combined with enthalpy increments and the standard entropy has been recalculated from the literature to give, for As2Se3(cr) only, ΔfHmo and the standard molar Gibbs energy of formation ΔfGmo as functions of temperature. Mean bond enthalpies have been deduced for As4Se3(g) and As4Se4(g) on the basis of the new ΔfHmo values, and the thermodynamic results are shown to be consistent with a linear structure for As2Se2(g) with a central AsAs bond. Bond dissociation enthalpies Dmo(As-X) are given, where X = O, S, Se, and Te.

Sodium and oxygen disorder in a scandium-substituted nasicon: A time of flight neutron powder diffraction study of Na2.5Zr1.8Sc0.2Si1.3P1.7O12

Abstract A Sc-substituted NASICON of composition Na 2.5 Zr 1.8 Sc 0.2 Si 1.3 P 1.7 O 12 has been prepared and characterized by neutron powder diffraction and conductivity measurements. Time-of-flight neutron powder diffraction data were collected at 26, 100, 200, and 400°C. Satisfactory Rietveld refinements were obtained for all temperatures using the rhombohedral space group R 3 c. The novel aspect of this structure is the simultaneous presence of partially occupied intersitial sodium and oxygen sites that are disordered with the regular Na(2) and O(1) sites in the known rhombohedral NASICON structure. The results are compared with recent findings of defect structures in other NASICON materials. Conductivity measurements in the range 30–350°C reveal an activation energy of 0.30 eV for Na + conduction but conductivity values were found to change with temperature of sample preparation.