S. Susman

Single crystal neutron diffraction study of sodium cyanide

The neutron diffraction pattern of a single crystal of cubic NaCN has been measured at 295°K. These data and earlier results for KCN at 180 and 295°K have been analyzed in terms of a model which expands the angular distribution of CN− orientations in symmetry‐adapted spherical harmonics. From this analysis, the distributions of CN− in these crystals have been derived, and these indicate a monotonic trend in the series KCN (295°K) thru KCN (180°K) to NaCN (295°K) involving a shift in preferential orientation from the [111] toward the [100] directions in the crystal. Unusually large translational Debye‐Waller factors have also been derived for NaCN and KCN from the fitting procedure, in agreement with earlier results and analysis on KCN. In addition, an average C≡N bond distance of 1.18 A is obtained.

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.

The structure of phosphorus-selenium glasses I. Concentration dependence of the short- and intermediate-range order☆

Abstract The structures of phosphorus-selenium glasses PxPe1−x with x=0 (pure selenium), 0.05, 0.15, 0.25, 0.40 and 0.50 have been investigated with neutron diffraction, extented, X-ray absorption fine structure measurements, and Raman and infrared spectroscopy. The behavior of the near-neighbor coordination at r∼2.3 A as a function of x suggests a picture in which P4Sen molecular units are linked together by Se atoms or chains. The diffraction patterns exhibit anomalous peaks at Q ∼ 1.2A which rise dramatically as P is added to the Se glass; these peaks can also be understood on a molecular picture. The Raman spectra at the larger P concentrations are characterized by series of relatively sharp peaks which also suggest molecular units and for x = 0.5 bear a close correspondence to the spectrum of crystalline P4Se3.

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).

Crystal dynamics of KCN and NaCN in the disordered cubic phase

The lattice dynamics of KCN and NaCN in their disordered cubic phase has been studied by coherent inelastic neutron scattering and infrared reflection measurements. Acoustic phonon branches for cyanide single crystals were measured in the [001] direction, with more limited measurements taken in the [110] direction. The one‐phonon scattering generally appeared as weak peaks superimposed on a large background of non‐one phonon scattering, particularly as measurements were extended to higher energy. No peaks clearly assignable to optical phonons were observed in the neutron measurements but the infrared reflection spectra for polycrystalline pellets were analyzed to derive TO and LO phonon energies (at q=0) of 17.0±0.6 and 28.5±2.5 meV. These measurements (and a comparison with the rather easily measured acoustic and optical phonons in a KBr crystal of similar volume) indicate that phonons do not exist as well‐characterized excitations in the cyanides at higher frequencies and finite wave vectors. The unusua...

Single Crystal Neutron Diffraction Study of Potassium Cyanide

Neutron diffraction measurements were made on single crystals of KCN to obtain information about the disorder of the CN− ions in the cubic phase. The crystals were obtained by pulling from the melt by a modified Czochralski method. Data were collected on two four‐circle diffractometers at room temperature and at 180°K, just above the cubic‐orthorhombic phase transition. Structure factors were obtained for 24 inequivalent reflections. Free rotation of the CN− ions is ruled out by the different structure factors obtained for reflections with equal values of (h2+k2+l2) but different (h, k, l). However, models fitted to the data assuming orientations along a particular symmetry direction did not establish a preferred direction. The fits indicated that both vibrational and librational thermal amplitudes are large and relatively independent of temperature. These amplitudes could not be reproduced by a dynamical model assuming harmonic forces even when rotational‐translational coupling was included. A tentative ...

The structure of silicon-selenium glasses: I. Short-range order☆

Abstract The structures of SixSe1−x glasses with 0.0 ⩽ x ⩽ 0.40 have been investigated with neutron diffraction and Se K-edge extended X-ray absorption fine-structure measurements. Nearest-neighbor bonding is characterized by coordination numbers for Si and Se of 4 and 2, independent of concentration. Heteropolar bonding is highly favored. The second-neighbor coordination shell undergoes a systematic increase in area with increasing x and indicates, near the stoichiometric SiSe2 composition, a structure that is consistent with the presence of edge-sharing of mSi ( Se 1 2 ) 4 tetrahedra. “First sharp diffraction peaks” are well defined for the x ⩽ 0.17 glasses and exhibit remarkable systematics.

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.

Intermediate-range order in glasses and liquids

Many glasses and liquids exhibit a first sharp diffraction peak that is evidence for intermediate-range order caused by regularities in the packing of structural units. These tend to cluster around particular values of the wavevector when scaled by the nearest-neighbour distance or the mean inter-atomic spacing. The first sharp diffraction peak is distinguished from the pre-peak in binary metallic glasses, which occurs at higher wavevectors when scaled in this way.

Fast ion transport in the NASICON analog Na3Sc2(PO4)3: Structure and conductivity☆

The room temperature modification of stoichiometric NASICON(Sc) is monoclinic Cc. At 64°C there is a first order transition to a normal-conducting rhombohedral form R3c. Na(1) sites are fully occupied whereas Na(2) sites are partially occupied. At 167°C there is a transition to asuperionic phase, but the structure remains rhombohedral R3c. Vacancies are now shared equally by Na(1) and Na(2) sites. Fast Na-ion motion in stoichiometric Na3Sc2(PO4)3 arises from vacancy motion in a “dogleg” path between Na(1) and Na(2) sites.