Bernhardt J. Wuensch

Single-crystal neutron diffraction study of the fast-ion conductor β-Ag2S between 186 and 325°C

Abstract Partial Fourier synthesis of the silver ion density in bcc β-Ag2S at several temperatures reveals cation delocalization in bands along 〈100〉. This density may be analytically represented by use of higher-rank tensors, with partial occupancy of both octahedral and tetrahedral interstices below 200°C, but only tetrahedral occupancy at higher temperatures. R values range from 2.6 to 5.9%. The distribution of mobile ions differs greatly in α-AgI and β-Ag2S, despite similar anion arrays, reflecting the different silver ion concentrations and characteristics of the bonding present in the respective low-temperature phases.

Distribution of Atoms in High Chalcocite, Cu2S

In spite of its simple composition, the structure of chalcocite, CU2S, has long defied analysis. In high chalcocite the sulfur atoms are in hexagonal close-packing, while three varieties of copper atoms, with four-fold, three-fold, and two-fold coordinations respectively, are in disorder in the interstices.

Single crystal neutron diffraction analysis of the cation distribution in the high-temperature phases α-Cu2−xS, α-Cu2−xSe, and α-Ag2Se

Abstract The distribution and thermal vibration of the mobile cations have been studied for α -Cu 2− x S (200°C), α -Cu 2− x Se (160, 210, and 360°C), and α -Ag 2 Se (135, 150, 200, and 300°C). Both α -Cu 2− x S and α -Cu 2− x Se, space group Fm3m, have an fcc anion array within which anharmonically-vibrating Cu atoms are displaced from the centers of the tetrahedra towards the vacant octahedral interstices. α -Ag 2 Se, space group Im3m, has Se in bcc packing and displays a cation distribution which is delocalized in channels along 〈100〉 similar to β-Ag 2 S. Local maxima in the Ag density occur in pairs about the center of the tetrahedral interstice.

Order–disorder transformations induced by composition and temperature change in (SczYb1−z)2Ti2O7 pyrochlores, prospective fuel cell materials

Abstract The structures and site occupancies of Sc–Yb titanate solid solutions with the A 2 B 2 O 7 pyrochlore structure type were determined by Rietveld analysis of powder-diffraction profiles obtained from a pulsed neutron source. The results are the first in situ studies of order–disorder transformations in pyrochlore that are induced by temperature increase (25 T 3+ site. Solid solution was found to extend to z R A and increase in the temperature (20 T R A and R B .

An improved method, based on Whipple’s exact solution, for obtaining accurate grain‐boundary diffusion coefficients from shallow solute concentration gradients

An equation developed by Le Claire is widely used to obtain a grain‐boundary diffusion product, aD′, from the measured solute concentration gradients produced under conditions of constant surface concentration in grain‐boundary diffusion experiments. However, a numerical assessment of the accuracy of Le Claire’s equation has revealed errors as large as 70% when applied outside of its range of validity to the shallow gradients (∼102 nm) that are provided by high‐resolution analytical methods. To provide a relation applicable to this region, a numerical analysis of the variation of computed values of ∂ ln C/∂η6/5 with β has been used to develop a new and greatly improved expression of the form aD′=D3/2t1/2[10A(−∂ ln C/∂η6/5)B ], where A and B are parameters whose value depends on the experimental value of ∂ ln C/∂η6/5. The relation is valid for the experimentally useful range of solute penetrations 6√Dt∼10√Dt and for 1≤β≤105, and has been shown to produce a grain‐boundary diffusion product accurate to wi...

Mobile ion distribution and anharmonic thermal motion in fast ion conducting Cu2S

A unique model was determined for the mobile copper ion disorder in hexagonal Cu/sub 2/S between 120/sup 0/ and 325/sup 0/C via single crystal neutron diffraction. The copper ions partially occupy two sets of three-coordinated sites within the HCP sulfur array and display anharmonic thermal motion. The results suggest that the conductivity is two dimensional in nature.

Lattice diffusion, grain boundary diffusion and defect structure of Zno

The rates of 65Zn2+ self-diffusion and Ni2+ diffusion have been measured along the a axis and the c axis of single-crystal ZnO over temperature ranges of 758–1267°C and 900–1550°C, respectively. Samples were encapsulated in polycrystalline disks during annealing to inhibit loss of solute or host material through vaporization. Cation self-diffusion in ZnO is isotropic within experimental error and may be described by an activation energy of 1.80 ± 0.09 eV and a pre-exponential term, Do, of 7.26 × 10−6 cm2 s−1. Results for Ni2+ diffusion provide an activation energy of 2.06 eV and a small dependence on crystallographic direction, Do being 9.89 × 10−5 and 3.16 × 10−5 cm2 s−1 along the a and c axes, respectively. Diffusion of Zn2+ and Ni2+ in polycrystalline ZnO is strongly enhanced along grain boundaries. At temperatures up to 1300°C the temperature dependence of δDB for Ni2+ provides the same activation energy as found for volume diffusion. The enhanced transport is attributed to higher defect concentrations near the boundary. At temperatures > 1300°C, δ seems to change with temperature due to incomplete equilibration of the sample, an interpretation supported by an observed increase of δDB with the time of equilibration with a reducing atmosphere. The increase of δDB with decrease in oxygen partial pressure supports the assignment of doubly-ionized interstitial zinc ions as the predominant point defect but leaves problematic the isotropy observed for both Do and the activation energy for migration.

Structure of Na3NdSi6O15.2H2O - A Layered Silicate with Paths for Possible Fast-Ion Conduction

Hydrothermal investigations of the high silica region of the Na₂0-Nd₂0₃-SiO₂ system, carried out in the search for new fast-ion conductors (FICs), yielded the compound Na₃NdSi₆O₁₅.2H₂0 (sodium neodymium silicate). Single-crystal X-ray methods provided lattice constants of a=7.385(2), b=30.831(7) and c= 7.1168 (13)A, space group Cmm2, and 22 atoms in the asymmetric unit. With four formula units per unit cell, the calculated density is 2.68gcm⁻³. Refinement was carried out with 1113 independent structure factors to a weighted residual wR(F) of 2.63% [8.09% for wR(F²)] using anisotropic temperature factors for all atoms. The structure, based on corrugated Si₆O₁₅ layers containing four-, five-, six- and eight-membered rings, is related to that of a model previously reported for a compound assigned the composition NaNdSi₆O₁₃(OH)₂.nH₂0. Our structure differs in the placement of sodium ions and water molecules, and contains no hydroxyl groups. We believe that both studies examined the same phase.

Structure of Na3YSi6O15 – a unique silicate based on discrete Si6O15 units, and a possible fast-ion conductor

Hydrothermal investigations in the high silica region of the Na₂O-Y₂0₃-Si₆O₁₅ system, carried out in a search for novel fast-ion conductors (FICs), yielded the new compound trisodium yttrium hexasilicate, Na₃YSi₆O₁₅. Single-crystal X-ray methods revealed that Na₃YSi₆O₁₅ crystallizes in space group Ibmm, has lattice constants a=10.468 (2), b=15.2467 (13) and c=8.3855 (6) A, Z=4, and 11 atoms in the asymmetric unit. Refinement was carried out to a weighted residual of 3.53% using anisotropic temperature factors for all atoms. The structure is unique in that the silica tetrahedra form isolated SiSi₆O₁₅⁶⁻ double dreier-rings, rather than layers as might be expected from the Si to O ratio of 0.4. No isomorphs to Na₃YSi₆O₁₅ have been reported.

Structure, phase transitions and ionic conductivity of K3NdSi6O15·xH2O. I. α-K3NdSi6O15·2H2O and its polymorphs

Hydrothermally grown crystals of α-K3NdSi6O15·2H2O, potassium neodymium silicate, have been studied by single-crystal X-ray methods. The compound crystallizes in space group Pbam, contains four formula units per unit cell and has lattice constants a = 16.008 (2), b = 15.004 (2) and c = 7.2794 (7) A, giving a calculated density of 2.683 Mg m−3. Refinement was carried out with 2161 independent structure factors to a residual, R(F), of 0.0528 [wR(F2) = 0.1562] using anisotropic temperature factors for all atoms other than those associated with water molecules. The structure is based on highly corrugated (Si2O52−)∞ layers which can be generated by the condensation of xonotlite-like ribbons, which can, in turn, be generated by the condensation of wollastonite-like chains. The silicate layers are connected by Nd octahedra to form a three-dimensional framework. Potassium ions and water molecules are located in interstitial sites within this framework, in particular, within channels that extend along [001]. Aging of as-grown crystals at room temperature for periods of six months or more results in an ordering phenomenon that causes the length of the c axis to double. In addition, two phase transitions were found to occur upon heating. The high-temperature transformations, investigated by differential scanning calorimetry, thermal gravimetric analysis and high-temperature X-ray diffraction, are reversible, suggesting displacive transformations in which the layers remain intact. Conductivity measurements along all three crystallographic axes showed the conductivity to be greatest along [001] and further suggest that the channels present in the room-temperature structure are preserved at high temperatures so as to serve as pathways for easy ion transport. Ion-exchange experiments revealed that silver can readily be incorporated into the structure.