Volume Effect or Paddle‐Wheel Mechanism—Fast Alkali‐Metal Ionic Conduction in Solids with Rotationally Disordered Complex Anions

Abstract

Dedicated to Professor Karl Heinz Buchel on the occasion of his 60th birthday Transport phenomena in the solid state are of equal importance in basic research and practice. In the past two decades particular interest has been directed towards so-called "fast" or "super" ionic conductors because of their attractive potential applications. We have synthesized highly conductive alkali-metal ionic conductors based on ionic crystals in which, on the one hand, high concentrations of the charge carriers can be realized by doping (point defects in the cation substructure) and, on the other, the activation energy of the interchange of sites is decreased by translationally fixed but rotationally mobile complex anions. Mixed crystals with Na,PO, or Na,AIF, structures have proven especially suitable in this connection. With the object of establishing a broader experimental foundation for clarifying the controversially discussed question of whether the higher free transport volume or the rotational motion of the anions is responsible for the high cation mobility in these rotary phases we have systematically varied the type of anions and concentration of defects and monitored the resulting changes in conduo tivity. Although the macroscopical characteristics investigated are not suitable for explaining mechanisms in detail at the atomic level, the results afford clear support for the assumption of a "paddle wheel mechanism"; but also effects of the enlarged transport volume are not to be disputed. Both these effects enhancing the cationic conductivity are concomitantly operative in amounts varying from system to system; they cannot be totally separated from each other.

DFT-basedab initiostudy of structural and electronic properties of lithium fluorooxoborate LiB6O9F and experimentally observed second harmonic generation

“…5 that SHG in LBOF is expected to occur, because the same magnitude of the refractive indices n x = n z is found for the reference and double frequencies ω, e.g., forhω = 1.85 eV and 2hω = 3.70 eV. Another precondition for SHG is a comparatively high second order nonlinear electronic susceptibility χ (2) , 40 which can be evaluated using the electric field dependence of the dielectric susceptibility χ (F) = ε(F) − 1, 41…”

“…LBO has a somewhat similar chemical composition to that of LBOF, possesses the same space group, and has been well studied because of its wide applications as a nonlinear optical material. 33,34 KDP Fdd2 displays the same point group of symmetry mm2 (and therefore the same set of nonzero components of the tensors χ (2) ij k and χ (3) ij kl ). KDP I42d shows the point group symmetry42d.…”

“…The tensor χ (1) ij has three principal components, χ (1) 11 , χ (1) 22 , and χ (1) 33 , corresponding to the three principal refractive indices. 35 In the crystals with inversion symmetry or along the nonpolar directions, in the case of polar crystals, the susceptibility χ does not change when inverting the electric field; therefore, the even-order susceptibilities χ (2) , χ (4) , . .…”

“…1 The fundamental prerequisites for high conductivity are well understood. 2 The anionic matrix needs to provide two dimensionally, or better three dimensionally, branched migration paths along which the ions experience a flat electrostatic potential profile. Researchers must avoid accumulations of negative charges on the walls of the pathways that might trap the moving cations, and LBOF was recently suggested as a candidate material.…”

Molten Carbonates as an Effective Oxygen Reduction Catalyst for 550–650°C Solid Oxide Fuel Cells

“…The migration of CO 4 2− from the O 2 /MC surface of chemisorption to the MC/LSCF interface of CO 4 2− reduction can be achieved through a cooperative "cogwheel" (or "paddle-wheel") mechanism shown in Figure 8, involving the breaking and reforming O-CO 3 2− bond. Similar mechanism has also been previously proposed for the superionic conduction phenomenon in fast alkali-metal ionic conductors with rotationally disordered complex anions [40][41][42] and Ga-based oxides with tetrahedral moieties. 43 3 2− ions return to the MC bulk to replenish the preceding reactions.…”