Order–disorder and ionic conductivity in calcium nitride-hydride
Gavin J. Irvine, Ronald I. Smith, Martin O. Jones, John T. S. Irvine
Abstract
Abstract Recently nitrogen-hydrogen compounds have successfully been applied as co-catalysts for mild conditions ammonia synthesis. Ca 2 NH was shown to act as a H 2 sink during reaction, with H atoms from its lattice being incorporated into the NH 3 (g) product. Thus the ionic transport and diffusion properties of the N–H co-catalyst are fundamentally important to understanding and developing such syntheses. Here we show hydride ion conduction in these materials. Two distinct calcium nitride-hydride Ca 2 NH phases, prepared via different synthetic paths are found to show dramatically different properties. One phase ( β ) shows fast hydride ionic conduction properties (0.08 S/cm at 600 °C), on a par with the best binary ionic hydrides and 10 times higher than CaH 2 , whilst the other ( α ) is 100 times less conductive. An in situ combined analysis techniques reveals that the effective β-phase conducts ions via a vacancy-mediated phenomenon in which the charge carrier concentration is dependent on the ion concentration in the secondary site and by extension the vacancy concentration in the main site.