POWDER CONDUCTIVITIES OF 3 ELECTRIDES

Electrical measurements on electride samples show a wide range of behavior in this novel class of compounds. K+(C222)e-, in which C222 represents cryptand [2.2.2], is highly conductive. Most of the measured resistance and the nonohmic behavior observed in two-probe measurements with inert electrodes result from either a Schottky barrier or a resistive barrier at the sample-electrode interface. By use of potassium metal on the electrodes, this effect was nearly eliminated, and the electrode impedance decreased by 4 orders of magnitude. An upper limit 0.2-OMEGA-cm for the resistivity of K+(C222)e- at 130 K was obtained through the use of the van der Pauw technique on pellets and apparent bandgaps of 0.086 and 0.055 above and below 135 K, respectively, were determined. The residual resistivity could be due to activated transport of electrons across grain boundaries. In contrast to the high electronic conductivity of K+(C222)e-, the electrides Cs+(15C5)2e- and Cs+(18C6)2e-, in which the cations is present in a crown ether sandwich, exhibit very low electronic conductivities but appear to be the first examples of ionically conductive electrides. The mechanism may involve release of the cesium cation from its crown ether cage, coupled with its reduction to Cs0 by a trapped electron. Ionic conductivity could then result from the transfer of Cs+ between anionic sites in the lattice. The activation energies for ionic conductivity are about 1.1 eV for Cs+(18C6)2e- and 0.6 eV for Cs+(15C5)2e-. The presence of small amounts of excess cesium metal in Cs+(18C6)2e-, to provide a mixed ceside-electride, leads to greatly increased conductivity and is probably the origin of semiconductor behavior previously seen for this compound.