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Electrical conductivity of NH<inf>4</inf>Cl and NH<inf>4</inf>Br crystals in phases I and II
Date Issued
01-01-1975
Author(s)
Murti, Y. V.G.S.
Prasad, P. S.
Abstract
The electrical conductivity of single crystals of ammonium chloride, ammonium bromide and of the complete range of their solid solutions is measured from the simple cubic (sc, CsCl type) to the face centred cubic (fcc, NaCl type) phases through the structural transition. The effects of added divalent cationic impurities in NH4Cl and of purification by recrystallization are studied. The sc → fcc transition of NH4Br crystals is marked by the onset of a pronounced conductivity peak in the conventional log (σT) vs. 1/T plot. NH4Cl crystals show a marked drop in conductivity at the transition point. The λ-type conductivity peak persists in the solid solutions with large bromide content. The activation energy and preexponential factors of conductivity are markedly less in the higher-temperature phase than in phase II (sc) for undoped NH4Cl and undoped NH4Br. The conductivity as well as the activation energy for NH4Cl are decreased in phase II but increased in phase I as a result of the doping. In the case of NH4Br the conductivity and activation energy are gradually increased in successive stages of purification. These results in phase II of the two salts are interpreted in terms of an anion vacancy mechanism of conductivity in the pure crystals and extrinsic cation vacany transport in the impure materials. The excess conductivity at the transition is ascribed to proton release from freely rotating ammonium ions. The observed variation of the transition temperature with composition of NH4Cl1-xBrx fits well into an ideal two-component solid-solution model. © 1975.
Volume
79