Mat. Res. Bull., Vol. 17, pp. 363-368, 1982. Printed in the USA. 0025-5408/82/030363-06503.00/0 Copyright (c) 1982 Pergamon Press Ltd. E.S.R. STUDY OF CHEMICAL COLORATION OF AND ~" ALUMINATES BY METALLIC SODIUM D. Gourier ~, A. Wicker Compagnie G~n~rale d'Electricit~, Laboratoires de Marcoussis Route de Nozay, 91460 Marcoussis, France D. Vivien Laboratoire de Chimie Appliqu~e de l'Etat Solide, L.A. 302 E.N.S.C.P. II, rue Pierre et Marie Curie, 75231 Paris C~dex 05, France. (Received January 4, 1982; Communicated by P. HaFenmuHer) ABSTRACT E.P.R. study of B and ~" aluminate ceramics and B aluminate single crystals colored by contact with molten sodium reveals the forma- tion of collo~dal sodium centers located in the conduction planes of the aluminate structure. The chemical coloration strongly reduces the lifetime of the electrolyte in sodium - sodium cells, and is probably at the origin of the propagation of sodium - filled cracks through the electrolyte under current flow. Introduction A large number of studies have been devoted to B and ~" aluminates as solid electrolytes for sodium-sulfur cells. It has long been recognized that the limiting factor for the practical application of this material is its rather short lifetime. The breakdown of ~ alumina often appears as a rapid sodium-filled crack propagation through the electrolyte (1) (2). Recently a slow degradation has also been recognized (3). It proceeds by a layer-like propagation through the electrolyte of a dark coloration starting from the sodium-Na~ aluminate interface. Both mechanisms are associated with current flow through the electrolyte. Besides, coloration of the electrolyte also occurs by chemical interaction between molten sodium and B aluminate without any current passage (4). The purpose of the present work is to study the chemical process and its eventual correlation with the current-assisted degradation mechanisms, using electron spin resonance as the principal method of investigation. Experimental procedure Standard ceramic tubes of B and ~" sodium aluminates have been pro- vided by the Laboratoires de Marcoussis (C.G.E.). Their compositions (weight %) are : 8 Na20 92 AI203 for B aluminate and 0.76 Li20 9.90 Na20 89.32 AI203 363