JOURNAL OF SOLID STATE CHEMISTRY 21,283-292 (1977) Structural Investigation of K+ and Tl+ P-Aluminas G. COLLIN,* J. P. BOILOT,T,$ A. KAHN,? J. THERY,? AND R. COMES* *Laboratoire de Physique desSolides, Unirersitt! de Paris Sud, 91405 Orsay, France, and 7E.R.A. 387-ENSCP, II, rue Pierre et Marie Curie, Chimie Appliquee de 1’Etat Solide, 75005Paris, France Received December 14, 1976; in revised form February 17, 1977 Structures of K+ and Tl+ aluminas were investigated by single-crystal X-ray diffraction. In the final refinements, corresponding to R values of 0.034 and 0.058, respectively, the distribution of conduct- ing ions was found to be similar to that of Na b-alumina. In K+ b-alumina clear evidence of addi- tional electronic density in the spinel-like blocks is obtained and is interpreted as due to a Frenkel defect. Introduction /?-Alumina compounds are double oxides with the general formula nA,O,-B,O where A = A13+, Ga3+, Fe3+ B = Na+, Ag+, K+, Rb+, Tl+. The structure has hexagonal symmetry (space group P6Jmmc). The outstanding fea- ture is the stacking of close-packed spine1 blocks separated by mirror planes perpendicu- lar to the c axis and containing B+ ions (I, 2). The great interest in thesecompounds arises from their very high ionic conductivity in the solid state, which allows them to be put among superionic conductors (3). This conductivity is due to the diffusion of B+ ions in the inter- mediate conduction planes; major questions to be answered concern the origin of the tem- perature dependance of their electrical pro- perties and their variation with regard to temperature. An X-ray diffuse scattering in- vestigation of the distribution conducting ions (4-6) revealed the existence of two-dimen- sional ordered microdomains, the size of which varies with temperature and cation type For silver p-alumina, the average distribu- $ To whom correspondence should be addressed. tion of electron density in the mirror planes, obtained by X-ray diffraction study (7), al- lowed a semiquantitative analysis of diffuse intensities and gave a description of cation distribution within the microdomains (8). To account for the variations in the scat- tered intensity distribution (related to local order changes) which are observed for different types of B+ ions and preparation conditions (5,6), it is necessary to have a good knowledge of the mean structures of those different compounds. This paper reports an X-ray diffraction in- vestigation of the nature and mean occupancy factors of the sites occupied by conducting ions in potassium and thallium /3-aluminas. Experimental Sodium p-alumina crystals are obtained by the slow cooling of a melt heated by direct induction at high frequency through the oxide (Kyropoulos self-crucible method). Their composition, deducedfrom neutron activation analysis, is about 11 Al&-l .3 & 0.03 Na,O. Replacement of Na+ by K+ or Tl+ is obtained by an exchange reaction in molten salts (9). p-alumina single crystals grow in the shape of easily cleavable thin platelets. The choice of Copyright 0 1977 by Academic Press, Inc. 283 All rights of reproduction in any form reserved. Printed in Great Britain ISSN 00224596