Pergamon Solid State Communications, Vol. 93, No. 12, pp. 10 I3- IO 17, I995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved OO38-1098(94)OO920-1 0038 1098/95 $9.50+.00 zyxwvutsr ELECTRICAL CONDUCTIVITY OF o?-LiIO, ACID TYPE CRYSTALS AT 1 kHz C. Galez, C. Rosso, Y. Teisseyre Equipe de Recherche sur les Materiaux pour la Ferroelectricicte et 1’Optique non Liieaire, Universite de Savoie - ESIA, 41 avenue de la Plaine, BP806,74016 Annecy cedex, France J.M. Crettez Laboratoire de Physique de 1’UniversitC de Bourgogne, URA CNRS 1796, UFR Sciences et Techniques, BP138 21004 Dijon, France P. Bourson*, G. Medeiros-Ribeiro, A. Righi, RL. Moreira Departamento de Fisica - ICEx - Universidade Federal de Minas Gerais, Brazil zyxwvutsrqponmlkjihgfedcba (Received 29 September 1994 by P. B&et) The temperature dependence of the electrical conductivity of a-LiIOs acid type crystals is studied. By applying a very low amplitude electric field at 1 kHz and performing a continuous sampling of measurements, differences, reproducible for all the investigated samples, appeared between the Grst and subsequent heatings The anomalies occurring during the first heating are attributed mainly to inclusions of mother liquor, HI03 and Lii. &IGs. The ‘intrinsic’ conductivity is measured after a first annealing at about 470 K ; the activation energies are then calculated. Keywords : B. crystal growth, D. dielectric response, D. phase transitions zyxwvutsrqponmlkjihgfedcbaZYXWVUTS 1. Introduction Crystals of hexagonal lithium iodate (a-LiIO3, space group P63) exhibit large piezoelectric [l], elasto-optic [2] and non linear optical effects [3,4]. They are of great interest for application in laser technology and in opto- electronic devices, but such applications need strict requirements on their quality and also stability of their properties. Upon heating the a-LiIO3 crystal undergoes two phase transitions according to the sequence a - y -f l3 which limit the range of applications from the low temperature up to the temperature of cracking of the sample. This cracking is accompanied by the loss of homogeneity, the drop of optical transmission and a structural change. It can occur in a range extending from 480 K to 530 K depending on various parameters such as the acidity of the growth mother solution, the size of the crystal, previous mechanical or thermal treatments [ 51. Another kind of instability, within the existence range of the c1 phase, is the change of properties when varying temperature and under the action of electric fields. The latter instability is mainly governed by a process of charge * Laboratoire MOPS, CLOES-SUPELEC, Universite de Metz, 2 rue E. Belin 57078 Metz cedex 3, France transfer and by the possibility of formation of space charges. The electrical conductivity ~33 along the polar c- axis being several orders of magnitude higher than (~11 along the perpendicular direction, a-LiIOa is a quasi-one- dimensional conductor [l]. Finally, it seems now well established that the intrinsic ionic conduction is due to two kinds of carriers, One of them is the interstitial Li’ ion which hops among interstices in zigzag channels along the c-axis, the other is the Li’ vacancy which exchanges positions with Li’ ions at lattice sites along the c-axis [6]. From a survey of the literature one can notice that the dielectric behaviour along the c-axis, which is often the only one to be considered, has to be classified in three different regions according to the frequency of the applied voltage : i) With DC and low frequency (f < 100 Hz) the behaviour is determined rather by electrolytic phenomena near the electrodes and space charges than by mechanisms of charge transfer in the bulk. Indeed when a DC field is used, with a relatively high amplitude (1 kV/cm or more), deposition of layers of decomposition products occurs behind the [7,8]. Such a phenomenon is evidently time dependent [9,10] and furthermore is a function of the nature of the electrodes [ 11,12]. 1013