Volume 27A, number 5 PHYSICS LETTERS 15 July 1968 zyxwvuts CRYSTALLOGRAPHIC STUDY OF THE TRANSITION IN Ti203* C. N. R. RAO** , R. E. LOEHMAN and J. M. HONIG Department of Chemistry, Pdue University, Lafayette, Indiana 47907, USA Received 10 June 1968 X-ray crystallographic studies of highly purified Ti203 show abrupt changes in the lattice parameters with temperature in the range 390-4700K. The distension of the unit cell in this temperature range is readily correlated with corresponding changes in resistivity. Because of discrepancies in the literature [1,2], we have reinvestigated the crystallography of Ti203 as a function of temperature, T; such a study is also important to an understanding of the mechanism of the electrical transition in this compound [3,4]. It is generally agreed that Ti203 crystallizes in the corundum structure. However, a rather marked distension in the unit cell is reported by Pearson [l] to commence at a temperature rather different from that at which resistivity changes set in; furthermore, his data show two breaks in the dependence of the c parameter on zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDC T. The c parameter of Newnham and de Haan quoted by Goodenough [2], on the other hand, do not level off with rising temperature. X-ray diffraction patterns of Ti203 were re- corded in the temperature range 290~540°K in an atmosphere of Ar, employing a Philips diffracto- meter fitted with a Stone and Co. high tempera- ture attachment. The sample was mounted on a base-plate heater assembly of which a calibrated Pt-Pt/lO% Rh thermocouple was an integral part; the sample temperature was thus known rather accurately. High purity Ti203 was prepared by arc melting [5] TiO2 and Ti (of better than 99.99% purity) in the proper stoichiometric proportions under an atmosphere of gettered Ar. The variations of a, c, a, trigonal unit cell vol- ume, and resistivity with temperature [6,7] are shown in fig. 1. As is seen, there are differences between the lattice parameters determined in our study and those quoted in the earlier work. Our data show that a rather abrupt change in param- eter sets in at 390°K, in good agreement with the * Supported on NASA Multidisciplinary Institutional Grant NGR 15-005-021. ** Permanent address: Department of Chemistry, Indian Institute of Technology, Kanput, India. Temp *K Fig. 1. Variation of a, c, (r (trigonal) and trigonal unit cell volume of Ti203 with temperature. Gpen circles from present work; full circles from Pearson’s work [l]. Resistivity data from ref.6. 271