X-RAY STUDY OF THE MARTENSITIC-TRANSFORMATION IN AN INDIUM-THALLIUM ALLOY

Single crystal X-ray measurements of the temperature dependence of the lattice parameters of In0.73 Tl0.27 between 25 and 300 K were perform ed in order to search for a pretransformation tetragonal phase above t he fc.c.-fc.t. martensitic transformation at 165 K, as previously foun d for Fe-Pd by Seto et al. [H. Seto, Y. Noda and Y. Yamada, J. Phys. S oc. Japan 57, 3668 (1988); 59, 965 (1990)] and in order determine, in conjunction with ultrasonically measured room temperature elastic data , the parameters of the Landau free energy for the O(h)-D4h proper fer roelastic transformation, The mechanical stability limit of the cubic phase and the third and fourth order elastic moduli depend on thermal cycling through the transformation, but their ratio and the transforma tion temperature T0 remained constant. In the as-grown state of the cr ystal all model parameters are consistent with those measured ultrason ically by Brassington and Saunders [Proc. R. Soc. A387, 289 (1983)]. T he directly measured volume change at T0 was found to be negative, con sistent with the positive pressure coefficient of T0 for In0.77Tl0.23 [G. A. Saunders, J. D. Comins, J. E. Macdonald and E. A. Saunders, Phy s. Rev. B34, 2064 (1986)]. No pretransformation tetragonal phase about T0 was found.