Single crystal X-ray measurements of the temperature dependence of the lattice parameters of In0.73Tl0.27 between 25 and 300 K were performed in order to search for a pretransformation tetragonal phase above the f.c.c.-f.c.t. martensitic transformation at 165 K, as previously found for FePd by Seto et al. [H. Seto, Y. Noda and Y. Yamada, J. Phys. Soc. 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 OhD4h proper ferroelastic 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 transformation temperature T0 remained constant. In the as-grown state of the crystal all model parameters are consistent with those measured ultrasonically by Brassington and Saunders [Proc. R. Soc. A387, 289 (1983)]. The directly measured volume change at T0 was found to be negative, consistent with the positive pressure coefficient of T0 for In0.77Tl0.23 [G. A. Saunders, J. D. Comins, J. E. Macdonald and E. A. Saunders, Phys. Rev. B34, 2064 (1986)]. No pretransformation tetragonal phase about T0 was found.
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