The martensitic transformation in In-Tl alloys revisited

Abstract Indium-Thallium (In-Tl) alloys in the composition range In-xat%Tl, for 15.5 ≦ x ≦ 31, are known to undergo a martensitic transformation on cooling below an M s temperature with 425K ≧ M s ≧ 0K. The traditional view of this transformation as being via a double {110}〈1-10〉 shear driven by a softening of low-q [ζζ0][ζ-ζ0] phonons, on the basis of a pseudopotential calculation which relied on ultrasonically measured elastic constants, has been questioned, on the basis of recent phonon dispersion data. Measurements of the shear modulus, (c 1 1 - c 1 2 )/2, by the resonant ultrasound spectroscopy (RUS) method, are reported for some single crystals of Indium-Thallium (In-Tl) alloys in this composition range. The new RUS results together with the [ζζ0][ζ-ζ0] phonon dispersion data, support an alternative mechanism for the transformation, which is consistent with most recent phonon dispersion data for the [ζζζ]T phonon branch.