Nature of structural instabilities in superconducting Sr₃Ir₄Sn₁₃

In order to obtain microscopic insights into phase transition phenomena in Sr₃Ir₄Sn₁₃, complementary neutron and inelastic x-ray scattering experiments were carried out. Neutron scattering measurements in Sr₃Ir₄Sn₁₃ revealed a superlattice reflection at the M point, (7/2, 7/2, 1), below T^* ≃ 147 K. The second-order nature of the structural transition at T^* is corroborated by the continuous evolution of the superlattice peak below T^* without any thermal hysteresis. A gradual increase of critical scattering upon cooling towards T^* also supports the second-order transition. Inelastic X-ray scattering experiments at (11/2, 13/2, 0) revealed several low energy phonon modes below 8 meV at room temperature. The lowest energy phonon mode at 2.7 meV softens with decreasing temperature, and extrapolates to zero at around T^*. This feature clarifies that the structural transition at T^* in Sr₃Ir₄Sn₁₃ is driven by the soft phonon at the M point. (author)