  We investigated the crystal structure and lattice excitations of the ternary
intermetallic stannide Ca3Ir4Sn13 using neutron and x-ray scattering
techniques. For T > T* ~ 38 K the x-ray diffraction data can be satisfactorily
refined using the space group Pm-3n. Below T* the crystal structure is
modulated with a propagation vector of q = (1/2, 1/2, 0). This may arise from a
merohedral twinning in which three tetragonal domains overlap to mimic a higher
symmetry, or from a doubling of the cubic unit cell. Neutron diffraction and
neutron spectroscopy results show that the structural transition at T* is of a
second-order, and that it is well described by mean-field theory. Inelastic
neutron scattering data point towards a displacive structural transition at T*
arising from the softening of a low-energy phonon mode with an energy gap of
Delta(120 K) = 1.05 meV. Using density functional theory the soft phonon mode
is identified as a 'breathing' mode of the Sn12 icosahedra and is consistent
with the thermal ellipsoids of the Sn2 atoms found by single crystal
diffraction data.
