Crystal structure and phonon softening in Ca3Ir4Sn13

TL;DR

This study reveals a second-order structural transition in Ca3Ir4Sn13 at T* ~ 38 K, driven by phonon softening of a breathing mode of Sn12 icosahedra, combining experimental and theoretical insights.

Contribution

It provides a detailed analysis of the crystal structure change and phonon softening mechanism in Ca3Ir4Sn13 using neutron, x-ray scattering, and density functional theory.

Findings

Structural transition at T* is second-order and well described by mean-field theory.

Phonon softening of a low-energy mode causes the structural change.

The soft mode is identified as a breathing mode of Sn12 icosahedra.

Abstract

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.