Crystal structure and phonon softening in Ca$_3$Ir$_4$Sn${13}$

Abstract : We investigated the crystal structure and lattice excitations of the ternary intermetallic stannide Ca$_3$Ir$_4$Sn${13}$ using neutron and x-ray scattering techniques. For $T$ > $T*$ approximate to 38 K, the x-ray diffraction data can be satisfactorily refined using the space group Pm3n. Below T*, the crystal structure is modulated with a propagation vector of (q) over right arrow = (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 Sn$_{12}$ icosahedra and is consistent with the thermal ellipsoids of the Sn2 atoms found by single-crystal diffraction data.
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Submitted on : Wednesday, March 14, 2018 - 6:04:12 PM
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D. G. Mazzone, S. Gerber, J. L. Gavilano, R. Sibille, M. Medarde, et al.. Crystal structure and phonon softening in Ca$_3$Ir$_4$Sn${13}$. Physical Review B : Condensed matter and materials physics, American Physical Society, 2015, 92, pp.024101. ⟨10.1103/PhysRevB.92.024101⟩. ⟨cea-01734608⟩

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