Description
The term "soft phonon mode" was first introduced by Cochran in describing the lattice dynamical behaviour for certain ferroelectric crystals. Within the past forty years or so, the term has been applied, perhaps somewhat loosely, to describe almost any section of a phonon dispersion relation for which frequency decreases as temperature decreases. So the real relationship between "phonon softening anomalies" and "phase transformations" has been a point of confusion in the literature, as was discussed some years ago by Professor Jim Krumhansl. The term has been continually applied to ferroelastic materials, certain superconductors, shape memory alloys and, in particular. some magnetic shape memory materials, as well as various inorganic crystals.
In this presentation, I shall review some of the observations of anomalous phonon softening and the extent to which such features of the lattice dynamics, relate to a phase transformation of the crystal of interest. Such transformations range from a quite simple cubic to tetragonal transformation involving an associated softening of the low-q, acoustic phonons, as in the A-15 structure compound, V3Si, to the more complex crystallographic changes resulting in modulated structures related to phonon anomalies at specific values of q along a dispersion relation, as in the magnetic shape memory Ni-Mn-Ga alloys.
In our own recent research on In-Tl alloys which exhibit a cubic to tetragonal transformation on cooling, which old literature attributed to a low-q phonon anomaly on the T2 branch, there is no phonon softening along this branch. Rather, a slight softening of the [111]T zone-boundary phonon with deceasing temperature was suggested to be consistent with a transformation mechanism involving defect formation on {111} conjugate crystal planes connecting the cubic and tetragonal crystal structures.
| I am the presenting author | Yes |
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