Anomalous phonon Grüneisen parameters studied by Raman spectroscopy and X-ray diffraction

Phonon Grüneisen parameter is a dimensionless quantity measuring the strength of lattice anharmonic effects in a crystal. We study the temperature dependence of the lattice structure and dynamics for valence-fluctuating metal EuPd2Si2 and semiconductor Ta2NiS5 to examine their Grüneisen parameters. In these two cases, we find that even when the system is only in proximity to a structural instability, without any symmetry-breaking phase transition happening, certain optical phonon modes could have a frequency change an order of magnitude larger than the unit-cell volume change, resulting in an anomalously large phonon Grüneisen parameter.