Infrared(IR) study of strong negative thermal expansion(NTE) material Scandium trifluoride(ScF3)

Strong, thermally persistent, isotropic NTE is a rare phenomenon that has been observed in ScF3.We have previously reported unusual softening of an optical mode branch along the edges of the BZ and strong quasi-elastic peak appearance at low temperatures were observed in an IXS, providing copious evidence of a structural quantum phase transition near the ground state of this system[1].
Here, we present an IR study on single crystal ScF_3 using FTIR and bolometric detection. Temperature-dependent reflectivities were successfully fit and zone center mode frequencies were determined.
We also identify additional absorption arising from modes away from zone center, associated with well-known multi-phonon absorption seen in other polar crystals which is attributed to second-order dipole moment and/or third order anharmonicity. In either case, the net result is a weakly-dispersing TO phonon is excited at momentum q while another, lower energy phonon is excited at momentum -q. The multi-phonon absorption is therefore sensitive to modes extending throughout the BZ, including the zone edge where the NTE or incipient modes reside. In this way, we have identified a link between optical probes and the incipient NTE modes of interest to this field.
[1] Handunkanda, et al PRB 92, 134101 (2015)