Isotropic Zero Thermal Expansion and Local Vibrational Dynamics in (Sc,Fe)F₃

Abstract

Scandium fluoride (ScF) exhibits a pronounced negative thermal expansion (NTE), which can be suppressed and ultimately transformed into an isotropic zero thermal expansion (ZTE) by partially substituting Sc with Fe in (ScFe)F (Fe20). The latter displays a rather small coefficient of thermal expansion of -0.17 × 10/K from 300 to 700 K. Synchrotron X-ray and neutron pair distribution functions confirm that the Sc/Fe-F bond has positive thermal expansion (PTE). Local vibrational dynamics based on extended X-ray absorption fine structure indicates a decreased anisotropy of relative vibration in the Sc/Fe-F bond. Combined analysis proposes a delicate balance between the counteracting effects of the chemical bond PTE and NTE from transverse vibration. The present study extends the scope of isotropic ZTE compounds and, more significantly, provides a complete local vibrational dynamics to shed light on the ZTE mechanism in chemically tailored NTE compounds.

Controlling the Phase Behavior of Low and Negative Thermal Expansion ReO3-Type Fluorides using Interstitial Anions: Sc1–xZrxF3+x

“…Several different strategies have been explored to tune their thermal expansion. These include (a) forming solid solutions by isovalent cation substitution to make Sc 1– x Y x F 3 , Sc 1– x Al x F 3 , Sc 1– x Ti x F 3 , Sc 1– x Fe x F 3 , , Sc 1– x (Ga/Fe) x F 3 , and Sc 1– x (Al/Fe) x F 3 ; (b) preparing cation-ordered double ReO 3 -type materials such as CaZrF 6 , other M II ZrF 6 ’s, , CaTiF 6 , M II Nb IV F 6 , and solid solutions based on these materials; , (c) including guests in the open A-sites of the ReO 3 -structure by redox insertion or high-pressure gas treatment; and (d) deliberately introducing excess fluoride by aliovalent cation substitution to make materials such as Sc 1– x Zr x F 3+δ , YbZrF 7 , Ti 1– x Zr x F 3+ x , and [Mg 1– x Zr x ]­ZrF 6+2 x . Isovalent cation substitution and the creation of stoichiometric double ReO 3 -type materials often lead to compositions that undergo an undesirable structural phase transition from the cubic phase, which has interesting thermal expansion characteristics, to a lower symmetry or disordered material upon cooling or modest compression.…”

EXAFS spectroscopy: a powerful tool for the study of local vibrational dynamics

“…due to the large error bars in the Sc-Sc perpendicular MSRD, the combination of these findings indicates the presence of a "guitar-string" effect [108] as the local mechanism responsible for NTE in ScF 3 . Soon after, EXAFS was used to investigate the isotropic zero thermal expansion of (Sc 0.8 Fe 0.2 )F 3 , where the NTE of ScF 3 was suppressed by partially substituting Sc with Fe [109] . The EXAFS study of Purans et al [110] on ReO 3 , which is isostructural to ScF 3 , supports the concept that NTE is due to correlated movements of Re atoms in the transverse direction with respect to the -Re-O-Re-chains.…”

Controlling thermal expansion within mixed cocrystals by tuning molecular motion capability

“…Alternatively, negative TE (NTE) is rarer [4][5][6] and corresponds to a decrease in size upon heating. Lastly, near-zero or zero TE materials are quite rare [7][8][9] and are characterized by minimal to no change in response to temperature.…”