Anomalous thermal expansion in YbPdIn and YbAl2 compounds: is there a relationship between charge transfer and local vibrational dynamics?

Thermal expansion is a problem in many technological applications and its control represents a challenge for the material design. An intriguing thermal expansion behaviour is displayed by both YbPdIn and YbAl2 compounds, i.e., their volume thermal expansion shows an anomalous deflection at around room temperature (Figure 1). This phenomenon is due to a charge transfer process driven by Yb atoms [1]. However, the effects of charge transfer phenomena on the local vibrational dynamics, in particular on the transverse atomic vibrations related to negative thermal expansion (NTE), are still unknown. Accordingly, we aim to shed light on this unclear relationship between charge transfer and local dynamics by means of temperature-dependent XAFS spectroscopy.