Vibrational dynamics of $A$V$_{\mathrm{2}}$Al$_{\mathrm{20}}$ ($A \quad =$ Sc, La and Ce) cage compounds

We report on the inelastic response of $A$V$_{\mathrm{2}}$Al$_{\mathrm{20}}$ (with $A \quad =$ Sc, La and Ce) probed by high-resolution inelastic neutron scattering experiments [1]. Intense signals associated with the dynamics of Sc, La and Ce are identified in the low-energy range at 6-14 meV in ScV$_{\mathrm{2}}$Al$_{\mathrm{20}}$ and at 8-16 meV in LaV$_{\mathrm{2}}$Al$_{\mathrm{20}}$ and CeV$_{\mathrm{2}}$Al$_{\mathrm{20}}$. Their response to temperature changes between 2 and 300 K reveals a very weak softening of the modes upon heating in LaV$_{\mathrm{2}}$Al$_{\mathrm{20\thinspace }}$and CeV$_{\mathrm{2}}$Al$_{\mathrm{20}}$ and a distinguished blue shift by about 2 meV in ScV$_{\mathrm{2}}$Al$_{\mathrm{20}}$. By means of density functional theory (DFT) and LDC we show that the unusual anharmonicity of the Sc-dominated modes is due to the local potential of Sc featured by a strong quartic term and compare it with anharmonic systems (Al,Ga)V$_{\mathrm{2}}$Al$_{\mathrm{20}}$ [2]. [1] M. M. Koza et al., Phys. Chem. Chem. Phys. \textbf{16,} 27119 (2014) [2] M. M. Koza et al., Phys. Chem. Chem. Phys. \textbf{17}, 24837 (2015)