Temperature-driven phase transformation in Y3Co: Neutron scattering and first-principles studies

Abstract

Contrary to previous studies that identified the ground state crystal structure of the entire ${R}_{3}$Co series ($R$ is a rare earth) as orthorhombic $Pnma$, we show that Y${}_{3}$Co undergoes a structural phase transition at ${T}_{t}\ensuremath{\simeq}160$ K. Single crystal neutron diffraction data reveal that at ${T}_{t}$ the trigonal prisms formed by a cobalt atom and its six nearest-neighbor yttrium atoms experience distortions accompanied by notable changes of the Y-Co distances. The formation of the low-temperature phase is accompanied by a pronounced lattice distortion and anomalies seen in heat capacity and resistivity measurements. Density functional theory calculations reveal a dynamical instability of the $Pnma$ structure of Y${}_{3}$Co. In particular, a transversal acoustic phonon mode along the $(00\ensuremath{\xi})$ direction has imaginary frequencies at $\ensuremath{\xi}<1/4$. Employing inelastic neutron scattering measurements we find a strong damping of the $(00\ensuremath{\xi})$ phonon mode below a critical temperature ${T}_{t}$. The observed structural transformation causes the reduction of dimensionality of electronic bands and decreases the electronic density of states at the Fermi level that identifies Y${}_{3}$Co as a system with the charge density wave instability.