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 $T_t\simeq 160$ 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 $\left(00\xi \right)$ direction has imaginary frequencies at $\xi <1/4$. Employing inelastic neutron scattering measurements we find a strong damping of the $\left(00\xi \right)$ 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.

• Received 27 January 2013

DOI:https://doi.org/10.1103/PhysRevB.88.024117