Temperature-driven Phase Transformation in Y$_3$Co: Neutron Scattering and DFT Studies

The effects of a crystal structure deformation due to subtle atomic displacements have attracted much attention because they can result in colossal changes of the electronic and magnetic properties of solids. The R$_3$Co binary intermetallic systems exhibit a number of complicated phenomena, including field-induced magnetic phase transitions (R=Er, Ho, Tb), giant magnetoresistance (R=Dy), a substantial magnetocaloric effect (R=Gd) and superconductivity (R=La). Contrary to previous studies that defined the ground state crystal structure of the entire R$_3$Co series as orthorhombic Pnma, we find that Y$_3$Co undergoes a structural phase transition upon cooling around Tc~160K. Density functional theory calculations reveal a dynamical instability of the Pnma structure of Y$_3$Co. Employing inelastic neutron scattering measurements we find a strong damping of the $(00\xi)$ acoustic phonon mode below the critical temperature Tc. We suggest that some other members of the R$_3$Co series (or even all of them) have ground state crystal symmetry lower than reported Pnma. This raises a question about the true magnetic structures and hence the influence of magnetic properties of the entire R$_3$Co series.