Temperature-driven Phase Transformation in Y_3Co: Neutron Scattering and First-principles Studies

TL;DR

This study reveals a temperature-induced structural phase transition in Y_3Co involving lattice distortions, phonon mode damping, and electronic structure changes, indicating charge density wave instability, supported by neutron scattering and first-principles calculations.

Contribution

It demonstrates a previously unrecognized phase transition in Y_3Co driven by phonon instability and provides detailed experimental and theoretical insights into its structural and electronic changes.

Findings

Structural phase transition at 160K in Y_3Co.

Damping of (00z) phonon mode below transition temperature.

Reduction in electronic density of states at Fermi level.

Abstract

Contrary to previous studies that identified the ground state crystal structure of the entire R_3Co series (R is a rare earth) as orthorhombic Pnma, we show that Y_3Co undergoes a structural phase transition at T_t=160K. 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_3Co. In particular, a transversal acoustic phonon mode along the (00z) direction has imaginary frequencies at z<1/4. Employing inelastic neutron scattering measurements we find a strong damping of the (00z) 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_3Co as a system with the charge density wave instability.