Thursday, July 18, 2013

1307.4736 (A. Podlesnyak et al.)

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

A. Podlesnyak, G. Ehlers, H. Cao, M. Matsuda, M. Frontzek, O. Zaharko, V. A. Kazantsev, A. F. Gubkin, N. V. Baranov
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.
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