Phonon dynamics in the layered negative thermal expansion compounds CuNi(CN)
arXiv:1906.05924 · doi:10.1103/PhysRevB.100.094312
Abstract
This study explores the relationship between phonon dynamics and negative thermal expansion (NTE) in CuNi(CN). The partial replacement of nickel (II) by copper (II) in Ni(CN) leads to a line phase, CuNi(CN) (x = 1), and a solid solution, CuNi(CN) (0 x 0.5). CuNi(CN) adopts a layered structure related to that of Ni(CN) (x = 0), and interestingly exhibits 2D NTE which is about 1.5 times larger. Inelastic neutron scattering (INS) measurements combined with first principles lattice dynamical calculations provide insights into the effect of Cu on the underlying mechanisms behind the anomalous thermal behavior in all the CuNi(CN) compounds. The solid solutions are presently reported to also show 2D NTE. The INS results highlight that as the Cu content increases in CuNi(CN), large shifts to lower energies are observed in modes consisting of localized in- and out-of-plane librational motions of the CN ligand, which contribute to the NTE in CuNi(CN). Mode Grüneisen parameters calculated for CuNi(CN) show that acoustic and low-energy optic modes contribute the most to the NTE, as previously shown in Ni(CN). However, mode eigenvectors reveal a large deformation of the [CuN4] units compared to the [NiC4] units, resulting in phonon modes not found in Ni(CN), whose NTE-driving phonons consist predominately of rigid-unit modes. The deformations in CuNi(CN) arise because the d square-planar center is easier to deform than the d one, resulting in a greater range of out-of-plane motions for the adjoining ligands.
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