Phonon Dynamics In The Layered Negative Thermal Expansion Compounds Cuxni2-X(Cn)(4)

This study explores the relationship between phonon dynamics and negative thermal expansion (NTE) in CuxNi2-x(CN)(4). The partial replacement of nickel (II) by copper (II) in Ni(CN)(2) leads to a line phase, CuNi(CN)(4) (x = 1), and a solid solution, CuxNi2-x(CN)(4) (0 <= x <= 0.5). CuNi(CN)(4) adopts a layered structure related to that of Ni(CN)(2)(x = 0), and interestingly exhibits two-dimensional (2D) NTE which is similar to 1.5 times larger. Inelastic neutron-scattering (INS) measurements combined with first-principles lattice dynamical calculations provide insights into the effect of Cu2+ on the underlying mechanisms behind the anomalous thermal behavior in all the CuxNi2-x(CN)(4) compounds. The solid solutions are presently reported to also show 2D NTE. The INS results highlight that as the Cu2+ content increases in CuxNi2-x(CN)(4), 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)(4). Mode Gruneisen parameters calculated for CuNi(CN)(4) show that acoustic and low-energy optic modes contribute the most to the NTE, as previously shown in Ni(CN)(2). 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)(2), whose NTE-driving phonons consist predominately of rigid-unit modes. The deformations in CuNi(CN)(4) arise because the d(9) square-planar center is easier to deform than the d(8) one, resulting in a greater range of out-of-plane motions for the adjoining ligands.