| Summary: | 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 ∼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 Grüneisen 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 d9 square-planar center is easier to deform than the d8 one, resulting in a greater range of out-of-plane motions for the adjoining ligands
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