Phonon dynamics in the layered negative thermal expansion compounds CuxNi2−x(CN)

Abstract

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 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 Cu 2+ 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 Cu 2+ 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 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.

Chemical Diversity for Tailoring Negative Thermal Expansion

“…On account of the lower stability of d 9 (Cu 2+ )-type square-planar configuration than the d 8 (Ni 2+ ) one, [CuN 4 ] units in CuNi(CN) 4 exhibit a large deformation, leading to greater out-of-plane motions of N atoms and an enhanced NTE (∼1.5 times). 35 Figure 2. Crystal structure of MCN (M = Cu, Ag, or Au).…”

“…Theoretical calculation confirms that the shrinkage of unit cell volume is accompanied by a change in the spin state (Figure 37d) (the LS Co 3+ atomic radius is smaller than that of HS Co 3+ ). 35 So, the giant NTE in (1 − x)PT−xBC compounds was considered to be a combined effect caused by ferroelectricity, magnetism, and structure phase transition, where SVFS plays a dominant role.…”

“…The cyanide ligands in CuNi­(CN) 4 are highly ordered with carbon bonding to Ni 2+ and nitrogen to Cu 2+ . On account of the lower stability of d 9 (Cu 2+ )-type square-planar configuration than the d 8 (Ni 2+ ) one, [CuN 4 ] units in CuNi­(CN) 4 exhibit a large deformation, leading to greater out-of-plane motions of N atoms and an enhanced NTE (∼1.5 times) …”

Anomalous thermal expansion in one-dimensional transition metal cyanides: Behavior of the trimetallic cyanide Cu1/3Ag1/3d'Ambrumenil1, Zbiri2, Hibble3 et al. 2019Phys. Rev. BSelf Cite51111View full textAdd to dashboardCite

“…Synchrotron-based pairdistribution-function (PDF) analysis and 13 C solid-state nuclear-magnetic-resonance (SSNMR) measurements were performed to build an adequate input structural model for the lattice-dynamical analysis and interpretation. This combined approach provides insight into the atomistic mechanisms of thermal expansion and has previously been used to study many other transitionmetal cyanides with unusual thermal properties [9][10][11][12][13][14].…”

Colossal negative thermal expansion in a cucurbit[8]uril-enabled uranyl-organic polythreading framework via thermally induced relaxation

“…However, a different electron-driven mechanism is also observed in functional materials that undergo temperature-related electron changes, such as semiconductors, Mott insulators and superconductors. 7 Since the first comprehensive research on the NTE properties of the ZrW 2 O 8 framework by Sleight et al , 15 an increasing number of framework-structured NTE materials have been developed over the past two decades, ranging from oxides to fluorides 10,16,17 and molecular framework materials, including Prussian blue analogues 18 (PBAs, mostly metal cyanides 19–23 ) and metal–organic frameworks (MOFs). 24–34 Compared with well-studied inorganic NTE materials, especially oxides, NTE materials with organic structures have not been fully developed, and more work on the optimization of the NTE properties of these materials needs to be carried out.…”