Lattice dynamics and negative thermal expansion in the framework compound ZnNi(CN)4 with two-dimensional and three-dimensional local environments

Abstract

ZnNi(CN)4 is a three-dimensional (3D) framework material consisting of two interpenetrating PtStype networks in which tetrahedral [ZnN4] units are linked by square-planar [NiC4] units. Both the parent compounds, cubic Zn(CN)2 and layered Ni(CN)2, are known to exhibit 3D and 2D negative thermal expansion (NTE), respectively. Temperature-dependent inelastic neutron scattering (INS) measurements were performed on a powdered sample of ZnNi(CN)4 to probe phonon dynamics. The measurements were underpinned by ab initio lattice dynamical calculations. Good agreement was found between the measured and calculated generalized phonon density-of-states, validating our theoretical model and indicating that it is a good representation of the dynamics of the structural units. The calculated linear thermal expansion coefficients are αa = -21.2 × 10 -6 K -1 and αc = +14.6 × 10 -6 K -1 , leading to an overall volume expansion coefficient, αV of -26.95 × 10 -6 K -1 , pointing towards pronounced NTE behavior. Analysis of the derived mode-Grüneisen parameters shows that the optic modes around 12 and 40 meV make a significant contribution to the NTE. These modes involve localized rotational motions of the [NiC4] and/or [ZnN4] rigid units, echoing what has previously been observed in Zn(CN)2 and Ni(CN)2. However, in ZnNi(CN)4, modes below 10 meV have the most negative Grüneisen parameters. Analysis of their eigenvectors reveals that a large transverse motion of the Ni atom in the direction perpendicular to its squareplanar environment induces a distortion of the units. This mode is a consequence of the Ni atom being constrained only in two dimensions within a 3D framework. Hence, although rigid-unit modes account for some of the NTE-driving phonons, the added degree of freedom compared with Zn(CN)2 results in modes with twisting motions, capable of inducing greater NTE.

Responsive Four‐Coordinate Iron(II) Nodes in FePd(CN)4

“…Notably, ZnM(CN) 4 (M=Pt, Pd, Ni) also exhibits a two‐fold interpenetrated PtS‐type topology network; however, it exhibits ideal tetrahedral Zn centers (average ϕ (T‐SP)=7.8 %) (Figure ) . In ZnNi(CN) 4 , the NTE behavior is observed with coefficients of α a = b =−21.2 MK −1 (M=10 −6 ), α c =+14.6 MK −1 , and α volume =−27.0 MK −1 ; where the coefficient of thermal expansion, α =d l / l d T ( l =lattice parameter, T =temperature) . These results led us to investigate the TE behavior of 1 by variable temperature synchrotron XRD measurements (wavelength=0.774638(1) Å) in the temperature range of 100–500 K (Figure ; Supporting Information, Figure S6 and Table S3).…”

“…Various cyanide‐bridged CPs, synthesized by combining cyanide‐metallate building blocks with metal ions, exhibit characteristic structure‐related properties, such as guest tunable magnetic properties, ion storage capabilities, and thermal expansion behaviors (TE), including negative thermal expansion (NTE) . The combination of Fe II with tetracyanide metallate units [M(CN) 4 ] 2− (M=Pt, Pd, Ni) could be a potential method for obtaining four‐coordinate Fe II nodes in FeM(CN) 4 ; however, such synthetic experiments often produced octahedral Fe II nodes coordinated to four equatorial cyanides and two other axial solvents/neutral ligands .…”

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].…”

Mechanical Properties of Cubene Crystals

“…The mechanical properties of molecular crystals may differ from those of elemental solids, since the rotational degrees of freedom of molecules may play an important role [ 21 , 22 , 23 , 24 , 25 ]. There exist many examples of molecular crystals where rotations of molecules were analyzed [ 26 , 27 , 28 , 29 , 30 ].…”