15 research outputs found
Lattice dynamics and negative thermal expansion in the framework compound ZnNi(CN)4 with 2-D and 3-D local environments
ZnNi(CN)4 is a 3-D framework material consisting of two interpenetrating PtS-type 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 3-D and 2-D 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 Ī±a = -21.2 Ć 10-6 K-1 and Ī±c = +14.6 Ć 10-6 K-1, leading to an overall volume expansion coefficient, Ī±V of -26.95 Ć 10-6 K-1, pointing towards pronounced NTE behaviour. Analysis of the derived mode-GrĆ¼neisen parameters shows that the optic modes around 12 and 40 meV make a significant contribution to the NTE. These modes involve localised 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 GrĆ¼neisen parameters. Analysis of their eigenvectors reveals that a large transverse motion of the Ni atom in the direction perpendicular to its square-planar environment induces a distortion of the units. This mode is a consequence of the Ni atom being constrained only in two dimensions within a 3-D 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
Anomalous thermal expansion in 1D transition-metal cyanides: what makes the novel trimetallic cyanide Cu1/3Ag1/3Au1/3CN behave differently?
The structural dynamics of a quasi-one-dimensional (1D) mixed-metal cyanide, Cu1/3Ag1/3Au1/3CN, with intriguing thermal properties is explored. All the current known related compounds with straight-chain structures, such as group 11 cyanides CuCN, AgCN, AuCN and bimetallic cyanides MxMā1-xCN (M, Mā = Cu, Ag, Au), exhibit 1D negative thermal expansion (NTE) along the chains and positive thermal expansion (PTE) perpendicular to them. Cu1/3Ag1/3Au1/3CN exhibits similar PTE perpendicular to the chains, however PTE, rather than NTE, is also observed along the chains. In order to understand the origin of this unexpected behavior, inelastic neutron scattering (INS) measurements were carried out, underpinned by lattice-dynamical density-functional-theory (DFT) calculations. Synchrotron-based pair-distribution-function (PDF) analysis and 13C solid-state nuclear-magnetic-resonance (SSNMR) measurements were also performed to build an input structural model for the lattice dynamical study. The results indicate that transverse motions of the metal ions are responsible for the PTE perpendicular to the chains, as is the case for the related group 11 cyanides. However NTE along the chain due to the tension effect of these transverse motions is not observed. As there are different metal-to-cyanide bond lengths in Cu1/3Ag1/3Au1/3CN, the metals in neighboring chains cannot all be truly co-planar in a straight-chain model. For this system, DFT-based phonon calculations predict small PTE along the chain due to low-energy chain-slipping modes induced by a bond-rotation effect on the weak metallophilic bonds. However the observed PTE is greater than that predicted with the straight-chain model. Small bends in the chain to accommodate truly co-planar metals provide an alternative explanation for thermal behavior. These would mitigate the tension effect induced by the transverse motions of the metals and, as temperature increases and the chains move further apart, a straightening could occur resulting in the observed PTE. This hypothesis is further supported by unusual evolution in the phonon spectra, which suggest small changes in local symmetry with temperature
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Intra- and interchain interactions in (Cu1/2Ā¬Au1/2)CN, (Ag1/2Ā¬Au1/2)CN, and (Cu1/3Ag1/3Au1/3)CN and their effect on one-, two- and three-dimensional order
Mixed-metal cyanides, (Cu1/2Au1/2)CN, (Ag1/2Au1/2)CN and (Cu1/3Ag1/3Au1/3)CN, adopt an AuCN-type structure in which metal-cyanide chains pack on a hexagonal lattice with metal atoms arranged in sheets. The interactions between and within the metal-cyanide chains are investigated using density-functional-theory (DFT) calculations, 13C solid-state NMR (SSNMR) and X-ray pair distribution function (PDF) measurements. Long-range metal and cyanide order is found within the chains: (āCuāNCāAuāCNā)ā, (āAgāNCāAuāCNā)ā and (āCuāNCāAgāNCāAuāCNā)ā. Although Bragg diffraction studies establish that there is no long-range order between chains, X-ray PDF results show that there is local order between chains. In (Cu1/2Au1/2)CN and (Ag1/2Au1/2)CN, there is a preference for unlike metal atoms occurring as nearest neighbours within the metal sheets. A general mathematical proof shows that the maximum average number of heterometallic nearest-neighbour interactions on a hexagonal lattice with two types of metal atom is four. Calculated energies of periodic structural models show that those with four unlike nearest neighbours are most favourable. Of these, models in space group Immm give the best fits to the X-ray PDF data out to 8 Ć
, providing good descriptions of the short- and medium-range structures. This result shows that interactions beyond those of nearest neighbours must be considered when determining the structures of these materials. Such interactions are also important in (Cu1/3Ag1/3Au1/3)CN, leading to the adoption of a structure in Pmm2 containing mixed Cu-Au and silver-only sheets arranged to maximise the numbers of CuĖĖĖAu nearest- and next-nearest-neighbour interactions
In Situ Investigation of the Thermal Decomposition of Ammonium Tetrathiomolybdate Using Combined Time-Resolved X-ray Absorption Spectroscopy and X-ray Diffraction
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Structures of Pd(CN)2and Pt(CN)2: intrinsically nanocrystalline materials?
Analysis and modeling of X-ray and neutron Bragg and total diffraction data show that the compounds referred to in the literature as āPd(CN)2āandāPt(CN)2ā are nanocrystalline materials containing of small sheets of vertex-sharing square-planar M(CN)4 units, layered in a disordered manner with an intersheet separation of 3.44 A at 300 K. The small size of the crystallites means that the sheetsā edges form a significant fraction of each material. The Pd(CN)2 nanocrystallites studied using total neutron diffraction are terminated by water and the Pt(CN)2 nanocrystallites by ammonia, in place of half of the terminal cyanide groups, thus maintaining charge neutrality. The neutron samples contain sheets of approximate dimensions 30 A x 30 A. For sheets of the size we describe, our structural models predict compositions of Pd(CN)2-xH2O and Pt(CN)2-yNH3 (x = y = 0.29). These values are in good agreement with those obtained from total neutron diffraction and thermal analysis, and are also supported by infrared and Raman spectroscopy measurements. It is also possible to prepare related compounds Pd(CN)2-pNH3 and Pt(CN)2-qH2O, in which the terminating groups are exchanged. Additional samples showing sheet sizes in the range 10 A x 10 A (y = 0.67) to 80 A x 80 A (p = q = 0.12), as determined by X-ray diffraction, have been prepared. The related mixed-metal phase, Pd1/2Pt1/2(CN)2-qH2O(q = 0.50), is also nanocrystalline (sheet size 15 A x 15 A). In all cases, the interiors of the sheets are isostructural with those found in Ni(CN)2. Removal of the final traces of water or ammonia by heating results in decomposition of the compounds to Pd and Pt metal, or in the case of the mixed-metal cyanide, the alloy, Pd1/2Pt1/2, making it impossible to prepare the simple cyanides, Pd(CN)2, Pt(CN)2 or Pd1/2Pt1/2(CN)2, by this method
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Mixed Copper, Silver, and Gold Cyanides, (MxM '(1-x))CN: Tailoring Chain Structures To Influence Physical Properties
Binary mixed-metal variants of the one-dimensional MCN compounds (M = Cu, Ag, and Au) have been prepared and characterized using powder X-ray diffraction, vibrational spectroscopy, and total neutron diffraction. A solid solution with the AgCN structure exists in the (CuxAg1āx)CN system over the range (0 ā¤ x ā¤ 1). Line phases with compositions (Cu1/2Au1/2)CN, (Cu7/12Au5/12)CN, (Cu2/3Au1/3)CN, and (Ag1/2Au1/2)CN, all of which have the AuCN structure, are found in the gold-containing systems. Infrared and Raman spectroscopies show that complete ordering of the type [MāCā”NāMā²āNā”Cā]n occurs only in (Cu1/2Au1/2)CN and (Ag1/2Au1/2)CN. The sense of the cyanide bonding was determined by total neutron diffraction to be [AgāNCāAuāCNā]n in (Ag1/2Au1/2)CN and [CuāNCāAuāCNā]n in (Cu1/2Au1/2)CN. In contrast, in (Cu0.50Ag0.50)CN, metal ordering is incomplete, and strict alternation of metals does not occur. However, there is a distinct preference (85%) for the N end of the cyanide ligand to be bonded to copper and for AgāCNāCu links to predominate. Contrary to expectation, aurophilic bonding does not appear to be the controlling factor which leads to (Cu1/2Au1/2)CN and (Ag1/2Au1/2)CN adopting the AuCN structure. The diffuse reflectance, photoluminescence, and 1-D negative thermal expansion (NTE) behaviors of all three systems are reported and compared with those of the parent cyanide compounds. The photophysical properties are strongly influenced both by the composition of the individual chains and by how such chains pack together. The NTE behavior is also controlled by structure type: the gold-containing mixed-metal cyanides with the AuCN structure show the smallest contraction along the chain length on heating
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New insights into the compressibility and high-pressure stability of Ni(CN)2: a combined study of neutron diffraction, Raman spectroscopy, and inelastic neutron scattering
Nickel cyanide is a layered material showing markedly anisotropic behaviour. High-pressure neutron diffraction measurements show that at pressures up to 20.1 kbar, compressibility is much higher in the direction perpendicular to the layers, c, than in the plane of the strongly chemically bonded metal-cyanide sheets. Detailed examination of the behaviour of the tetragonal lattice parameters, a and c, as a function of pressure reveal regions in which large changes in slope occur, for example, in c(P) at 1 kbar. The experimental pressure dependence of the volume data is fitted to a bulk modulus, B0, of 1050 (20) kbar over the pressure range
0ā1 kbar, and to 124 (2) kbar over the range 1ā20.1 kbar. Raman spectroscopy measurements yield additional information on how the structure and bonding in the Ni(CN)2 layers change with pressure and show that a phase change occurs at about 1 kbar. The new high-pressure phase, (Phase PII), has ordered cyanide groups with sheets of D4h symmetry containing Ni(CN)4 and Ni(NC)4 groups. The Raman spectrum of phase PII closely resembles that of the related layered compound, Cu1/2Ni1/2(CN)2, which has previously been shown to contain ordered Cā”N
groups. The phase change, PI to PII, is also observed in inelastic neutron scattering studies which show significant changes occurring in the phonon spectra as the pressure is raised from 0.3 to 1.5 kbar. These changes reflect the large reduction in the interlayer spacing which occurs as Phase PI transforms to Phase PII and the consequent increase in difficulty for out-of-plane atomic motions. Unlike other cyanide materials e.g. Zn(CN)2 and Ag3Co(CN)6, which show an amorphization and/or a decomposition at much lower pressures (~100 kbar), Ni(CN)2 can be recovered after pressurising to 200 kbar, albeit in a more ordered form
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Local and average structure in zinc cyanide: towards an understanding of the atomistic origin of negative thermal expansion
Neutron diffraction at 11.4 and 295 K and solid-state 67Zn NMR are used to determine both the local and average
structures in the disordered, negative thermal expansion (NTE) material, Zn(CN)2. Solid-state NMR not only
confirms that there is head-to-tail disorder of the Cā”N groups present in the solid, but yields information about the relative abundances of the different Zn(CN)4-n(NC)n tetrahedral species, which do not follow a simple binomial
distribution. The Zn(CN)4 and Zn(NC)4 species occur with much lower probabilities than are predicted by binomial
theory, supporting the conclusion that they are of higher energy than the other local arrangements. The lowest
energy arrangement is Zn(CN)2(NC)2. The use of total neutron diffraction at 11.4 K, with analysis of both the
Bragg diffraction and the derived total correlation function, yields the first experimental determination of the
individual ZnāN and ZnāC bond lengths as 1.969(2) and 2.030(2) Ć
, respectively. The very small difference in bond lengths, of ~0.06 Ć
, means that it is impossible to obtain these bond lengths using Bragg diffraction in isolation. Total neutron diffraction also provides information on both the average and local atomic displacements responsible for NTE in Zn(CN)2. The principal motions giving rise to NTE are shown to be those in which the carbon and nitrogen atoms within individual ZnāCā”NāZn linkages are displaced to the same side of the ZnĀ·Ā·Ā·Zn axis. Displacements of the carbon and nitrogen atoms to opposite sides of the ZnĀ·Ā·Ā·Zn axis, suggested previously in X-ray studies as being responsible for NTE behavior, in fact make negligible contribution at temperatures up to 295 K
Local and Average Structure in Zinc Cyanide: Toward an Understanding of the Atomistic Origin of Negative Thermal Expansion
Neutron
diffraction at 11.4 and 295 K and solid-state <sup>67</sup>Zn NMR
are used to determine both the local and the average structures
in the disordered, negative thermal expansion (NTE) material, ZnĀ(CN)<sub>2</sub>. Solid-state NMR not only confirms that there is head-to-tail
disorder of the Cī¼N groups present in the solid, but yields
information about the relative abundances of the different ZnĀ(CN)<sub>4ā<i>n</i></sub>(NC)<sub><i>n</i></sub> tetrahedral species, which do not follow a simple binomial distribution.
The ZnĀ(CN)<sub>4</sub> and ZnĀ(NC)<sub>4</sub> species occur with much
lower probabilities than are predicted by binomial theory, supporting
the conclusion that they are of higher energy than the other local
arrangements. The lowest energy arrangement is ZnĀ(CN)<sub>2</sub>(NC)<sub>2</sub>. The use of total neutron diffraction at 11.4 K, with analysis
of both the Bragg diffraction and the derived total correlation function,
yields the first experimental determination of the individual ZnāN
and ZnāC bond lengths as 1.969(2) and 2.030(2) Ć
, respectively.
The very small difference in bond lengths, of ā¼0.06 Ć
,
means that it is impossible to obtain these bond lengths using Bragg
diffraction in isolation. Total neutron diffraction also provides
information on both the average and the local atomic displacements
responsible for NTE in ZnĀ(CN)<sub>2</sub>. The principal motions giving
rise to NTE are shown to be those in which the carbon and nitrogen
atoms within individual ZnāCī¼NāZn linkages are
displaced to the same side of the ZnĀ·Ā·Ā·Zn axis. Displacements
of the carbon and nitrogen atoms to opposite sides of the ZnĀ·Ā·Ā·Zn
axis, suggested previously in X-ray studies as being responsible for
NTE behavior, in fact make negligible contributions at temperatures
up to 295 K