Anomalous thermal expansion in one-dimensional transition metal cyanides: Behavior of the trimetallic cyanide Cu1/3Ag1/3

Abstract

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 MxM'1-xCN (M, M' = 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 pairdistribution-function (PDF) analysis and 13 C 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 bondrotation 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 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.

Intra- and Interchain Interactions in (Cu1/2Au1/2)CN, (Ag1/2Au1/2)CN, and (Cu1/3Ag1/3Au1/3)CN and Their Effect on One-, Two-, and Three-Dimensional Order

“…Examination of the X-ray PDF of (Cu 1/3 Ag 1/3 Au 1/3 )CN shows that the model built in P 6 mm ( K ) containing three homometallic sheets can immediately be rejected because it is a poor description of the local order. This conclusion was also reached in our previous paper 16 because, in this model, the large number of Au–Au nearest neighbors produces far too much intensity in the peak centered around ∼3.35 Å corresponding to the distance between nearest-neighbor metals in the metal sheets. This result also tallies with those from DFT calculations, which found the energy of the P 6 mm structure to be significantly higher than that for the other three structures ( L – N in Table 2 ).…”

“… 4 , 8 The 13 C NMR spectrum reported previously for (Cu 1/3 Ag 1/3 Au 1/3 )CN is consistent with chains having the form (−Cu–NC–Ag–NC–Au–CN−) ∞ , with C bonded to Au and Ag and not to Cu. 16 The chemical shifts of 163 and 157 ppm are ascribed to 13 C in (NC–Au–CN) units and the C attached to Ag in (NC–Ag–NC) units, respectively.…”

“…Total-energy DFT calculations were performed on the four periodic structural models constructed for (Cu 1/2 Ag 1/2 Au 1/2 )CN ( K – N in Table 3 ) using the PBE-D3 method as described previously. 16 …”

“…Determining which of the models, L , M , or N , gives the best description of the local- and medium-range order is something that can be achieved using X-ray PDF studies but only by going to higher r (i.e., 7.0 Å) than was reported in our previous study. 16 Extending the maximum r examined to 7.0 Å means that the contribution of the highly significant M···M correlations between next-next-nearest neighbors at ∼6.75 Å is included in the calculation of the X-ray PDF. As we found above when examining possible structures for the bimetallic compounds, (Cu 1/2 Au 1/2 )CN and (Ag 1/2 Au 1/2 )CN, inclusion of next-next-nearest metal neighbors is a powerful discriminator between structures.…”

“…We have recently prepared a trimetallic cyanide, (Cu 1/3 Ag 1/3 Au 1/3 )CN, which also adopts an AuCN-related structure. 16 13 C solid-state NMR (SSNMR), DFT calculations, and a preliminary analysis of synchrotron X-ray PDF data established that the metal-cyanide chains are ordered as (−Cu–NC–Ag–NC–Au–CN−). This compound is unusual in that, unlike all of the group 11 parent and binary metallic cyanides, MCN, and (M x M′ 1– x )CN (M = Cu, Ag, Au), which exhibit negative thermal expansion along the chains, it exhibits positive thermal expansion in this direction.…”