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The structural dynamics of a 1D mixed-metal cyanide,Cu$_{1/3}$Ag$_{1/3}$Au$_{1/3}$CN, with intriguing thermal properties isexplored. All the current known related compounds with straight-chainstructures, such as CuCN, AgCN, AuCN and M$_x$M'$_{1-x}$CN (M, M' = Cu, Ag,Au), exhibit 1D negative thermal expansion (NTE) along the chains and positivethermal expansion (PTE) perpendicular to them. Cu$_{1/3}$Ag$_{1/3}$Au$_{1/3}$CNexhibits similar PTE perpendicular to the chains, however PTE, rather than NTE,is also observed along the chains. In order to understand the origin of thisunexpected behavior, inelastic neutron scattering (INS) measurements werecarried out, underpinned by DFT calculations. Synchrotron-based PDF analysisand 13C SSNMR measurements were also performed to build an input structuralmodel for the lattice dynamical study. The results indicate that transversemotions of the metal ions are responsible for the PTE perpendicular to thechains, as is the case for the related group 11 cyanides. However NTE along thechain due to the tension effect of these transverse motions is not observed.For this system, DFT-based phonon calculations predict small PTE along thechain due to low-energy chain-slipping modes induced by a bond-rotation effecton the weak metallophilic bonds. However the observed PTE is greater than thatpredicted with the straight-chain model. Small bends in the chain toaccommodate truly co-planar metals provide an alternative explanation forthermal behavior. These would mitigate the tension effect induced by thetransverse motions of the metals and, as temperature increases and the chainsmove further apart, a straightening could occur resulting in the observed PTE.This hypothesis is further supported by unusual evolution in the phononspectra, which suggest small changes in local symmetry with temperature.

Anomalous thermal expansion in one-dimensional transition-metal cyanides: Behavior of the trimetallic cyanide Cu$_{1/3}$Ag$_{1/3}$Au$_{1/3}$CN