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Anomalous thermal expansion in one-dimensional transition metal cyanides: Behavior of the trimetallic cyanide C u 1 / 3 A g 1 / 3 A u 1 / 3 CN

DOI: 10.1103/PhysRevB.100.174302 DOI Help

Authors: Stella D'Ambrumenil (Institut Laue-Langevin; University of Reading) , Mohamed Zbiri (Institut Laue-Langevin) , Simon J. Hibble (University of Oxford) , Ann M. Chippindale (University of Reading) , Dean S. Keeble (Diamond Light Source) , Camille Wright (University of Reading) , Nicholas H. Rees (University of Oxford)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Physical Review B , VOL 100

State: Published (Approved)
Published: November 2019
Diamond Proposal Number(s): 17685

Abstract: The structural dynamics of a quasi-one-dimensional (1D) mixed-metal cyanide, C u 1 / 3 A g 1 / 3 A u 1 / 3 CN , 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 M x M ′ 1 − x CN ( M , M ′ = Cu , Ag, Au), exhibit 1D negative thermal expansion (NTE) along the chains and positive thermal expansion (PTE) perpendicular to them. C u 1 / 3 A g 1 / 3 A u 1 / 3 CN 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 measurements were carried out, underpinned by lattice-dynamical density-functional-theory (DFT) calculations. Synchrotron-based pair-distribution-function analysis and 13 C solid-state nuclear-magnetic-resonance 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 C u 1 / 3 A g 1 / 3 A u 1 / 3 CN , the metals in neighboring chains cannot all be truly coplanar 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 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.

Journal Keywords: Crystal structure; Lattice dynamics; Thermal expansion; 1-dimensional systems; Crystalline systems; Density functional theory; Inelastic neutron scattering; Infrared spectroscopy; Nuclear magnetic resonance; Raman spectroscopy; X-ray diffraction

Subject Areas: Materials, Physics

Instruments: I15-1-X-ray Pair Distribution Function (XPDF)

Added On: 28/11/2019 09:35

Discipline Tags:

Physics Materials Science Metallurgy

Technical Tags:

Scattering Total Scattering