Ag3[Co(CN)6]: Giant Positive and Negative Thermal Expansion

Elizabeth A. ClarkMaterials 286GMay 26, 2010

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Thermal Expansion: A result of Anharmonicity in Bond Vibrations

http://www.ami.ac.uk/courses/topics/0197_cte/images/ch_cte_imga.gif

• Thermal expansion occurs in most materials because of the anharmonicity of the bond vibrations

•The bond length increases as temperature increases

r r

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Negative Thermal Expansion due to Transverse Vibrations

α ≈ -9x10-6 /K

ZrO6 OctahedraZrW2O8

J. S. O. Evans et al., Chem. Mater., 8 [12], 2809–2823

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Ag3[Co(CN)6]: Structure

A.L. Goodwin et al., Science, 319, 794-797

Cobalt sits in carbon octahedra. Planes of silver sit between the octahedra, and are connected by nitrogen.

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P3 1mRhombahedral

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Ag3[Co(CN)6]: Connectivity

CoNCNC CN

CN

CN

NCAg NC

Bonding along <101> directions

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Ag3[Co(CN)6]: Connectivity

Ag+ ions sit on the vertices of a Kagome lattice

Argentophilicity – weak Ag-Ag interactions, (In case of Au-Au, similar in strength to hydrogen bonds)

AgAg ≈ 3.4Å

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Ag3[Co(CN)6]: Colossal Thermal Expansion/Contraction

130x10-6/K < αa < 150x10-6/K-120x10-6/K < αc < -130x10-6/K

αAg…Co ≈ 0/KA.L. Goodwin et al., Science, 319, 794-797

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Argentophilic Interactions Drive Colossal PTE/NTE

A.L. Goodwin et al., Science, 319, 794-797

A: Lattice enthalpy from density functional calculations ()

B: Mean partial phonon frequencies: CN Δ Co Ag

9A.L. Goodwin et al., J. Am. Chem. Soc., 130 [30], 9660-9661

Argentophilic Interactions Drive Colossal PTE/NTE

10J.L. Korčok, M.J. Katz, D.B. Leznoff, J. Am. Chem. Soc., 131, 4866-4871

Strength of Metallophilic Interactions Affect Thermal Expansion

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• Increases in bond length with increasing temperature is a result of the anharmonicity of bond vibrations

• Transverse vibrations can cause negative thermal expansion• Ag3[Co(CN)6] exhibits colossal positive and negative thermal

expansion– Flexible framework– Ag…Ag interactions

Conclusions