Ag 3 [Co(CN) 6 ]: Giant Positive and Negative Thermal Expansion

1. Ag3[Co(CN)6]: Giant Positive and Negative Thermal Expansion Elizabeth A. Clark Materials 286G May 26, 2010

2. Thermal Expansion: A result of Anharmonicity in Bond Vibrations • Thermal expansion occurs in most materials because of the anharmonicity of the bond vibrations • The bond length increases as temperature increases r r http://www.ami.ac.uk/courses/topics/0197_cte/images/ch_cte_imga.gif

3. Negative Thermal Expansion due to Transverse Vibrations ZrO6Octahedra ZrW2O8 α ≈ -9x10-6 /K J. S. O. Evans et al., Chem. Mater., 8 [12], 2809–2823

4. Ag3[Co(CN)6]: Structure Rhombahedral Cobalt sits in carbonoctahedra. Planes of silver sit between the octahedra, and are connected by nitrogen. A.L. Goodwin et al., Science, 319, 794-797

5. Ag3[Co(CN)6]: Connectivity NC CN Ag Co CN NC NC NC CN Bonding along <101> directions

6. 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) AgAg ≈ 3.4Å

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

8. Argentophilic Interactions Drive Colossal PTE/NTE A: Lattice enthalpy from density functional calculations () B: Mean partial phonon frequencies:  CN ΔCo  Ag A.L. Goodwin et al., Science, 319, 794-797

9. Argentophilic Interactions Drive Colossal PTE/NTE A.L. Goodwin et al., J. Am. Chem. Soc., 130 [30], 9660-9661

10. Strength of Metallophilic Interactions Affect Thermal Expansion J.L. Korčok, M.J. Katz, D.B. Leznoff, J. Am. Chem. Soc., 131, 4866-4871

11. Conclusions • 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