Negative Thermal Expansion in LnCo(CN)6 (Ln=La, Pr, Sm, Ho, Lu, Y): Mechanisms and Compositional Trends


  • This research was supported by Australian Research Council grants DP120101445 and DP0985611 and fellowships FT100100514 and FF0561456. We acknowledge travel funding provided by the International Synchrotron Access Program (ISAP) managed by the Australian Synchrotron and funded by the Australian Government. S.G.D. acknowledges a postgraduate scholarship funded by the Australian Government and the Australian Institute of Nuclear Science and Engineering. We thank G. J. Halder, K. W. Chapman, P. D. Southon, D. J. Price, Y. Wu, and E. A. Carter for support in experimental work.


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RUM with a twist: The magnitude of negative thermal expansion (NTE) in the LnCo(CN)6 coordination frameworks increases with Ln ion radius rLn. The framework structure contains an unusual locally unstable trigonal prismatic LnN6 unit that participates in an NTE-contributing vibrational mode by twisting about its axis at low energies. This contrasts with the rigid unit modes (RUMs) prevalent in other systems.