Turning Negative Thermal Expansion

Negative thermal expansion (NTE, contraction upon heating) is of fundamental scientific interest as a rare material property, and may find applications in precision engineering, compensating for the often troublesome positive thermal expansion of other materials.

NTE has been identified in a broad family of cyanide coordination frameworks,1 including ErCo^III(CN)₆,² a member of the lanthanoid hexacyanometallates (general formula LnM^III(CN)₆: Ln = lanthanoid; M = trivalent transition metal). These materials form as hydrates, and the water can be reversibly removed, leaving a porous framework possessing NTE properties.

Polyhedral representation of the structure of LnM(CN)₆, with cyanide bridges linking the trigonal prismatic lanthanoids and octahedral transition metals in three dimensions.

The mechanism for NTE involves the transverse vibrations of the bridging cyanide ligands^.3 As these vibrational modes become thermally populated during heating, the increasing average transverse displacements of the cyanide bridges draw the metal atoms closer together, corresponding to a unit cell shrinkage.

Two of the low energy transverse cyanide vibrational modes that give rise to NTE by decreasing the metal‑metal distances.

 We can measure NTE through diffraction methods, following the evolution of the unit cell as a function of temperature. The sensitivity of neutrons to light atoms in the presence of such heavy metals is advantageous when the precise locations of cyanide molecules are of interest. Similarly, the strong contrast between the nitrogen and carbon afforded by neutrons help to further resolve the temperature-dependent positions of the cyanide units.

In this study, variable temperature powder diffraction was used to measure the thermal expansion of various other members of this series. Wombat’s exceptionally intense neutron beam and fast detector enabled the collection of high quality powder diffraction patterns in a few minutes rather than hours, making the collection of the large amount of data required for a variable temperature study of a series of samples feasible.

The sample environment gave temperature control between 10 K and 450 K, while sample cans with capillary attachments allowed in situ vacuum dehydration of the materials.

Unit cell volume vs temperature as a percentage of the 100 K volume for LaFe^III(CN)₆, HoFe^III(CN)₆, LuFe^III(CN)₆ and KHoFe^II(CN)₆. Each data point represents a full Rietveld structural refinement.

This work demonstrates tunable NTE, where isostructural frameworks, LaFe^III(CN)₆, HoFe^III(CN)₆ and LuFe^III(CN), display similar NTE behaviour and KHoFe^II(CN) displays positive thermal expansion. The presence of iron(II) rather than iron(III) leads to the inclusion of K+ in the pore centre to maintain charge neutrality. Coordination of these K+ ions to the cyanide nitrogen atoms results in a pronounced framework distortion and effectively hinders the transverse cyanide vibrations.

 
References

1. Kepert, C. J. Chem. Commun. 2006, 695.

2. Pretsch, T.; Chapman, K. W.; Halder, G. J.; Kepert, C. J. Chem. Commun. 2006, 1857.

3. Goodwin, A. L.; Kepert, C. J. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 71, 140301/1.