Zero Thermal Expansion Fluid and Oriented Film Based on a Bistable Metal-Cyanide Polymer

A zero thermal expansion (ZTE) material based on plate-shaped rubidium manganese hexacyanoferrate, Rb 0.97 Mn[Fe(CN) 6 ] 0.99 ·0.3H 2 O, is prepared using a polyethylene glycol monolaurate (PEGM) surfactant matrix. The prepared microcrystals show a charge transfer induced phase transition between the cubic Mn II –NC–Fe III and tetragonal Mn III –NC–Fe II phases. The Mn III –NC–Fe II phase exhibits a small negative thermal expansion (NTE) along the a LT and c LT axes with a thermal expansion coefficient of α ( a LT) = −1.40 ± 0.12 × 10 –6 K –1 and α ( c LT) = −0.17 ± 0.13 × 10 –6 K –1 over a wide temperature range of 15 K – 300 K. Such small |α| materials are classified as ZTE materials. The far-infrared spectra show that NTE originates from the transverse modes δ(Fe–C≡N–Mn)of the transverse translational mode around 304 cm –1, and transverse librational modes at 253 and 503 cm –1, which are assigned according to first principle calculations. Molecular orbital calculations indicate that ZTE and the charge transfer phase transition both originate from the transverse mode. Additionally, an oriented film on SiO 2 glass is prepared using a microcrystal dispersive methanol solution and a spin-coating technique. This is the first example of a ZTE film that maintains a constant film thickness over a wide temperature range of 300 K.