Guest-dependent negative thermal expansion in nanoporous Prussian Blue analogues M^IIPt^IV(CN)⁶ · x{H ₂O} (0 ≤ X ≤ 2; M = Zn, Cd)

The guest-dependent thermal expansion behavior of the nanoporous Prussian Blue analogues M^IIPt^IV(CN)₆·x{H₂O} (0 ≤ x ≤ 2; M = Zn, Cd) has been investigated using variable temperature single-crystal X-ray diffraction. The dehydrated phases M^IIPt^IV(CN)₆ were found to exhibit negative thermal expansion, attributed to thermal population of low energy transverse vibrations of the bridging cyanide ligands. The presence of guest molecules within the framework pore system was found capable of dampening the effect of these transverse vibrational modes. The guest-loaded ZnII phase, in which the available pore volume is commensurate with the volume occupied by individual water molecules, possesses a considerably higher coefficient of thermal expansion, with the material switching from positive to negative thermal expansion behavior upon guest removal.