Network dimensionalities and thermal expansion properties of metal nitroprussides

Abstract

We investigated the thermal expansion properties of metal nitroprussides, M_A[Fe(CN)₅NO]·zH₂O (M_A = Mn, Fe, Co, Ni, Cu, Zn, and Cd), which have various network structures, i.e. three-dimensional (3D) cubic (Fmm for M_A = Fe, Co, and Ni), distorted three-dimensional (distorted-3D) orthorhombic (Pnma for M_A = Mn, Zn, and Cd), and two-dimensional (2D) orthorhombic (Amm2 for M_A = Cu) structures. In the cubic system, in which the unbridged NO groups are randomly distributed, isotropic positive thermal expansion was observed. On the contrary, in the orthorhombic system, in which the unbridged NO groups are uniaxially distributed, positive (negative) thermal expansion was observed along (perpendicular to) the uniaxial direction. We interpreted the characteristic thermal responses in terms of the rotational vibrations of the [Fe(CN)₅NO] units as well as their steric hindrances due to the network dimensionality.