Universal thermal response of the Prussian blue lattice

Thermal responses of Prussian blue lattice, that is, a rocksalt-type network of metal ions bridged by cyano group, are systematically investigated for Cs,Rb xM FeIII CN 6 yzH2O M=Ni, Cu, Zn, and Cd and Cs,Rb xM FeII CN 6 yzH2O M=Fe and Co by means of the synchrotron-radiation x-ray powder diffraction. We found a universal relation between the coefficient d ln a /dT; a is lattice constant of the thermal expansion and magnitude of a: changes the sign from positive to negative with increase in a. We ascribed the negative thermal expansion observed in the large-a region to intrinsic rotational instability of the Fe CN 6 unit.