Periodic density functional theory study of spin crossover in the cesium iron hexacyanochromate prussian blue analog

Abstract

This paper presents a detailed theoretical analysis of the electronic structure of the CsFe [Cr (CN) 6] prussian blue analog with emphasis on the structural origin of the experimentally observed spin crossover transition in this material. Periodic density functional calculations using generalized gradient approximation (GGA) +U and nonlocal hybrid exchange-correlation potentials show that, for the experimental low temperature crystal structure, the t 2g 6 eg0 low spin configuration of FeII is the most stable and CrIII (S=3/2, t 2g 3 eg0) remains the same in all cases. This is also found to be the case for the low spin GGA+U fully relaxed structure with the optimized unit cell. A completely different situation emerges when calculations are carried out using the experimental high temperature structure. Here, GGA+U and hybrid density functional theory calculations consistently predict that the t 2g 4 eg2 FeII high spin configuration is the ground state. However, the two spin configurations appear to be nearly degenerate when calculations are carried out for the geometries arising from a GGA+U full relaxation of the atomic structure carried out at experimental high temperature lattice constant. A detailed analysis of the energy difference between the two spin configurations as a function of the lattice constant strongly suggests that the observed spin crossover transition has a structural origin with non-negligible entropic contributions of the high spin state. © 2009 American Institute of Physics.