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

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)e(g)(0) low spin configuration of Fe-II is the most stable and Cr-III (S=3/2, t(2g)(3)e(g)(0)) 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)e(g)(2) Fe-II 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.