Complex structures of different CaFe 2 As 2 samples

Abstract

The interplay between magnetism and crystal structures in three CaFe 2 As 2 samples is studied. For the nonmagnetic quenched crystals, different crystalline domains with varying lattice parameters are found, and three phases (orthorhombic, tetragonal, and collapsed tetragonal) coexist between T S = 95a €...K and 45a €...K. Annealing of the quenched crystals at 350°C leads to a strain relief through a large (∼1.3%) expansion of the c-parameter and a small (∼0.2%) contraction of the a-parameter, and to local ∼0.2a €...Å displacements at the atomic-level. This annealing procedure results in the most homogeneous crystals for which the antiferromagnetic and orthorhombic phase transitions occur at T N /T S = 168(1) K. In the 700°C-annealed crystal, an intermediate strain regime takes place, with tetragonal and orthorhombic structural phases coexisting between 80 to 120a €...K. The origin of such strong shifts in the transition temperatures are tied to structural parameters. Importantly, with annealing, an increase in the Fe-As length leads to more localized Fe electrons and higher local magnetic moments on Fe ions. Synergistic contribution of other structural parameters, including a decrease in the Fe-Fe distance, and a dramatic increase of the c-parameter, which enhances the Fermi surface nesting in CaFe 2 As 2, are also discussed.