Cause for the Orbital Ordering of Cs2AgF4 and Its Effect on Thermoelectric Properties.

The cause for the orbital-ordered structure of orthorhombic Cs2AgF4 and its effect on thermoelectric properties were probed by density functional theory calculations. The orbital-ordered structure of orthorhombic Cs2AgF4 has been regarded as caused by a cooperative Jahn-Teller (JT) distortion of the AgF6 octahedra of tetragonal Cs2AgF4. However, each AgF6 octahedron of tetragonal Cs2AgF4 is axially compressed and hence has no JT instability. The orbital-ordering transition of Cs2AgF4 is best described as a metal-to-insulator transition driven by a bandgap opening at the Fermi level. The orbital-ordering opens a bandgap at the Fermi level and generates peaks in the density of states at the conduction band minimum (CBM) and valence band maximum (VBM). Consequently, orthorhombic Cs2AgF4 has strongly enhanced Seebeck coefficients as compared to tetragonal Cs2AgF4.