Electronic Structures and Magnetic Order of Ordered-Fe-Vacancy Ternary Iron Selenides TlFe1.5Se2 and AFe1.5Se2 (A = K, Rb, or Cs)

By the first-principles electronic structure calculations, we find that the ground state of the Fe-vacancies ordered TlFe1.5Se2 is a quasi-two-dimensional collinear antiferromagnetic semiconductor with an energy gap of 94 meV, in agreement with experimental measurements. This antiferromagnetic order is driven by the Se-bridged antiferromagnetic superexchange interactions between Fe moments. Similarly, we find that crystals AFe(1.5)Se(2) (A = K, Rb, or Cs) are also antiferromagnetic semiconductors but with a zero-gap semiconducting state or semimetallic state nearly degenerated with the ground states. Thus, rich physical properties and phase diagrams are expected.