Structural, electronic and magnetic properties of TlFeSe2under high pressure.

The high-pressure (HP) properties of TlFeSe2are investigated based on the first-principles calculations combined with structure-searching method. The low-pressureC2/mphase will transform into the orthorhombicPnmaphase at 2 GPa, with 8% volume collapse, the insulator-metal transition and the bicollinear antiferromagnetic-to-nonmagnetic spin-crossover. At pressure higher than 8 GPa, the HPC2/mphase will become the ground state. BothPnmaphase and HPC2/mphase are constituted by one-dimensional chains of edge-sharing FeSe5tetragonal pyramids. Pressuring decrease the Se-Se bond length giving rise to the transition from [Se2]3-to [Se2]2-. Negative charge transfer causes the Fe2+with ∼2 μBmagnetic moment at ambient pressure and the nonmagnetic Fe1.5+at higher pressure. The Fermi surfaces of HP phases are also discussed.