Structure Evolution and Spin-Glass Transition of Layered Compounds ALiFeSe₂ (A = Na, K, Rb)

Three new layered compounds, namely NaLiFeSe₂, KLiFeSe₂, and RbLiFeSe₂, have been discovered. NaLiFeSe₂ adopts a trigonal CaAl₂Si₂-type structure with space group P3̅m1, while the other two possess a tetragonal ThCr₂Si₂-type structure with space group I4/mmm. Structural refinements reveal that Li and Fe atoms randomly occupy the same sites in all these compounds without ordering. It is found that the radius of the alkali metals plays a vital role in determining the symmetry of this series of compounds. The substitution of Li at the Fe site shortens the layer spacing and elongates the A-Se bond length in the ThCr₂Si₂-type structure. The elongated Na-Se bond length would destabilize the ThCr₂Si₂-type structure in NaLiFeSe₂, suggesting that Na_xFe_2-ySe₂ lies at the border of ThCr₂Si₂-type and CaAl₂Si₂-type structures. Magnetic and resistivity measurements demonstrate that these compounds exhibit anisotropic spin-glass and narrow-band-gap semiconducting characteristics. First-principles calculations indicate that the introduction of Li enhances strong localization and weakens the correlation of the 3d electrons of Fe, which are responsible for the observed spin-glass transition and semiconducting conductions.