Abstract
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 NaxFe2-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.
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