Jiangang Guo

Effects of Co and Mn doping in K0.8Fe2-ySe2 revisited.

Accumulated evidence indicates that phase separation occurs in potassium intercalated iron selenides, a superconducting phase coexisting with the antiferromagnetic phase K2Fe4Se5, the so-called 245 phase. Here, we report a comparative study of substitution effects by Co and Mn for Fe sites in K0.8Fe2-ySe2 within the phase separation scenario. Our results demonstrate that Co and Mn dopants have distinct differences in occupancy and hence in the suppression mechanism of superconductivity upon doping of Fe sites. In K0.8Fe2-xCoxSe2, Co prefers to occupy the lattice of the superconducting phase and suppresses superconductivity very quickly, obeying the magnetic pair-breaking mechanism or the collapse of the Fermi surface nesting mechanism. In contrast, in K0.8Fe1.7-xMnxSe2, Mn shows no preferential occupancy in the superconducting phase or the 245 phase. The suppression of superconductivity can be attributed to restraining of the superconducting phase and meanwhile inducing another non-superconducting phase by Mn doping.

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 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.

Crystal structures of carbonates Cs2Sr2(CO3)3 and Rb2Sr2(CO3)3 from powder data

Two carbonates Cs2Sr7(CO3)(3) and Rb2Sr2(CO3)(3) were synthesized by solid-state reaction at high temperatures, and their crystal structures were determined from X-ray powder diffraction data. Because of the presence of heavy atoms and heavy peak overlapping, proper restrains on light-atom groups were found essential to obtain satisfactory results. The title compounds are isostructural to an early reported compound Cs2Ba7(CO3)(3) and crystallize in cubic space group I2(1)3, with unit-cell dimensions a=-10.072 64(2) angstrom for the cesium phase and a=9.855 56(7) angstrom for the rubidium phase. Unlike most carbonates, the title compounds feature in mutually perpendicular CO3 atomic groups. Our results also indicate that the global instability index is a more sensitive parameter in evaluating the structure rationality over the agreement factors. Moreover, differential thermal analysis and thermogravimetric analysis measurements reveal that Cs2Sr7(CO3)(3) and Rb2Sr2(CO3)(3) are stable in air up to 796 and 880 degrees C, respectively