Exploration of new superconductors has always been one of the research directions in condensed matter physics. We report here a new layered heterostructure of [(Fe,Al)(OH)2][FeSe]1.2, which is synthesized by the hydrothermal ion-exchange technique. The structure is suggested by a combination of X-ray powder diffraction and the electron diffraction (ED).[(Fe,Al)(OH)2][FeSe]1.2 is composed of the alternating stacking of tetragonal FeSe layer and hexagonal (Fe,Al)(OH)2 layer. In [(Fe,Al)(OH)2][FeSe]1.2, there exists mismatch between the FeSe sub-layer and (Fe,Al)(OH)2 sub-layer, and the lattice of the layered heterostructure is quasi-commensurate. The as-synthesized [(Fe,Al)(OH)2][FeSe]1.2 is non-superconducting due to the Fe vacancies in the FeSe layer. The superconductivity with a Tc of 40 K can be achieved after a lithiation process, which is due to the elimination of the Fe vacancies in the FeSe layer. The Tc is nearly the same as that of (Li,Fe)OHFeSe although the structure of [(Fe,Al)(OH)2][FeSe]1.2 is quite different from that of (Li,Fe)OHFeSe. The new layered heterostructure of [(Fe,Al)(OH)2][FeSe]1.2 contains an iron selenium tetragonal lattice interleaved with a hexagonal metal hydroxide lattice. These results indicate that the superconductivity is very robust for FeSe-based superconductors. It opens a path for exploring superconductivity in iron-base superconductors.
Two kinds of ternary thorium nitride
compounds, ThNF and ThNCl, are synthesized. Via the refinement of
X-ray diffraction patterns, the accurate crystal structure of the
two compounds is solved. Although ThNF and ThNCl share a similar structure
with MNX (M = Ti,
Zr, Hf; X = Cl, Br) compounds, the interaction between
adjacent ThNF and ThNCl layers is not a van der Waals gap. For ThNF,
the strong electronegativity of F ions leads to the bonding of Th
to the F both in the nearest neighbor layer and the next nearest neighbor
layer, which results in the absence of a van der Waals gap between
ThNF layers. However, for ThNCl, the reason for the absence of a van
der Waals gap could be attributed to the large Th–Cl bond length
due to the partially covalent Th–Cl bond as well as the flat
ThN layer. It is the absence of van der Waals gap that results in
the failure of intercalating cations into ThNF and ThNCl. Our result
reveals the reason for unsuccessful intercalation in ThNF and ThNCl,
thereby providing a deeper understanding for the interlayer interaction
in ternary layer structures in metal nitride halides.
The superconductivity of the FeSe-based superconductor is extremely sensitive to Fe vacancies in the FeSe plane. In order to reduce the formation of Fe vacancies in the FeSe plane, various low-temperature methods are developed. However, it is still complex and time consuming to control the Fe vacancies concentration. Here, a hybrid iron-based superconductor, Li0.21Se0.05(EG)0.26FeSe (EG: Ethylene glycol, C2H6O2), is synthesized by a solvothermal ion-exchange technique using a nearly vacancy-free precursor of (TBA)0.3FeSe as the matrix. Bulk superconductivity with transition temperature (Tc) of 30 K is confirmed by the characterization with the magnetic susceptibility and resistivity measurements. Compared with the pristine FeSe single crystal, XPS results show a decrease of the Fe valence, indicating that electron doping to FeSe plane accounts for the increase of the Tc. Our result suggests that (TBA)0.3FeSe, a nearly vacancy-free precursor with a large distance between the adjacent FeSe planes, is a good template candidate for the synthesis of other hybrid iron-based superconductors using the ion exchange technique.
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