Layered double hydroxides (LDHs) are two-dimensional materials composed of positively charged metal hydroxide layers separated by charge-balancing anions. Given their cationic nature, LDH layers demonstrate great potential to intercalate anionic layers of iron-based superconductors [FeS]^δ- and [FeSe]^δ-. Through this intercalation chemistry, we can potentially enhance the superconducting critical temperature T_c. We present systematic studies to understand how variations in metal composition within the LDH layers and the Fe:Se ratio in the intercalated anionic layers affect the resulting superconducting properties. Through the hydrothermal synthesis, we prepare compounds with the general formula [M²⁺_1-xM³⁺_x(OH)₂][Fe_1-ySe_y], where M²⁺ = Mg, Mn, Fe and M³⁺ = Al, Fe. A range of divalent and trivalent metals can be incorporated into the LDH host layer, so one can tune the lateral dimensions of the unit cell and the overall cationic charge. Interestingly, while the LDH layer is based on a hexagonal brucite-type structure, that of the superconducting FeSe (or FeS) layers is based on a tetragonal layer. Hence, these heterostructured compounds can be viewed as misfit structures caused by the distortions necessary to fit a hexagonal-based lattice with a square-based one. We elucidate the structure through powder X-ray diffraction, electron diffraction, and transmission electron microscopy, and develop structure-property relationships by physical property measurements. Most importantly, we characterize the magnetic and superconducting properties via magnetic susceptibility measurements. We present the results of our multiple hydrothermal syntheses and formulate design rules by which to tailor the composition and properties of our heterostructured superconductors.

keke Lu

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