Intercallation of Li$_{1-x}$Fe$_{x}$O$_{2}$ in the superconducting FeSe

The intercallation of LiFeO${_2}$ in the tetragonal lattice of the 8 K superconductor Fe$_{1-y}$Se leads to a great enhancement of the superconducting transition temperature, T$_{C}$ $\sim$ 43 K, and to an antiferromagnetic transition at 8.5 K. While the LiFeO${_2}$ layer acts as a charge reservoir, its Fe$^{3+}$ ion (3d$^{5}$) is magnetic creating a magnetic buffer layer. Most recently, we developed a new synthesis method to control the Fe concentration in the intercallating layer as well as the filling ratio of the Li$_{1-x}$Fe$_{x}$O$_{2}$ : FeSe layers. Neutron scattering measurements were carried out on powder samples of (Li$_{1-x}$Fe$_{x}$O$_{2}$)${_y}$FeSe. With the intercallation, no crystal structural transition from the P4/nmm symmetry occurs but the c-axis lattice constant expands substantially, evidence of the intercallation. At the same time, the tetrahedral FeSe layers remain intact with no compression or expansion and free of vacancies. Moreover, the intercallation along the c-axis although not uniform leads to a reduction in T${_C}$ when the ratio of Li${_1-x}$Fe${_x}$O$_{2}$ : FeSe layers is about 1 to 3. Our results also indicate that the amount of Fe in the Li${_1-x}$Fe${_x}$O$_{2}$ layer has a direct correlation to the transition temperature as well.