Structure and superconducting properties of multiple phases of (NH₃)_y AE_x FeSe (AE: Ca, Sr and Ba)

We synthesized the alkaline-earth metal-doped FeSe compounds (NH3) y AE x FeSe (AE: Ca, Sr and Ba), using the liquid NH3 technique, to determine their superconducting properties and crystal structures. Multiple superconducting phases were obtained in each sample of (NH3) y Ca x FeSe and (NH3) y Ba x FeSe, which showed two superconducting transition temperatures (T c's) as high as 37-39 K and 47-48 K at ambient pressure, hereinafter referred to as the 'low-T c phase' and 'high-T c phase', respectively. The high-T c phases in (NH3) y Ca x FeSe and (NH3) y Ba x FeSe were metastable, and rapidly converted to their low-T c phases. However, T c values of 38.4 K and 35.6 K were recorded for (NH3) y Sr x FeSe, which displayed different behavior than (NH3) y Ca x FeSe and (NH3) y Ba x FeSe. The Le Bail fitting of X-ray diffraction (XRD) patterns provided lattice constants of c = 16.899(1) Å and c = 16.8630(8) Å for the low-T c phases of (NH3) y Ca x FeSe and (NH3) y Ba x FeSe, respectively. The lattice constants of their high-T c phases could not be determined due to the disappearance of the high T c phase within a few days. The XRD pattern for (NH3) y Sr x FeSe indicated the coexistence of two phases with c = 16.899(3) Å and c = 15.895(4) Å. The former value of c in (NH3) y Sr x FeSe is almost the same as those of the low-T c phases in (NH3) y Ca x FeSe and (NH3) y Ba x FeSe. Therefore, the phase with c = 16.899(3) Å in (NH3) y Sr x FeSe must correspond to the superconducting phase with the T c of 38.4 K, while the superconducting phase with T c = 35.6 K is assigned to the crystal phase with c = 15.895(4) Å. For (NH3) y Sr x FeSe, a high-T c phase with T c = 47-48 K has not yet been obtained, but a new phase showing the T c value of 35.6 K was clearly obtained. This is the first systematic study of the preparation, crystal structure, and superconductivity of alkaline-earth metal-doped FeSe, (NH3) y AE x FeSe.