Structure and superconducting properties of multiple phases of (NH)AEFeSe (AE: Ca, Sr and Ba).

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