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

We synthesized the alkaline-earth metal-doped FeSe compounds (NH₃)_yAE_xFeSe (AE: Ca, Sr and Ba), using the liquid NH₃technique, to determine their superconducting properties and crystal structures. Multiple superconducting phases were obtained in each sample of (NH₃)_yCa_xFeSe and (NH₃)_yBa_xFeSe, 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_cphase' and 'high-T_cphase', respectively. The high-T_cphases in (NH₃)_yCa_xFeSe and (NH₃)_yBa_xFeSe were metastable, and rapidly converted to their low-T_cphases. However,T_cvalues of 38.4 K and 35.6 K were recorded for (NH₃)_ySr_xFeSe, which displayed different behavior than (NH₃)_yCa_xFeSe and (NH₃)_yBa_xFeSe. The Le Bail fitting of x-ray diffraction (XRD) patterns provided lattice constants ofc= 16.899(1) Å andc= 16.8630(8) Å for the low-T_cphases of (NH₃)_yCa_xFeSe and (NH₃)_yBa_xFeSe, respectively. The lattice constants of their high-T_cphases could not be determined due to the disappearance of the highT_cphase within a few days. The XRD pattern for (NH₃)_ySr_xFeSe indicated the coexistence of two phases withc= 16.899(3) Å andc= 15.895(4) Å. The former value ofcin (NH₃)_ySr_xFeSe is almost the same as those of the low-T_cphases in (NH₃)_yCa_xFeSe and (NH₃)_yBa_xFeSe. Therefore, the phase withc= 16.899(3) Å in (NH₃)_ySr_xFeSe must correspond to the superconducting phase with theT_cof 38.4 K, while the superconducting phase withT_c= 35.6 K is assigned to the crystal phase withc= 15.895(4) Å. For (NH₃)_ySr_xFeSe, a high-T_cphase withT_c= 47-48 K has not yet been obtained, but a new phase showing theT_cvalue 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)_yAE_xFeSe.