  The discovery of ThCr2Si2-type AxFe2-ySe2 (A = K, Rb, Cs and Tl) with Tc ~
30K make much progress in iron-based superconducting field, but their
multiple-phase separations are disadvantageous for understanding the origin. On
the other hand, for small alkali metals, studies on (Li,Na)FeCu(S,Se)2 and
NaFe2-{\delta}S2 shows that these compounds possess CaAl2Si2-type structure,
implying that ThCr2Si2-type structure is unstable for small alkali-metal
intercalated FeSe under high-temperature. Here we report a new intercalate
Na0.65(1)Fe1.93(1)Se2 with Tc ~ 37 K, synthesized by low-temperature
ammonothermal method. The notable finding is that the Na0.65(1)Fe1.93(1)Se2
shows a ThCr2Si2-type structure, which is the first instance of small-sized
alkali metal intercalates without NH3 co-intercalation. Besides, the NH3-poor
Na0.80(4)(NH3)0.60Fe1.86(1)Se2 and NH3-rich phase with Tcs at 45 K and 42 K are
identified by tuning the concentration of Na-NH3 solutions. The modulation of
interlayer spacing reveals the versatile evolution of structural stability and
superconductivity in these intercalates.
