  We grew Lix(NH3)yFe2Te1.2Se0.8 single crystals successfully using the
low-temperature ammonothermal method and the onset superconducting transition
temperature Tc,onset is increased to 21 K when compared to 14 K in the parent
compound FeTe0.6Se0.4. The derived critical current density Jc increases
remarkably to 2.6*10^5 A/cm^2 at 2 K. Further analysis indicates that the
dominant pinning mechanism in Lix(NH3)yFe2Te1.2Se0.8 single crystal is the
interaction between vortex and surface-like defects with normal core, possibly
originating from the stacking faults along the c axis, by variations in the
charge-carrier mean free path l near the defects (delta l pinning). Moreover,
the flux creep is important to the vortex dynamics of this material.
