  We report the effect of Ni doping on superconductivity of FeSe0.5Te0.5. The
single crystal samples of series Fe1-xNixSe0.5Te0.5 (x=0.0, 0.01, 0.03, 0.05,
0.07, 0.10 and 0.20) are synthesized via vacuum shield solid state reaction
route and high temperature heating followed by slow cooling. All the crystals
of Fe1-xNixSe0.5Te0.5 series with x up to 0.20, i.e., 20% substitution of Ni at
Fe site are crystallized in single phase tetragonal structure with space group
P4/nmm. The electrical resistivity measurements revealed that Tc decreases fast
with increase of Ni concentration in Fe1-xNixSe0.5Te0.5. Namely the
superconducting transition temperature (Tc) being defined as resistivity =0
decrease from 12K to around 4K and 2K for x=0.01 and 0.03 samples respectively.
For x=0.05 (5at% Ni at Fe site) though Tconset is observed in resistivity
measurements but \r{ho}=0 is not seen down to 2K. For x more than 0.07, neither
the Tconset nor Tc\r{ho}=0 is seen down 2K in R-T measurements. It is
demonstrated that Ni doping at Fe site in FeSe0.5Te0.5 superconductor
suppresses superconductivity fast. The rate of Tc depression is albeit non
monotonic. Summarily, a systematic study on suppression of superconductivity
with Fe site Ni doping in flux free gown FeSe0.5Te0.5 single crystals is
presented in the current communication.
