Background: [Fe(bpp)2](NCS)2.2H2O exhibits a two-step spin transition between the 5T2
state (HS, S = 2) and the 1A1 state (LS, S = 0) below room temperature, both steps being associated
with thermal hysteresis. The present study explored the nature of spin transition (ST) in
[Fe(bpp)2](NCS)2.2H2O under pressure to determine if ST could be induced to go to completion and
whether a two-step process was then involved.
Methods: Magnetic measurements were performed on polycrystalline samples of [Fe(bpp)2]
(NCS)2.2H2O in the temperature range of 100 - 300 K under different applied pressures up to 0.5 GPa.
Results: [Fe(II)(bpp)2](NCS)2.2H2O under ambient pressure exhibited a two-step HS LS transition
with two thermal hysteresis loops while the second loop appears at γHS = 0.25. As the applied pressure
increased from ambient, γHS decreased, the high temperature thermal hysteresis gradually diminished
while the low temperature thermal hysteresis shifted to higher temperatures and vanished at 0.5 GPa
applied pressure. The γHS values estimated at 300 K during cooling and heating protocols followed an
exponential decay equation.
Conclusion: [Fe(bpp)2](NCS)2.2H2O at room temperature may have four high spin Fe(II) ions out of
which three are having nearly identical coordination environment, while the remaining rests in a very
rigid environment. Application of 0.5 GPa pressure on the compound below 250 K converts all the HS
states to LS state. Temperature dependent structural and Mössbauer spectroscopic studies may identify
the proposed crystallographically independent Fe(II) HS sites in [Fe(bpp)2](NCS)2.2H2O at higher
temperatures, and hence unearth the anomalous spin transition exhibited by this compound.