Pressure Effect Studies on the High Spin <=>Low Spin Transition Behavior Observed in [Fe(II)(bpp)2](NCS)2.2H2O

摘要

Background: [Fe(bpp)2](NCS)2.2H2O exhibits a two-step spin transition between the ~5T2 state (HS, 5 = 2) and the iAx 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 γ_(HS) = 0.25. As the applied pressure increased from ambient, γ_(HS) 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 γ_(HS) 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 Mossbauer 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.