Synchrotron powder-diffraction study of the spin transition compound [Fe(bpp)2](NCS)2·2H2O and soft X-ray-induced structural phase conversion

We have studied the high spin and low spin structural phases of the two-step spin transition compound [Fe(bpp)2](NCS)2·2H2O (bpp: 2,6-bis(pyrazol-3-yl)pyridine) of different sample history with X-ray synchrotron (λ ∼ 0.32 Å) powder diffraction at different temperatures as well as with usual X-ray (λ ∼ 1.54 Å) powder diffraction at room temperature by performing rapid repeated scans at regular intervals of time. A structural phase transition occurs only during the first step of the spin transition. Time dependent interconversions of the two crystallographic phases have been observed at room temperature for this material. The results indicate that a soft X-ray (Cu-Kα)-induced structural change occurs in this material at room temperature. Possible mechanisms for this unusual observation are discussed.