Investigation on photo-induced phenomena by means of SR x-ray powder diffraction

Photo-induced phase transitions (PIPT) have been classified into three cases depending on the duration of transformed states. 1. Permanent PIPT. Photo-irradiation causes a permanently transformed state. 2. Dynamical PIPT. The transformed state is observed only in the duration of photo-irradiation and depends on the excitation power of light. 3. Transient PIPT. The notable response for photo-irradiation is observed only immediately after the irradiation of very short pulsed light. The three PIPTs have been studied by means of synchrotron radiation x-ray powder diffraction at SPring-8 BL02B2. A Co-Fe cyanide film, Na0.42Co[Fe(CN)6]0.78·4.64H2O, shows the permanent PIPT from the low-spin to the high-spin phase by photo-irradiation at 91 K. A transition metal cyanide, RbMn[Fe(CN)6], shows the charge-transfer transition from the nominal Mn2+-Fe3+ configuration to the Mn3+-Fe2+ on at low temperatures. The charge-transfer causes the structural transition from the cubic to the tetragonal phase, and Jahn-Teller distortion of MnN6 octahedra. A spin crossover complex, [Fe(ptz)6](BF4)2 (ptz=1-propyltetrazole), is studied under photo-irradiation at 77 K. When the excitation power exceeds 1.5 mW/mm2, the spin state of the Fe2+ ions changes from the low spin to the high spin state, which thermally relaxes to the low spin state. The restricted thermal vibration is a characteristics of the high spin state caused by photo-irradiation. For the study of the transient PIPT, a time resolved powder x-ray diffraction system with a x-ray pulse selector is installed at a high flux beam line. (Y.K.)