A spin state transition in a square planar coordinated iron was found in SrFeO2 and Sr3Fe2O5 under high pressure. This transition occurs as a consequence of enhanced Fe-Fe direct interactions between adjacent face-to-face FeO4 square planar units. We also found a pressure-induced structural transition in Sr3Fe2O5, which is viewed as a B1（NaCl structure） to B2（CsCl structure） structural transition. Such a B1-B2 structural transition generally occurs in intergrowth system consisting of rock-salt blocks and square-planar blocks, as found in A2MO3 （A＝Ca, Sr; M＝Cu, Pd）. An empirical relation between the structural transition pressure Ps and the ionic radius for the binary system holds well for the intergrowth structure, which indicates that the square-planar block is highly compressible along the out-of-plane direction.
We have determined the crystal structure at 0.48 Å resolution of high-potential iron-sulfur protein, HiPIP, which is a small soluble protein playing as an electron carrier in photosynthetic bacteria. The ultra-high resolution structure of HiPIP enabled us to perform charge-density analyses where distributions of valence electrons were clearly visualized as the first case of metalloproteins. A topological analysis of the charge density provided electronic structure information for the iron-sulfur （Fe4S4） cluster as well as the peptide portion.
In-situ synchrotron diffraction experiments were conducted in order to clarify the formation process of α'-martensite from the γ-phase induced by external strain, combined with Lorentz transmission electron microscopy （TEM） and high-resolution TEM observations. Lorentz TEM observation revealed that α'-martensite exist near the defect structures such as dislocations and stacking faults in the parent γ-phase. In addition, it is clearly demonstrated that ε-martensite with hexagonal symmetry appears as an intermediate phase during the plastic deformation of SUS304 stainless steel. Our experimental results suggested that the interfaces between twin structures of the γ-phase presumably play a crucial role in the formation of ε-martensite.