THE CRYSTAL-STRUCTURE OF A NEW CRYSTALLINE PHASE IN THE AL-PD-CR ALLOY SYSTEM, STUDIED BY HIGH-RESOLUTION ELECTRON-MICROSCOPY

The structure of a new crystalline phase (O-phase) in an Al72Pd18Cr10 alloys, which is closely related to an Al3Mn crystalline phase and the Al-Pd-Mn decagonal quasicrystal, was investigated by electron diffraction and high-resolution electron microscopy. The O-phase has an orthorhombic structure with the lattice parameters a = 1.25 nm, b = 3.28 nm and c = 2.38 nm, and the space group is Pmmm (no. 59). This structure is characterized as a periodic tiling constructed with two types of tiles, a squashed hexagon and a star-shaped pentagon, formed with pentagonal atom columns which are structural units in the Al3Mn crystalline phase and in Al-Pd-Mn decagonal quasicrystal. Structural relationships among the O-phase, the decagonal quasicrystal and some crystalline phases are discussed.