The orthorhombic (Al₁₁mn₄)-pd decagonal approximant

We present the results of a transmission electron microscopy study of an Al-Mn-Pd orthorhombic crystalline approximant to the decagonal phase that is isostructural to Al₁₁Mn₄. Selected area diffraction (SAD) and convergent beam electron diffraction (CBED) are discussed. The stereographic projection of the CBED Kikuchi bands is presented. While this phase exhibits a symmetry similar to the decagonal phase in the same alloy, the decagonal symmetry is broken by a splitting of some prominent Kikuchi bands. The intensity modulations of the SAD patterns strongly resemble those found in the 12 Å periodic Al-Mn and Al-Mn-Pd decagonal phases. These observations suggest the existence of clusters of atoms with a distorted icosahedral symmetry within the crystallographic unit cells. Most importantly, these distortions are consistent with the cluster distortions present in the Al-Mn and Al-Mn-Pd decagonal phases.