Hexagonal close-packed structure twinning

Fig. 2.56 Twin structure of hexagonal close packing projected on (OlO)oh - Twin plane is (101),. Note that a new type of interstice (trigonal prism) appears on the twin plane.

A second group of M3P phases has the cementite FesC stmcture (Table 3) built ft-om distorted trigonal prisms and octahedra of M atoms, the former centered by P as well as M. The complete structure can be derived ft om a hexagonal close-packed M structure by twinning. The Pd4,8P structure is a modified FesC type. 

Recently, Rocha and Zanchet have studied the defects in silver nanoprisms in some detail and have shown that the internal structure can be very complex with many twins and stacking faults [107]. These defects are parallel to each other and the flat 111 face of the nanoprism, subdividing it into lamellae which are stacked in a <111> direction, and are also present in the silver seeds. In that paper, it was demonstrated how the planar defects in the <111> direction could give rise to local hexagonally close-packed (hep) regions. These could in turn explain the 2.50 A lattice fringes that are observed in <111> orientated nanofrisms, which have hitherto been attributed to formally forbidden 1/3(422 reflections as mentioned above. 

Third, the notion (and reality) of such structural infractions as twins and coherent intergrowths - as is seen by Yacaman et al. in a 923-atom nanoalloy of AuPd [35] - is meaningless in our molecular bimetallic nanoparticles. In the nanoaUoys of Yacaman et al. [35, 42] and others [43], one may discern directly, by aberration-corrected electronic microscopy, thin bands of hexagonal close-packed and face-centered cubic packed sheets. In a typical molecular nanoparticle of the kind that we have studied (also by aberration-corrected electron microscopy [39]), it is directly established (in line with theoretical predictions [44]) that a single bimetallic cluster of RUj Pt does indeed possess molecular character. Furthermore, when six or more such clusters coalesce into larger entities containing ca 200 atoms they adopt the regular crystalline, and faceted state of a bulk metal. 

Fig. 438 Mode] for a structure giving rise to the diffraction pattern in Fig. 4.39 (Phoon et al. 1993). (a) Hexagonal ABC structure (not close-packed) (b) top view (d) front view, showing lattice vectors and some of the lattice planes (c) orientation of unit cell vectors (ajb,c) with respect to the flow direction, rotation axis and the direction of the neutron beam (e) the Bragg diffraction pattern from a twinned structure.

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