On the structure of the hexagonal μ-Al4Cr and its relation to the monoclinic η-Al11Cr2

The crystal structure of the hexagonal μ-Al4Cr was determined by single-crystal X-ray diffraction analysis: P63/mmc (No. 194), a = 2.01911(15) nm and c = 2.48540(19) nm. Final R = 0.0563 for 4336 reflections with Fo > 4σ(Fo) and 0.0852 for all 6163 reflections, goodness of fit S = 1.068. There are 42 (32 Al + 10 Cr) independent atomic positions in the unit-cell, of which 9 Cr atom sites and 3 Al atom sites are icosahedrally coordinated. Two independent layer blocks of μ-Al4Cr crystal structure, (P1′F1P1) and (P2F2P2m), are reexamined by analyzing characteristic configurations of icosahedral clusters, where F and P denote flat and puckered atomic layers respectively, and superscript ′ and m indicate a 21 rotation and a mirror operation separately. Icosahedral clusters 3I3 (3 mutually penetrated I3, I3: three vertex-connected icosahedra), I6 (6 face-sharing icosahedra) in (P1′F1P1) and 4I3 (4 face-sharing I3) in (P2F2P2m) of the hexagonal μ-Al4Cr are found to coexist within the unique layer block (PFP′) of the monoclinic η-Al11Cr2. In addition, it is revealed that periodically extracting sections along 〈100〉 directions in the (P1′F1P1)μ would lead to the structure of (PFP')η. An orientation relationship is thus suggested as 〈100〉μ//[101¯]η and [001]μ//[101]η.