New crystal structure of Nd₂Ni₇ formed on the basis of stacking of block layers

The new crystal structure of the intermetallic compound (Nd,Mg)₂(Ni,Al)₇ has been determined by scanning transmission electron microscopy and electron diffraction. This crystal structure is different from that (either 2H or 3R) usually reported for the intermetallic compound R₂T₇ (R: rare-earth elements and T: transition-metal elements) in the constitution of block layers. While the block layer for the crystal structure usually reported is of the R₂T₇ stoichiometric composition consisting of one R₂T₄ unit layer and two RT₅ unit layers, the block layer for the new crystal structure consists of a sub-block layer of the RT₃ stoichiometry (formed with one R₂T₄ unit layer and one RT₅ unit layer) and a sub-block layer of the R₅T₁₉ stoichiometry (formed with one R₂T₄ unit layer and three RT₅ unit layers), which alternately stack on top of each other. The crystal structure is described based on the order-disorder (OD) theory and the simplest crystal structures (polytypes) among many other polytypes belonging to the same OD family is deduced to be of the 2H-type with the space group of P6¯m2 and of the 6R-type with the space group of R3¯m. The experimentally observed polytype corresponds to the second simplest form of the 6R-type.