The novel intermetallic phases TbNiMg and Tb4+xNi2Mg3−x (x = 0.2): Synthesis, crystal structure and peculiarities

The novel intermetallic phases TbNiMg and Tb4+xNi2Mg3−x (x = 0.2) were synthesized by heating in a resistance furnace evacuated quartz vials containing Nb-crucibles with element pieces. Scanning electron microscopy coupled with electron microprobe analysis was performed to check microstructure and phases composition. The crystal structures were refined from X-ray single crystal diffraction data. TbNiMg crystallizes in the MoAlB structure type – orthorhombic, Cmcm, oS12, a = 0.36684(4), b = 1.7721(2), c = 0.39855(4) nm, Z = 4, wR2 = 0.0861, 171 F2 values, 14 variables; Tb4+xNi2Mg3−x crystallizes in the Ru4Al3B2 structure type – tetrahedral, P4/mmm, tP18, a = 1.08878(2), c = 0.36290(1) nm, Z = 2, wR2 = 0.0995, 310 F2 values, 24 variables. Interatomic distances in the studied phases indicate metallic type bonding with strong Tb–Ni interactions. Both structures are two-layers. The crystal chemical peculiarities of these compounds are described and compared applying the intergrowth concept on the base of AlB2, CsCl and α-Fe simple fragments. Several two-layer structures, including CrB, ZrSi2, MoAlB and CuGe4Hf2, are described as isomorphic group-subgroup transformation derivatives of the α-U structure by means of the Bärnighausen formalism.