THE INTERMETALLIC COMPOUND Mn8Ga27Zn14: SEPARATED Zn13-CLUSTERS INTERSPERSED IN A PRIMITIVE CUBIC SUPERLATTICE
U. Häussermann1, S. Lidin1, P. Viklund2, C. Svensson2, S. Eriksson3
1Department of Inorganic Chemistry,
Stockholm University, Sweden
2Department of Inorganic Chemistry
2, Lund University, Sweden
3Studsvik Neutron Research
Laboratory, Uppsala University, Sweden
The new ternary compound Mn8Ga27Zn14 has been discovered in the gallium-rich corner of the phase diagram gallium-manganese-zinc.
Crystals were obtained from a gallium-zinc melt prepared in a evacuated and sealed quartz tube heated to 800oC for 24 hours and then slowly cooled.
The rhombohedral structure was determined by single crystal X-ray diffraction methods (space group R3_, a= 13.6029(8) A , c = 14.604(2) A, Z = 3) and represents a ternary variant of the binary V8Ga41 type [1].
The structure of V8Ga41 can be built up from the simple PtHg4 type. PtHg4 corresponds to a defect CsCl structure where 3/4 of the Cs atoms has been removed in such a way that an array of corner connected cubes [PtHg8] is formed\cite{sten}. The final V8Ga41 structure is obtained by replacing 1/9 of the transition metal atoms with centered cuboctahedra Ga13 (see Figure). A primitive cubic lattice is a perfect host for cuboctahedral units where square antiprims of flexible size serve as an interface and thus also allow some flexibility in the size of the cuboctahedra.
In the ternary compound Mn8Ga27Zn14 these centered cuboctahedra are very regular and are formed by Zn atoms, which thus represent small pieces of fcc metal. This subtle ordering tendency was confirmed by neutron powder diffraction experiments and reflects the different bonding behaviour of Ga and Zn, namely the tendency of Ga to form homonuclear Ga-Ga and heteronuclear T-Ga bonds and the tendency of Zn to segregate into more metallic bonded entities.