  We describe the crystal structure and elementary magnetic properties of a
previously unreported ternary intermetallic compound, Cr4PtGa17, which
crystallizes in a rhombohedral unit cell in the noncentrosymmetric space group
R3m. The crystal structure is closely related to those of XYZ half-Heusler
compounds, where X, Y and Z are reported to be single elements only, occupying
three different face-centered cubic sublattices. The new material, Cr4PtGa17,
can be most straightforwardly illustrated by writing the formula as
(PtGa2)(Cr4Ga14)Ga (X=PtGa2, Y = Cr4Ga14, Z = Ga), that is, the X and Y sites
are occupied by clusters instead of single elements. The magnetic Cr occupies a
breathing pyrochlore lattice. Ferromagnetic ordering is found below TC ~61 K,
by both neutron diffraction and magnetometer studies, with a small, saturated
moment of ~0.25 muB/Cr observed at 2 K, making Cr4PtGa17 the first
ferromagnetically ordered material with a breathing pyrochlore lattice. A
magnetoresistance of ~140% was observed at 2 K. DFT calculations suggest that
the material has a nearly-half-metallic electronic structure. The new material,
Cr4PtGa17, the first realization of both a half-Heusler-type structure and a
breathing pyrochlore lattice, might pave a new way to achieve novel types of
half-Heusler compounds.
