MCo1.5Sn (M = Ti, Zr, and Hf) ternary compounds: a class of three-quarter Heusler compounds

Heusler-based compounds are a class of important inorganic materials with potential applications in spintronics, solar cells, thermoelectrics, optoelectronics, and so on. Previous studies mainly focus on half-Heusler compounds or full-Heusler compounds. In this work, a class of MCo1.5Sn (M = Ti, Zr, and Hf) compounds, which are named as three-quarter Heusler compounds, has been synthesized. Their formation mechanism, crystal structure, and physical properties have been systematically investigated. Both the high-angle annular dark field images and X-ray refinement confirm that their crystal structures can be viewed as the ordered half-Heusler sublattice with the four empty tetrahedral holes being disorderedly occupied by two extra Co atoms. Electronic structure calculations and experimental results reveal that these compounds are metallic ferromagnets. The magnetic ordering below the Curie temperature greatly affects the electrical transports due to the scattering by spin fluctuations. The three-quarter Heusler compounds have ultralow lattice thermal conductivities compared with other typical Heusler-based compounds due to the partial occupation of Co at the 4d sites. This work enriches the investigation on the Heusler-based compounds.