True colours shining through: Determining site distributions in coloured Li-containing quaternary Heusler compounds

The compounds LiMgPdSn, LiMgPtSn, LiMgPdGe, and LiMgCdGe were prepared by induction heating followed by annealing at 600 ​°C. The cubic structure of LiMgPdSn, which serves as the parent structure type for equiatomic quaternary Heusler compounds, was confirmed to contain an ordered site distribution (or “colouring”) by powder X-ray diffraction, 7Li solid-state nuclear magnetic resonance spectroscopy, and electronic structure calculations. The Li atoms occupy one site in LiMgPdSn, LiMgPtSn, and LiMgCdGe, but more than one site in LiMgPdGe. These compounds are rare examples of coloured metallic materials, with LiMgPdSn, LiMgPtSn, and LiMgPdGe appearing red and LiMgCdGe appearing blue. Their colours were quantified by extracting chromaticity coordinates from optical reflectance spectra. As a test of machine-learning models that propose the formation of quaternary Heusler compounds, the new compound LiMgPtZn was prepared, but other predictions were wrong.