Unprecedented Site Preference of Sn in the Structure of CoSn-Type NiIn_1-Sn (x < 0.7)

A series of compositions NiIn_1-Sn (x = 0-1) were synthesized by conventional high-temperature synthesis, and as-synthesized samples were checked by powder X-ray diffraction experiments. NiIn_1-Sn (x < 0.7) mainly forms the ternary variant of the CoSn-type structure (P6/mmm), whereas, x = 0.7-0.9, NiIn_1-Sn forms predominantly an orthorhombic (Pnma) phase. To resolve the accurate crystal structure of hexagonal NiIn_1-Sn, a combination of single-crystal X-ray diffraction and neutron powder diffraction techniques is employed. It is revealed that in the crystal structure of NiIn_1-Sn (x < 0.7), the Sn gradually substitutes one of the two In sites present in the NiIn (CoSn-type) and forms the pseudobinary NiIn_1-Sn (x < 0.7). These experimental findings on the specific site substitution are supplemented by first-principles density functional theory (DFT) calculations. Mulliken's and Löwdin's population and Bader charge analyses further support the unexpected substitution of Sn for one In site in the structure of hexagonal NiIn_1-Sn. The structure can be viewed as an alternating arrangement of two flat atomic layers in the [001]: Kagomé layer of Ni atoms with indium at the center of the hexagons and honeycomb nets of In/Sn. Density of state (DOS) calculations, crystal orbital Hamilton population (COHP) calculations, and crystal orbital bond index (COBI) calculations further elucidate the stability and bonding scenario in the hexagonal phases. Interestingly, gradual Sn inclusion into the CoSn-type NiIn downshifts the 3d band center from the Fermi level that could influence the catalytic performance as well as many intriguing properties. These findings not only contribute to the fundamental understanding of atomic ordering between neighboring elements in NiIn_1-Sn but also pave the way for designing fascinating physical and chemical properties.