We have studied the electronic structure of a class of half-Heusler compounds M NiPn, where M is Y, La, Lu, Yb, and Pn is a pnicogen As, Sb, Bi. All these systems excepting Yb are narrow-gap semiconductors and are potential candidates for high-performance thermoelectric materials. The Yb system shows heavy fermion characteristics. Calculations were carried out within density-functional theory ͑generalized gradient approximation͒ using self-consistent full-potential linearized augmented plane-wave method. Comparison of the electronic structures of isoelectronic systems YNiSb and ZrNiSn, another narrow-gap semiconductor, brings out the role of hybridization on the energy gap formation. We also find that in YNiPn systems, the gap narrows as we go from As to Bi, a result of relativistic lowering of the Pn valence s band and its influence on the lowest conduction band. Our band-structure results for YbNiSb differs drastically from a previous calculation using a different method, but agrees closely with a similar mixed valence system YbPtBi. ͓S0163-1829͑99͒03324-X͔

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Two ternary silicides, MNiSi3 (M = Sm, Y), have been synthesized from Sm, Ni, and Si in molten Ga at 850 °C in sealed silica tubes. Both compounds form black shiny crystals and are stable even in aqua regia. The structures, determined by single-crystal X-ray diffraction, are orthorhombic, Cmmm (No. 65) with Z = 4, and have lattice parameters a = 3.965(2) Å, b = 21.144(2) Å, c = 4.007(1) Å for M = Sm and a = 3.930(2) Å, b = 21.021(2) Å, c = 3.960(1) Å for M = Y, respectively. Refinement based on F o 2 yielded R 1 = 0.0319 and wR 2 = 0.0712 [I > 2 σ(I)] for M = Sm and R 1 = 0.0267 and wR 2 = 0.0688 [I > 2σ(I)] for M = Y. The compounds adopt the SmNiGe3 structure type with zigzag Si chains and Si dimers and exhibit metallic p-type electrical conductivity. Variable temperature magnetic susceptibility data suggest that Sm is 3+ and Ni has no magnetic moment. SmNiSi3 has an antiferromagnetic transition at 12 K and follows the modified Curie−Weiss law above 12 K. Band structure calculations using density functional theory, generalized gradient approximation, full potential LAPW method, and also extended Hückel tight binding theory show that the materials are metallic and suggest that Ni is either neutral or in a reduced oxidation state. Additional insight into the bonding was obtained by extended Hückel calculations carried out on the [NiSi3]3- framework under the assumption that Y is mostly 3+. These results suggest that the Si zigzag chain contains single bonds with a partial double bond character.