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.

Molten Ga is an excellent solvent for the preparation of the polygallide Sm2NiGa12. The compound is metallic and adopts an unusual three-dimensional structure composed of two different kinds of alternating segments. One segment has Ni and Ga atoms and is reminiscent of the KCu4S3 structure type. The other has only Ga atoms and adopts an elaborate Ga network.

The four new intermetallic aluminum silicides RE 4 Fe 2+x Al 7-x Si 8 (RE ) Ce, Pr, Nd, Sm) crystallize from the reaction of Si, Fe, and RE (or rare earth oxides) in molten Al at 850 °C. All compounds share the same structure type as determined by single-crystal X-ray diffraction analysis. They form in the space group Cmmm (No. 65) with cell constants of a ) 10.909(2) Å, b ) 16.265(3) Å, c ) 4.0804(8) Å, R1 ) 0.0196, and wR2 ) 0.0486 for the Sm analogue. The crystal structure is a complex three-dimensional network comprised of repeating layers containing Al, Si, and Fe connected by atoms between the layers. The RE 3+ ions are then located within tunnels of the three-dimensional network, running parallel to the c axis with a coordination number of 14. Magnetic susceptibility measurements indicate that the rare earth ions are in a 3+ oxidation state, whereas the Fe atoms are in an effective diamagnetic state. Electronic band structure calculations, carried out on the hypothetical analogue Y 4 Fe 2 Al 7 Si 8 , predict metallic behavior and suggest Fe to be in a reduced state with almost filled d orbitals. Variable temperature electrical conductivity and thermopower measurements confirm the metallic nature of the compounds. The charge transport and magnetic properties of the Ce analogue are anomalous and indicative of f 1/0 valence fluctuations at T < 100 K.

The RNiSb compounds (R=Ho, Er, Tm, Yb and Y) and some selected solid solution members such as (Zr1-xErx)Ni(Sn1-xSbx) and ErNiSb1-xPnx (Pn=As, Sb, Bi) have been studied. They all crystallize in the MgAgAs structure type, which can be considered as a NaCI structure type in which half of the interstitial tetrahedral sites are occupied by Ni atoms. The measured values of the Seebeck coefficients, at room temperature, are positive for RNiSb (R=Ho, Er, Yb and Y) compounds and ErNiSb1-xPnx (Pn=As, Sb, Bi) solid solutions, but for (Zr1-xErx)Ni(Sn1-xSbx) members vary from negative to positive values when 0 < x < 1. Some of these compounds show metallic conductivity while others exhibit thermally activated charge transport. Solid solutions of these materials have lower thermal conductivities than the pure members, RNiSb (R=Ho, Er, Yb and Y) and ZrNiSn. The electronic structures of RNiSb compounds, where R is Y, La, Lu, and Yb, have been studied with density functional theory. The results of the calculations for these systems, except for the Yb compound, indicate narrow gap semiconductors with large effective masses near the conduction band extrema. The Yb system is expected to show heavy fermion characteristics.

There is considerable current effort to discover new thermoelectric materials with a high figure of merit Z. Some of these new materials are narrow-gap semiconductors with rather complex crystal structures. In this paper we discuss the results of electronic structure calculations in two classes of such systems. The first class consists of BaBiTe3, a structural and chemical derivative of the well-studied Bi2Te3. Similarities and differences in the band structures of these two systems are discussed. The second class consists of half-Heusler or “stuffed”-NaCl compounds MNiX, where M is Y, La, Lu, Yb, and X is a pnictogen; As, Sb, Bi. To understand the physical reason behind the energy gap formation, we compare the electronic structure of YNiSb with that of an isoelectronic system ZrNiSn, another isostructural compound of thermoelectric interest. These calculations were carried out within density functional theory (in generalized gradient approximation) using self-consistent full-potential LAPW method. Energy gaps and effective masses associated with the conduction band minimum and valence band maximum have been calculated and these quantities have been used to estimate transport properties. Large room temperature thermopower values in Bi2Te3 and BaBiTe3 can be understood in terms of multiple conduction and valence band extrema whereas similar large values in ZrNiSn and other half-Heusler compounds can be ascribed to large electron and hole effective mass.

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