Article pubs.acs.org/IC
Hybridization Gap in the Semiconducting Compound SrIr4In2Ge4 Nicholas P. Calta,†,‡ Jino Im,§ Lei Fang,†,∥ Thomas C. Chasapis,† Daniel E. Bugaris,∥ Duck Young Chung,∥ Wai-Kwong Kwok,∥ and Mercouri G. Kanatzidis*,†,∥ †
Department of Chemistry, Northwestern University, Evanston, Illinois, United States Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, California, United States § Center for Molecular Modeling and Simulation, Chemical Infrastructure Division, Korea Research Institute of Chemical Technology, Daejeon, Ulsan, South Korea ∥ Materials Science Division, Argonne National Laboratory, Lemont, Illinois, United States ‡
S Supporting Information *
ABSTRACT: Large single crystals of SrIr4In2Ge4 were synthesized using the In flux method. This compound is a hybridization gap semiconductor with an experimental optical band gap of Eg = 0.25(3) eV. It crystallizes in the tetragonal EuIr4In2Ge4 structure type with space group I42̅ m and unit cell parameters a = 6.9004(5) Å and c = 8.7120(9) Å. The electronic structure is very similar to both EuIr4In2Ge4 and the parent structure Ca3Ir4Ge4, suggesting that these compounds comprise a new family of hybridization gap materials that exhibit indirect gap, semiconducting behavior at a valence electron count of 60 per formula unit, similar to the Heusler alloys.
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INTRODUCTION Most intermetallic compounds exhibit metallic electronic structures and therefore metallic behavior. However, a small number of intermetallic compounds exhibit semiconducting behavior and are broadly referred to as hybridization gap materials. Hybridization gap compounds, while rare, are relatively diverse in terms of chemical composition, crystal structure, and physical properties. Their band gaps range from roughly 0.11,2 to ∼0.5 eV,3 and their compositions range from relatively simple binary phases such as FeSi1 and TiSi24 to more complex ternary half Heusler alloys5 as well as quaternary compounds such as EuIr4In2Ge4.6 In addition to the range of band gaps and compositions, some hybridization gap materials exhibit exceptionally high Seebeck coefficients,7 which makes them attractive options for thermoelectric applications.8 Hybridization gap compounds present a fundamental challenge, as the mechanism for band gap formation varies from material to material and is not always well-understood. The simplest mechanism for hybridization gap formation is strong metal−metal bonding, in which the orbitals near the Fermi level are involved in bonds strong enough to form a true band gap with zero density of states rather than the more typical pseudogap observed in most intermetallic compounds.9 Such bonding is often a feature of the structure type, and many hybridization gap materials that exhibit this type of gap fall into well-defined families, where for a given structure type a hybridization gap is always present.10 At a specific electron count the Fermi level falls inside the gap, leading to semiconducting behavior.11 The half Heusler alloys are the largest and most well-known family of hybridization gap © XXXX American Chemical Society
materials that exhibit this type of gap formation mechanism. At an electron count of 18 valence electrons per formula unit, any intermetallic that adopts the half Heusler structure behaves as a semiconductor.5 Other families of hybridization gap materials also behave this way, including compounds with the TiSi2 structure type4 at a valence electron count of 14 such as TiSi2, RuAl2,12 and RuGa2.13 FeGa3 and RuGa3 comprise a similar small family of hybridization gap materials that are semiconducting at a valence electron count of 17.14 In this Article, we report the synthesis and properties of SrIr4In2Ge4, a new hybridization gap compound that is isostructural and isoelectronic with the recently reported EuIr4In2Ge4.6 The existence and physical properties of SrIr4In2Ge4, in conjunction with the electronic structure of its parent Ca3Ir4Ge4 compound,15 indicate that these are the first members of a new family of hybridization gap compounds whose band gaps arise as a consequence of strong Ir−Ir bonding in a tetrahedral Ir4 cluster, analogous to the mechanism of gap formation in the half Heusler alloys.
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EXPERIMENTAL DETAILS
Synthesis. SrIr4In2Ge4 was synthesized using the In flux technique, which yielded large single crystals (Figure 1a). The best synthesis method used a 1 mmol scale. Ir powder (1 mmol, 0.1922 g; 99.95%, American Elements), 1 mmol of Ge pieces manually crushed to coarse powder (0.726 g; 99.999%, Plasmaterials), and 30 mmol of In teardrops (3.445 g; 99.99%, Plasmaterials) were added to an alumina Received: October 27, 2016
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DOI: 10.1021/acs.inorgchem.6b02617 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
presented in Table 1, with atomic positions and displacement parameters listed in Table 2.
Table 1. Single Crystal X-ray Diffraction Refinement Details for SrIr4In2Ge4 formula weight, Z wavelength, temperature crystal system space group unit cell dimensions volume density F(000) crystal size θ range for data collection index ranges reflections collected, independent completeness to θ = 36.36° refinement method data/restraints/parameters goodness-of-fit final Ra indices [>2σ(I)] R indices [all data] extinction coefficient largest diff. peak and hole
Figure 1. Crystal structure of SrIr4In2Ge4. (a) Photograph of a crystal and residual In flux stuck to the stainless steel filter immediately after centrifuging. The filter is 12 mm in diameter. (b) The structure viewed approximately down the a axis and (c) the c axis with Ir−Ir bonds highlighted in orange. (d) View down the a axis with Ir4 tetrahedra highlighted and only the Ir−Ge 3D network shown.
crucible with an inert cement base outside a glovebox. The crucible was then transferred inside a nitrogen glovebox to prevent oxidation of the air-sensitive Sr metal. In the glovebox, Sr pieces (1 mmol, 0.088 g; 99%, Aldrich) were added to the crucible. This crucible was placed inside a fused silica tube. A 100 mesh stainless steel filter and small alumina counterweight was placed on top of the crucible. The fused silica tube was evacuated to
