Crystal Growth and Characterization of the Narrow-Band-Gap Semiconductors OsPn 2 (Pn = P, As, Sb) Daniel E. Bugaris, † Christos D. Malliakas, †,‡ Daniel P. Shoemaker, † Dat T. Do, § Duck Young Chung, † Subhendra D. Mahanti, § and Mercouri G. Kanatzidis* ,†,‡ † Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States ‡ Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States § Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, United States * S Supporting Information ABSTRACT: Using metal fluxes, crystals of the binary osmium dipnictides OsPn 2 (Pn = P, As, Sb) have been grown for the first time. Single-crystal X-ray diffraction confirms that these compounds crystallize in the marcasite structure type with orthorhombic space group Pnnm. The structure is a three- dimensional framework of corner- and edge-sharing OsPn 6 octahedra, as well as [Pn 2 4- ] anions. Raman spectroscopy shows the presence of P-P single bonds, consistent with the presence of [Pn 2 -4 ] anions and formally Os 4+ cations. Optical-band-gap and high-temperature electrical resistivity measure- ments indicate that these materials are narrow-band-gap semiconductors. The experimentally determined Seebeck coefficients reveal that nominally undoped OsP 2 and OsSb 2 are n-type semiconductors, whereas OsAs 2 is p-type. Electronic band structure using density functional theory calculations shows that these compounds are indirect narrow-band-gap semiconductors. The bonding p orbitals associated with the Pn 2 dimer are below the Fermi energy, and the corresponding antibonding states are above, consistent with a Pn-Pn single bond. Thermopower calculations using Boltzmann transport theory and constant relaxation time approximation show that these materials are potentially good thermoelectrics, in agreement with experiment. ■ INTRODUCTION Interest in transition-metal pnictides has revived with the discovery of high-temperature superconductivity in iron arsenides. 1 Although superconductivity has mostly been limited to ternary and quaternary transition-metal pnictides, the binary phases have been studied extensively as well, albeit for different reasons. One particular class of binary transition-metal pnictides that has drawn significant attention is the transition- metal dipnictides, MPn 2 (M = transition metal; Pn = P, As, Sb), many of which crystallize in the marcasite-type structure. Named for the mineral marcasite (FeS 2 ), this structure type 2 encompasses a wide array of transition-metal pnictides and chalcogenides. The marcasite-type compound FeSb 2 has been shown to be a rare example of a highly correlated diamagnetic narrow-band-gap semiconductor and/or a Kondo insulator (similar to FeSi). 3 Because of its colossal thermoelectric power factor (∼2300 μWK -2 cm -1 at 12 K and ∼8000 μWK -2 cm -1 at 28 K), 4 FeSb 2 has been investigated as a cryogenic thermoelectric material. However, its thermoelectric figure of merit (ZT) remains limited by the relatively high thermal conductivity. Various strategies have been pursued to improve the magnetotransport properties of FeSb 2 , including doping with cobalt, 5 chromium, 6 arsenic, 7 tin, 8 and tellurium, 9 as well as spark-plasma sintering of nanoparticles, 10 but none have yet resulted in a significant improvement in ZT. In contrast to the considerable effort spent on studying FeSb 2 and to a lesser extent its lighter group V analogues, FeP 2 (a possible electrode for lithium batteries) 11 and FeAs 2 (a thermoelectric material), 12 there has been minimal inves- tigation of the isostructural ruthenium and osmium pnictide marcasites. Although the syntheses of RuP 2 , OsP 2 , and RuAs 2 were first reported in the 1930s, 13 the unit cell lattice parameters and marcasite crystal structure for these compounds and the arsenide/antimonide variants were not determined by powder X-ray diffraction until the 1960s 14 and were confirmed by powder neutron diffraction in the 1970s. 15 Measurements revealed all six compounds to be diamagnetic, small-band-gap semiconductors below room temperature. 16 Herein we report a more detailed investigation of the marcasite-type OsPn 2 (Pn = P, As, Sb) binary compounds. To the best of our knowledge, this is the first instance where these materials have been grown as single crystals, which has been accomplished via the use of metal fluxes. The marcasite-type crystal structure previously determined by powder X-ray and neutron diffraction has been confirmed via single-crystal X-ray diffraction. The presence of a P-P single bond in OsP 2 is observed with Raman spectroscopy. This is consistent with the Received: July 18, 2014 Published: August 27, 2014 Article pubs.acs.org/IC © 2014 American Chemical Society 9959 dx.doi.org/10.1021/ic501733z | Inorg. Chem. 2014, 53, 9959-9968