Crystal Structure, Electrical Transport, and Magnetic Properties of Niobium Monophosphide J. Xu, M. Greenblatt,* T. Emge, and P. Ho1 hn Department of Chemistry, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08855-0939 T. Hughbanks* and Y. Tian Department of Chemistry, Texas A & M University, College Station, Texas 77843-3255 ReceiVed July 7, 1995 X Large single crystals of NbP have been prepared. A single-crystal X-ray diffraction study shows that it crystallizes in tetragonal symmetry with space group I4 1 md (No. 109) and lattice parameters a ) 3.3324(2) Å, c ) 11.3705- (7) Å, and Z ) 4. A full matrix least-squares refinement based on a unique data set of 285 reflections (I > 2σ(I)) yielded R(F) ) 0.017 and R w (F 2 ) ) 0.046 for nine variables. The unit cell consists of one unique Nb and one P, each in trigonal prismatic coordination with the other element. There are two short and four long bond distances of Nb-P. The Nb-Nb bond distances are significantly shorter than R c ) 4.09 Å, the critical distance required for good Nb-Nb 4d orbital overlap for niobium metal-metal bonds. NbP shows metallic behavior with F) 4.5 × 10 -5 Ω cm at room temperature. Magnetic susceptibility measurements on a collection of randomly oriented single crystals indicate very weak Pauli paramagnetism (∼10 -5 emu/mol). A discussion of the structure as well as the physical properties of NbP compared with those of previous results are presented. The band structure of NbP based on the extended Hu¨ckel (tight-binding) calculations is presented along with an analysis that reveals that the valence band is built up from three center bonds localized within Nb 3 triangles. Introduction A number of investigators have prepared polycrystalline niobium monophosphide by solid state reaction methods and investigated its properties by chemical analysis, powder X-ray diffraction, NMR, magnetic susceptibility, and electrical trans- port measurements. The structure of NbP has been studied: first by Scho¨nberg 1 in 1954, followed by Boller and Parthe, 2 Furuse and Kjekshus, 3 and finally Willerstrom in 1984. 4 On the basis of powder X-ray diffraction analysis, it appears that NbP is isostructural with NbAs. 2 The electrical resistivity as a function of temperature reported by Ripley 5 indicated that NbP was a metallic conductor with room temperature resistivity ∼10 -2 -10 -3 Ω cm. However, magnetic susceptibility reported by Scott et al. 6 showed that NbP was diamagnetic with a small orbital Knight shift, which suggested a near zero band gap. In order to clarify some of the controversy in the previously reported data, we had three major objectives: (1) to carry out a single-crystal X-ray structure analysis to determine accurate bond distances and angles (2) to obtain oriented single-crystal resistivity measurements on NbP, since prior resistivity mea- surements were made on polycrystalline pellets, 5 and (3) to clarify the relationship between structure and electrical transport and magnetic properties, as well as to construct a schematic band diagram. Experimental Section Sample Preparation. During an investigation of the La-Nb-P-O system, shiny, silver single crystals of unknown composition were obtained as a result of a chemical vapor transport reaction. The synthetic procedure was performed in two steps. A reaction mixture containing La 2(C2O4)3‚10H2O (Alfa, 99.99%), Nb2O5 (Alfa, 99.5%), and (NH 4)2HPO4 (Fisher, 99.7%) in a molar ratio of 1.5:2.3:4 was ground in an agate mortar and heated at 600 °C in a porcelain crucible for 4 h to decompose La 2(C2O4)3‚10H2O and (NH4)2HPO4 and to remove CO 2,H2O, and NH3. The decomposition temperature of the mixture was determined by TGA. The resulting powder was then mixed with the required amount of niobium metal powder (Johnson Matthey Electronics, 99.8%) to achieve a composition of La 3Nb6P4O26. NH4Cl (5 mol%) was added as chemical transport agent. The mixture was pelletized and sealed in an evacuated (∼10 -6 Torr) quartz tube. The quartz tube was placed horizontally inside a muffle furnace and heated at 1250 °C for a week, and then it was slowly cooled (5 °C/h) to 900 °C, followed by 50 °C/h to 500 °C and then quenched to room temperature. La 2O3 white powder was observed in the hot end, while shiny silver crystals, subsequently determined to be NbP, deposited in the cool end of the quartz tube. Single-Crystal X-ray Diffraction Data. A single crystal of NbP with dimensions 0.040 × 0.076 × 0.078 mm 3 was mounted ap- proximately along the a axis. The data were collected on an Enraf- Nonius CAD4 diffractometer with graphite monochromatized Mo KR radiation at room temperature. The 1179 intensity data points were measured up to θ ) 45° with ω scans. The unit cell parameters were determined by the least-squares fit of 25 peaks with 20.0 < 2θ e 33.1°. Of the 306 independent reflections, 285 had I > 2σ(I). The intensity of three standard reflections varied less than 1% during the data collection. The data were corrected for Lorentz effects (polarization and extinction); absorption corrections were performed based on a ψ-scan (MOLEN). 7 The cell parameters and detailed setting of data collection are given in Table 1. Determination and Refinement of the Structure. Shelxs-86 8 (direct methods) and Shelxl-93 9 packages were used for the crystal structure solution and refinement, respectively. The general reflection * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, January 15, 1996. (1) Scho¨nberg, N. Acta Chem. Scand. 1954, 8, 226. (2) Boller, H.; Parthe, H. Acta Crystallogr. 1963, 16, 1095. (3) Furuseth, S.; Kjekshus, A. Acta Crystallogr. 1964, 17, 1077. (4) Willerstrom, J.-O. J. Less-Common Met. 1984, 99, 273. (5) Ripley, R. L. J. Less-Common Met. 1962, 4, 496. (6) Scott, B. A.; Eulenberger, G. R.; Bernheim, R. A. J. Chem. Phys. 1968, 48, 1, 263. (7) Fair, C. K. MOLEN, Enraf-Nonius, Delft Instruments X-Ray Diffrac- tion B. V., Rontgenweg 1, 2624BD Delft, The Netherlands, 1990. 845 Inorg. Chem. 1996, 35, 845-849 0020-1669/96/1335-0845$12.00/0 © 1996 American Chemical Society