Timothy Hughbanks

On the structure and composition of carbon nitride

The role of the nonbonded NN repulsions of the recently proposed “β-C3N4” structure is considered. The importance of these repulsions raises questions about the composition of carbon nitride samples so far prepared, with the suggestion that nitrogen content of crystalline particles in such preparations is substantially below the 57% limit implied by the β-C3N4 formulation. A modification of the proposed structure is offered in which nitrogen centers subject to the greatest crowding in β-C3N4 are replaced by CC bonded dimers.

Low bandgap ladder polymers

Abstract Unique new ladder polymers consisting of condensed succession of six and five membered conjugated carbon rings have been made recently by Scherf and Mullen. Seven topological isomers of these polyindenoindenes are considered theoretically. The results are analyzed in terms of topological bandgaps and geometry relaxation. Three isomers are expected to have a bandgap smaller than 0.2 eV, although the possibility of a ferrimagnetic ground state for these can not be ruled out.

Electronic origin of the thermochromic effect in 2,2',5,5'-tetramethylbistibole

The observed thermochromic effect in 2,2’,5,5’-tetramethylbistibole is explained by the one-dimensional electronic band structure for the material. The experimental spectral red shift is well accounted for by the calculations. An alternative “regular” structure for the material is discussed, and suggestions for stabilizing this alternative are given. Recently Ashe et al. synthesized an air-stable distibine, 2,2’,5,5’-tetramethylbistibole (l), and found it to exhibit a remarkable thermochromic effect.’ Crystals of 1 reflect iridescent

Exploring the metal-rich chemistry of the early transition elements

Abstract The early transition metals, especially Zr, Hf, Hb and Ta, exhibit a metal-rich chemistry that is often surprising in its structural and physical aspects. Unfamiliarity with this chemistry is illustrated by the discovery of several new binary compounds in the Ta-S, Ta-Se, Ta-Te, and Hf-Te systems within the past few years. Some striking differences observed between the metal-rich chalcogenides of Zr and Hf or between Nb and Ta challenge basic presumptions about the similarity of these congeneric pairs. The factors controlling the structural anisotropy of a new class of tetragonal layered compounds that includes Ta 2 Se, Ta 2− x Nb x S, Hf 3 Te 2 , and ZrZTe (Z  Si, Ge, Sn) are discussed. Strongly early-late transition intermetallic bonding leads to the formation of an expanding class of compounds that includes Ta 9 M 2 S 6 (M  Fe, Co, Ni), Ta 11 M 2 Se 8 (M  Fe, Co, Ni), Ta 8 NiSe 8 and the newly discovered hafnium tellurides, Hf 8 MTe 6 (M  Mn, Fe, Co, Ni, Ru) and Hf 5 MTe 3 (M  Fe, Co). Our efforts to dismantle solid-state Zr-halide cluster compounds is described. Ambient temperature molten salts help us achieve the controlled excision of [(Zr 6 Z)CL 18 ] n− from solid state precursors; we describe the applications of electronic and NMR spectroscopies in characterizing clusters in solution. Finally, we discuss bonding in metal-rich systems, with particular emphasis on localized bonding descriptions for metalmetal bonds in extended metal-linked networks. Such localized descriptions increase our understanding of otherwise anomalous properties and illuminate the artificiality of separate “metallic” and “covalent” bonding concepts.

Superdegeneracies in Extended Systems; a Prerequisite for π Ferromagnets?

Abstract Using the concept of “superdegenerate” orbitals in extended systems, we examine the feasibility of organic (π) ferromagnetism. A reinvestigation of Matagas prototype π systems indicates that they would, if synthesized, have ferromagnetic (or ferrimagnetic) insulating ground states. The nonbonding orbitals in these systems are seen to be superdegenerate as we have defined this term in earlier work. Bands in extended systems need not be nonbonding to have superdegenerate character and therefore potential π ferromagnets need not possess half-filled nonbonding bands. This suggests the possibility of synthesizing more stable polymers without radical character but that will be ferromagnetic when heavily doped with donors and/or acceptors. Some tests of these ideas have been carried out using spin-polarized self-consistent field calculations on promising one- and two-dimensional candidates.

Synthesis, structure and bonding of Gd6MTe2 (M=Co, Ni), Er6RuTe2

Abstract Three new rare earth metal-rich compounds, Gd6MTe2 (M=Co, Ni) and Er6RuTe2, were synthesized in direct reactions between the R, R3M, and R2Te3 (R=Gd, Er, M=Co, Ni, Ru). These materials all adopt the same Zr6CoAl2 structure type with space group P 6 2m (No. 189, Z=1). Single crystal structures of Gd6CoTe2 and Er6RuTe2 were determined and lattice parameters are a=b=8.3799(5), c=3.9801(4) A, and a=b=8.1473(5) A, c=3.9962(4) A, respectively. Gd6NiTe2 was characterized by X-ray powder diffraction; lattice parameters are a=b=8.412(2), c=3.9577(9) A. Metal–metal bonding correlations were analyzed using the empirical Pauling bond order concept.

Subtleties of structure and bonding in Ta−S−Se and Ta−Nb−S solid solutions

Substitution of selenium by sulfur and tantalum by niobium into the 1 x [Ta 5 Ta] chains characteristic of Ta 3 S 2 and Ta 2 S is attempted in an effort to understand the structural diversity of a metal-rich chalcogenides. Neither Ta 2 S nor Ta 3 S 2 incorporates a significant amount of selenium, while Ta 2 Se-like structures are found to persist in Ta 2 S 1−x Se x for 0.2≤x≤1.0, a and c progressively increasing with x . The Ta 2 Se-like structure is stable to annealing at temperatures ≥1000°C for 0.5≤ x ≤1.0, and compositions with x ≤0.5 disproportionate to Ta 3 S 2 , Ta 6 S, and Ta 1− x Se 2 on annealing. Both our work and research completed in Franzens laboratories show that niobium substitution into the 1 x [Ta 5 Ta] chains does not occur to any large extent, instead layered Ta 2 Se-like structures ( M 4 S 2 and M 5 S 2 ) are stabilized. At the composition Ta 2− x Nb x S ( x =0.6) as-cast samples are virtually single phase, adopting a Ta 2 Se-like structure. A single crystal structure determination for a crystal with composition Ta 1.4 Nb 0.6 S was carried out: space group P 4/ nmm (No. 129), a =3.339(1), c =9.089(7) A, V =101.33(9) A 3 , Z =2. While electronic structure calculations nicely rationalize the metal-metal bonding in any of these structures, the ability to predict which structures will be stabilized for which systems remains out of reach.

d-Electron mediated 4f7–4f7 exchange in Gd-rich compounds; spin density functional study of Gd2Cl3

Abstract Spin-density functional theory (SDFT) calculations of the d–f exchange coupling for the pseudo-one-dimensional (pseudo-1-D) chain compound Gd2Cl3 has been carried out using the 1-D model, Gd8Cl12(OPH3)4, by considering seven variations in the ordering of the 4 f 7 moments. The calculations indicate that this semiconducting system should exhibit antiferromagnetic ordering of the 4 f 7 moments in a pattern consistent with published neutron diffraction data. An attempt to account for the calculated magnetic energies of spin patterns using an Ising model was unsuccessful, indicating that the latter model is inappropriate. The qualitative features can be interpreted using a perturbative molecular orbital model that focuses on the influence of the 4 f 7 –d exchange interaction on the d-based molecular orbitals. Fundamental to the d-electron-mediated exchange mechanism is the intra-atomic 4 f 7 –d exchange interaction. The essence of this interaction is present in the Gd atom [4 f 7 5d 1 6s 2 ] , which is computationally investigated within SDFT. In Gd2Cl3, the d-electron-mediated f–f exchange interaction was interpreted using basic perturbation theory. Computed density of states and spin polarization information was used to support the perturbation-theoretic analysis.

Hf2.64Zr0.36Te4 and MIxZr3Te4 (MI = Na, K, Rb, Cs), new compounds of the Nb3Te4-structure type

Abstract The synthesis of the pseudo-binary telluride Hf2.64Zr0.36Te4 and new ternary tellurides MIx)Zr3Te4 (MIx = Na, K, Rb, Cs; 0.2 a = 10.892(1) A , c = 3.9084(5) A for Hf 2.64 Zr 0.36 Te 4 , a = 11.009(2) A , c = 3.9374(8) A for Na 0.42 Zr 3 Te 4 ; a = 11.034(2) A , c = 3.9340(6) A for K 0.25 Zr 3 Te 4 ; a = 11.061(1) A , c = 3.9278(5) A for Rb 0.29 Zr 3 Te 4 ; and a = 11.100(1) A , c = 3.9188 (5) A for Cs 0.35 Zr 3 Te 4 . All compounds adopt the Nb3Te4 structure type, which is built up by edge and face sharing of distorted MTe6 (M = Zr, Hf) octahedra. MTe6 octahedra share edges to form zigzag M-M chains running along the c-axis. Distorted MTe6 octahedra in these zigzag metal-metal chains are further condensed by sharing faces. MM bond distances in these tellurides are quite a bit longer (≈0.5 A) than those of Nb3Te4, as expected for compounds with fewer electrons for metal-metal bonding. Large hexagonal channels are partially occupied by alkali cations in the ternary zirconium tellurides. Four-probe single crystal resistivity measurements (77–300 K) show these ternary tellurides to be metallic with resistivities of 1.2 × 10 −3 Ω- cm (Na0.42Zr3Te4), 2.8 × 10 −4 Ω- cm (K0.25Zr3Te4), 4.9 × 10 −4 Ω- cm (Rb0.29Zr3Te4), and 1.7 × 10 −4 Ω- cm (Cs0.35Zr3Te4) at 273 K. Hf2.64Zr0.36Te4 also shows metallic behavior.

Rules for Understanding and Designing Novel Molecule-Based Rare-Earth Magnetic Compounds

Results of SDFT calculations were used to construct and check features of a generally applicable qualitative approach to understanding magnetic coupling in rare-earth-rich compounds. Using fragments based on structures of metal-rich lanthanide compounds, we have investigated molecular and low-dimensional extended structures, including Gd 3 I 6 (OPH 3 ) 12 , Gd 6 I 12 Co(OPH 3 ) 6 , and Gd 2 Cl 3 . Open- d -shell clusters facilitate strong ferromagnetic coupling whereas in the closed- d -shell systems prefer antiferromagnetic coupling. The f - d exchange interaction, mediated by spin polarization of both filled and partially-filled metal-metal bonding orbitals, was described for the model system Gd 3 I 6 (OPH 3 ) 12 n+ using basic perturbation methods. This method has been successful for predicting the magnetic ground state for models of Gd[Gd 6 I 12 Fe] and Gd 2 Cl 3 .