Structural Diversity and Electron Confinement in Li4N: Potential for 0-D, 2-D, and 3-D Electrides.

In pursuit of new lithium-rich phases and potential electrides within the Li-N phase diagram, we explore theoretically the ground-state structures and electronic properties of Li4N at P = 1 atm. Crystal structure exploration methods based on particle swarm optimization and evolutionary algorithms led to 25 distinct structures, including 23 dynamically stable structures, all quite close to each other in energy, but not in detailed structure. Several additional phases were obtained by following the imaginary phonon modes found in low-energy structures, as well as structures constructed to simulate segregation into Li and Li3N. The candidate Li4N structures all contain NLin polyhedra, with n = 6-9. They may be classified into three types, depending on their structural dimensionality: NLin extended polyhedral slabs joined by an elemental Li layer (type a), similar structures, but without the Li layer (type b), and three-dimensionally interconnected NLin polyhedra without any layering (type c). We investigate the electride nature of these structures using the electron localization function and partial charge density around the Fermi level. All of the structures can be characterized as electrides, but they differ in electronic dimensionality. Type-a and type-b structures may be classified as two-dimensional (2-D) electrides, while type-c structures emerge quite varied, as 0-D, 2-D, or 3-D. The calculated structural variety (as well as detailed models for amorphous and liquid Li4N) points to potential amorphous character and likely ionic conductivity in the material.

Ionic N-B-N- and B-N-B-substituted benzene analogues: a theoretical analysis.

Calculations are presented on six-π-electron N-B-N- and B-N-B-substituted benzene rings, [C(3)BN(2)H(6)](+) and [C(3)NB(2)H(6)](-), and their isomers. These compounds display a wide range of thermodynamic stability in those molecules, with N-B-N connectivity favored strongly in the cation, B-N-B in the anion. That stability order is easily understood using the charge distribution in a benzene polarized by heteroatom substitutions or the underlying allyl anion and cation. Deprotonation at N in [C(3)BN(2)H(6)](+) leads to a set of BN-substituted pyridines. The calculations predicted three B-N-substituted pyridines clearly more stable thermodynamically than those synthesized so far. The order of stability of the B-N-B-substituted benzenoid systems, which are as yet not well known experimentally, shows similar features. We investigated in a preliminary way the reactivity and potential stabilization by substitution of the energetically most stable structures and by examining possible escape routes by dimerization. Our study suggests new N-B-N and B-N-B molecules that could be made.

Synthesis, crystal structure and magnetic properties of the new one-dimensional manganate Cs3Mn2O4.

Cs(3)Mn(2)O(4), a new member of the small family of ternary manganese (II/III) mixed-valent compounds, has been synthesized via the azide/nitrate route and studied using powder and single crystal X-ray diffraction, magnetic susceptibility measurements and density functional theory (DFT). Its crystal structure (P2(1)/c, Z = 8, a = 1276.33(1) pm, b = 1082.31(2) pm, c = 1280.29(2) pm, ß = 118.390(2)°) is based on one-dimensional MnO(2)(1.5-) chains built up from edge-sharing MnO(4) tetrahedra. The title compound is the first example of an intrinsically doped transition metalate of the series A(x)MnO(2), (A = alkali metal) where a complete 1:1 charge ordering of Mn(2+) and Mn(3+) is observed along the chains (-Mn(2+)-Mn(3+)-Mn(2+)-Mn(3+)-). From the magnetic point of view it basically consists of ferrimagnetic MnO(2) chains, where the Mn(2+) and Mn(3+) ions are strongly antiferromagnetically coupled up to high temperatures. Very interestingly, their long-range three-dimensional ordering below the Néel temperature (T(N)) ~12 K give rise to conspicuous field dependent magnetic ordering phenomena, for which we propose a consistent picture based on the change from antiferromagnetic to ferromagnetic coupling between the chains. Electronic structure calculations confirm the antiferromagnetic ordering as the ground state for Cs(3)Mn(2)O(4) and ferrimagnetic ordering as its nearly degenerate state.