Exotic magnetism on the quasi-FCC lattices of the d3d^3 double perovskites La2_2NaB′'O6_6 (B′' == Ru, Os)

We find evidence for long-range and short-range ($\zeta$ $=$ 70 \AA~at 4 K) incommensurate magnetic order on the quasi-face-centered-cubic (FCC) lattices of the monoclinic double perovskites La$_2$NaRuO$_6$ and La$_2$NaOsO$_6$ respectively. Incommensurate magnetic order on the FCC lattice has not been predicted by mean field theory, but may arise via a delicate balance of inequivalent nearest neighbour and next nearest neighbour exchange interactions. In the Ru system with long-range order, inelastic neutron scattering also reveals a spin gap $\Delta$ $\sim$ 2.75 meV. Magnetic anisotropy is generally minimized in the more familiar octahedrally-coordinated $3d^3$ systems, so the large gap observed for La$_2$NaRuO$_6$ may result from the significantly enhanced value of spin-orbit coupling in this $4d^3$ material.Comment: 5 pages, 4 figure

Hydroflux Crystal Growth of Platinum Group Metal Hydroxides: Sr₆NaPd₂(OH)₁₇, Li₂Pt(OH)₆, Na₂Pt(OH)₆, Sr₂Pt(OH)₈, and Ba₂Pt(OH)₈

Crystals of five complex metal hydroxides containing platinum group metals were grown by a novel low-temperature hydroflux technique, a hybrid approach between the aqueous hydrothermal and the molten hydroxide flux techniques. Sr₆NaPd₂(OH)₁₇ (<b>1</b>) crystallizes in orthorhombic space group <i>Pbcn</i> with lattice parameters <i>a</i> = 19.577(4) Å, <i>b</i> = 13.521(3) Å, and <i>c</i> = 6.885(1) Å. This compound has a three-dimensional framework structure with Sr­(OH)_<i>n</i> polyhedra, Na­(OH)₆ octahedra, and Pd­(OH)₄ square planes. Isostructural phases Li₂Pt­(OH)₆ (<b>2</b>) and Na₂Pt­(OH)₆ (<b>3</b>) crystallize in trigonal space group <i>P</i>-3 with lattice parameters of <i>a</i> = 5.3406(8) Å and <i>c</i> = 4.5684(9) Å and <i>a</i> = 5.7984(8) Å and <i>c</i> = 4.6755(9) Å, respectively. Structures of these materials consist of layers of A­(OH)₆ (A = Li (<b>2</b>), Na (<b>3</b>)) and Pt­(OH)₆ octahedra. Sr₂Pt­(OH)₈ (<b>4</b>) crystallizes in monoclinic space group <i>P</i>2₁/<i>c</i> with lattice parameters <i>a</i> = 5.9717(6) Å, <i>b</i> = 10.997(1) Å, <i>c</i> = 6.0158(6) Å, and β = 113.155(2)°, while Ba₂Pt­(OH)₈ (<b>5</b>) crystallizes in orthorhombic space group <i>Pbca</i> with lattice parameters <i>a</i> = 8.574(2) Å, <i>b</i> = 8.673(2) Å, and <i>c</i> = 10.276(2) Å. Both of these compounds have three-dimensional structures composed of Pt­(OH)₆ octahedra surrounded by either Sr­(OH)₈ or Ba­(OH)₉ polyhedra. Decomposition of these materials into condensed metal oxides, which is of importance to possible catalytic applications, was monitored via thermogravimetric analysis. For example, Na₂Pt­(OH)₆ (<b>3</b>) converts cleanly via dehydration into α-Na₂PtO₃

Metallic Borides, La₂Re₃B₇ and La₃Re₂B₅, Featuring Extensive Boron–Boron Bonding

La₂Re₃B₇ and La₃Re₂B₅ have been synthesized in single-crystalline form from a molten La/Ni eutectic at 1000 °C in the first example of the flux crystal growth of ternary rare-earth rhenium borides. Both compounds crystallize in their own orthorhombic structure types, with La₂Re₃B₇ (space group <i>Pcca</i>) having lattice parameters <i>a</i> = 7.657(2) Å, <i>b</i> = 6.755(1) Å, and <i>c</i> = 11.617(2) Å, and La₃Re₂B₅ (space group <i>Pmma</i>) having lattice parameters <i>a</i> = 10.809(2) Å, <i>b</i> = 5.287(1) Å, and <i>c</i> = 5.747(1) Å. The compounds possess three-dimensional framework structures that are built up from rhenium boride polyhedra and boron–boron bonding. La₃Re₂B₅ features fairly common B₂ dumbbells, whereas La₂Re₃B₇ has unique one-dimensional subunits composed of alternating triangular B₃ and trans-B₄ zigzag chain fragments. Also observed in La₃Re₂B₅ is an unusual coordination of B by an octahedron of La atoms. Electronic band structure calculations predict that La₂Re₃B₇ is a semimetal, which is observed in the electrical resistivity data as measured on single crystals, with behavior obeying the Bloch–Grüneisen model and a room-temperature resistivity ρ_300 K of ∼375 μΩ cm. The electronic band structure calculations also suggest that La₃Re₂B₅ is a regular metal