Spin and orbital quantum numbers play a key role in the physics of Mott insulators, but in most systems they are connected only indirectly -via the Pauli exclusion principle and the Coulomb interaction. Iridium-based oxides (iridates) introduce strong spin-orbit coupling directly, such that the Mott physics has a strong orbital character.In the layered honeycomb iridates this is thought to generate highly spin-anisotropic magnetic interactions, coupling the spin orientation to a given spatial direction of exchange and leading to strongly frustrated magnetism. Here we report a new iridate structure that has the same local connectivity as the layered honeycomb and exhibits striking evidence for highly spin-anisotropic exchange. The basic structural units of this material suggest that a new family of three-dimensional structures could exist, the 'harmonic honeycomb' iridates, of which the present compound is the first example.

The Cu(I)-tantalate, Cu 2 Ta 4 O 11 , has been synthesized by flux methods in high purity and characterized by singlecrystal X-ray diffraction (space group R3̅ c (167), a = 6.219(2) Å, c = 37.107(1) Å). The compound is a new n = 1 member of the Cu(I)-tantalate Cu x Ta 3n+1 O 8n+3 series of structures and can be prepared in a molten CuCl flux within a relatively low temperature range of ∼625−700 °C, in comparison to the synthesis of Cu 5 Ta 11 O 30 (n = 1.5) and Cu 3 Ta 7 O 19 (n = 2) at ∼800 to 1000 °C. The structure consists of layers of TaO 7 pentagonal bipyramids that alternate with layers of isolated TaO 6 octahedra and linearly coordinated Cu(I) cations. An increasing Cu-site vacancy across this series from Cu 3 Ta 7 O 19 (100%), to Cu 5 Ta 11 O 30 (83.3%), to Cu 2 Ta 4 O 11 (66.7%) leads to an increasing fraction of O atoms that are not locally charge balanced by the Ta(V)/Cu(I) cations and thus yields decreased stability of Cu 2 Ta 4 O 11 . Thermal analysis shows that Cu 2 Ta 4 O 11 decomposes in air or under flowing nitrogen at temperatures above ∼550 °C (in the absence of the CuCl flux) into a mixture of known tantalates and Cu(II)tantalate phases. The compound exhibits a bandgap size of ∼2.55 eV (indirect), with higher-energy direct transitions starting at ∼2.73 eV. Electronic structure calculations confirm the indirect nature of the lowest-energy bandgap transition, which arises from valence and conduction band states that are primarily composed of Cu 3d 10 and Ta 5d 0 orbital contributions, respectively.

A new low-temperature polymorph of the copper(I)-tantalate, α-Cu 2 Ta 4 O 11 , has been synthesized in a molten CuCl-flux reaction at 665 o C for 1 h and characterized by powder X-ray diffraction Rietveld refinements (space group (#9), a = 10.734(1) Å, b = 6.2506(3) Å, c = 12.887(1) Å, β = 106.070(4) o). Th e α-Cu 2 Ta 4 O 11 phase is a lower-symmetry monoclinic polymorph of the rhombohedral Cu 2 Ta 4 O 11 structure (i.e., β-Cu 2 Ta 4 O 11 space group 3 � (#167), a = 6.2190(2) Å, c = 37.107(1) Å), and related crystallographically by a hex = a mono /√3, b hex = b mono , and c hex = 3c mono sinβ mono. Its structure is similar to the rhombohedral β-Cu 2 Ta 4 O 11 and is composed of single layers of highly-distorted and edge-shared TaO 7 and TaO 6 polyhedra alternating with layers of nearly linearly-coordinated Cu(I) cations and isolated TaO 6 octahedra. Temperature dependent powder X-ray diffraction data show the α-Cu 2 Ta 4 O 11 phase is relatively stable under vacuum at 223 K and 298 K, but reversibly transforms to β-Cu 2 Ta 4 O 11 by at least 523 K and higher temperatures. The symmetry-lowering distortions from β-Cu 2 Ta 4 O 11 to α-Cu 2 Ta 4 O 11 arise from the out-of-center displacements of the Ta 5d 0 cations in the TaO 7 pentagonal bipyramids. The UV-Vis diffuse reflectance spectrum of the monoclinic α-Cu 2 Ta 4 O 11 shows an indirect bandgap transition of ~2.6 eV, with the higher-energy direct

Because the natural mechanical strength of materials is highly dependent on the crystal structure, four different silicate-derived ceramicsdiopside, akermanite, monticellite, and merwinitehave been synthesized and evaluated for their potential as bone augments and grafts. This sparks our interest in the fabrication of polycaprolactone (PCL)/ceramic composites for potential use as scaffolds. Diopside, which possesses the most three-dimensional structure among the four, shows the highest mechanical strength and stable structure in physiological solution when added to polycaprolactone in high concentration. In turn, the incorporation of merwinite into composite scaffolds led to materials that had poor mechanical strength and were unstable in physiological environments when ceramic concentration reaches over 50%. Cyto-compatibility and osteogenic studies of the four ceramics revealed that each ceramic is cytocompatible and supports human adipose derived stem cells (hASCs) proliferation in stromal medium. Akermanite and monticellite exhibit better osteogenic properties than diopside and merwinite, suggesting that they might be the optimal material for fabricating bone scaffolds.