Tuning the hysteresis of a metal-insulator transition via lattice compatibility.

Structural phase transitions serve as the basis for many functional applications including shape memory alloys (SMAs), switches based on metal-insulator transitions (MITs), etc. In such materials, lattice incompatibility between transformed and parent phases often results in a thermal hysteresis, which is intimately tied to degradation of reversibility of the transformation. The non-linear theory of martensite suggests that the hysteresis of a martensitic phase transformation is solely determined by the lattice constants, and the conditions proposed for geometrical compatibility have been successfully applied to minimizing the hysteresis in SMAs. Here, we apply the non-linear theory to a correlated oxide system (V1-xWxO2), and show that the hysteresis of the MIT in the system can be directly tuned by adjusting the lattice constants of the phases. The results underscore the profound influence structural compatibility has on intrinsic electronic properties, and indicate that the theory provides a universal guidance for optimizing phase transforming materials.

Oxidative C(sp3)-H bond cleavage, C-C and C=C coupling at a boron center with O2 as the oxidant mediated by platinum(II).

Dimethyl- and diphenylplatinum(II) fragments Pt(II)R2 (R = Me, Ph) enable facile and efficient oxidative C(sp(3))-H bond cleavage and stepwise C-C and C=C coupling at the boron atom of a coordinated 1,5-cyclooctanediyldi(2-pyridyl)borato ligand with O2 as the sole oxidant.

Superconductivity at 23 K in Pt doped BaFe2As2 single crystals.

We report superconductivity in single crystals of the new iron-pnictide system BaFe(1.90)Pt(0.10)As(2) grown by a self-flux solution method and characterized via x-ray, transport, magnetic and thermodynamic measurements. The magnetic ordering associated with a structural transition at 139 K present in BaFe(2)As(2) is completely suppressed by substitution of 5% Fe with Pt and superconductivity is induced at a critical temperature T(c) = 23 K. Full diamagnetic screening in the magnetic susceptibility and a jump in the specific heat at T(c) confirm the bulk nature of the superconducting phase. All properties of the superconducting state-including the transition temperature T(c), the lower critical field H(c1) = 200 mT, the upper critical field H(c2)≈ 65 T, and the slope ∂H(c2)/∂T-are comparable in value to those found in other transition metal-substituted BaFe(2)As(2) series, indicating the robust nature of superconductivity induced by substitution of Group VIII elements.

Superconducting and ferromagnetic phases induced by lattice distortions in stoichiometric SrFe2As2 single crystals.

Single crystals of SrFe2As2 grown using a self-flux solution method were characterized via x-ray, transport, and magnetization studies, revealing a superconducting phase below Tc=21 K characterized by a full electrical resistivity transition and partial diamagnetic screening. The reversible destruction and reinstatement of this phase by heat treatment and mechanical deformation studies, along with single-crystal x-ray diffraction measurements, indicate that internal crystallographic strain originating from c-axis-oriented planar defects plays a central role in promoting the appearance of superconductivity under ambient-pressure conditions in approximately 90% of as-grown crystals. The appearance of a ferromagnetic moment with magnitude proportional to the tunable superconducting volume fraction suggests that these phenomena are both stabilized by lattice distortion.

Hydrothermal synthesis and characterization of a layered cobalt phenylphosphonate, Co(PhPO3)(H2O).

We report the hydrothermal synthesis and characterization of a layered cobalt phenylphosphonate. Unlike most metal phosphonates reported to date, the structure was solved by single crystal X-ray diffraction (SC-XRD). Co(ii) centres are hexa-coordinated by oxygen and the octahedra corner-share into a layer. The layers are capped by phenylphosphonate groups, where the phenyl groups define a hydrophobic bilayer region. The material was also characterized by powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA) and SQUID (superconducting quantum interference device) magnetometry. The material undergoes an antiferromagnetic transition at a relatively low Néel temperature of 4.0 K, while the Curie-Weiss temperature of -76.5 K reflects the low-dimensionality of the magnetic structure. The effective magnetic moment of 5.01 micro(B) per Co(2+) verifies a high-spin configuration and an octahedral coordination of the metal centres. This layered material was correctly predicted in the literature from powder data, adds to the structural diversity of the cobalt phosphonates, and may be useful as an intercalation or exfoliation compound.

A layered tin(II) phosphonate, [Sn(C6H5O3P)]n.

Poly[tin(II)-mu-phenylphosphonato], [Sn(C(6)H(5)O(3)P)](n), was synthesized solvothermally at 423 K and crystallized in the monoclinic system, space group Cc. The inorganic layers consist of alternating pyramidal Sn and tetrahedral P centers, joined by doubly bridging O atoms. The corner-sharing SnO(3) and PO(3)C(6)H(5) polyhedra define a corrugated layer of six-membered rings. The layers are connected along the unique b axis by interdigitated phenyl rings of the phenylphosphonate groups.

Li(2)Sn(OH)(6).

Hydrothermally prepared dilithium tin hexahydroxide crystallizes in the monoclinic system (space group P2(1)/n), with the Sn atom at a site with -1 symmetry and all other atoms in general positions. The Sn coordination polyhedron is made up of six hydroxide groups. The Li atom is tetrahedrally coordinated by oxygen, with the tetrahedra sharing two corners and one edge with the adjacent Sn octahedra. Hydrogen bonds between the OH groups provide additional bonds in the framework.

Sodium trivanadium(III) bis(sulfate) hexahydroxide.

The title compound, NaV(3)(SO(4))(2)(OH)(6), was synthesized and found to crystallize in the trigonal space group R-3m with the Na atom at a site with 3;m symmetry, the V atom at a site with 2/m symmetry, and the S and a terminal O atom at sites with 3m symmetry; the remaining two O atoms and the H atom are at sites with m symmetry. Octahedrally coordinated vanadium and tetrahedral sulfate ions share corners to form layers that are held together by hydrogen bonds and by sodium ions, which are located between the layers.