Spin chirality on a two-dimensional frustrated lattice.

The collective behaviour of interacting magnetic moments can be strongly influenced by the topology of the underlying lattice. In geometrically frustrated spin systems, interesting chiral correlations may develop that are related to the spin arrangement on triangular plaquettes. We report a study of the spin chirality on a two-dimensional geometrically frustrated lattice. Our new chemical synthesis methods allow us to produce large single-crystal samples of KFe3(OH)6(SO4)2, an ideal Kagomé lattice antiferromagnet. Combined thermodynamic and neutron scattering measurements reveal that the phase transition to the ordered ground-state is unusual. At low temperatures, application of a magnetic field induces a transition between states with different non-trivial spin-textures.

Spin frustration in 2D kagomé lattices: a problem for inorganic synthetic chemistry.

A kagomé antiferromagnet presents an ideal construct for studying the unusual physics that result from the placement of magnetically frustrated spins on a low-dimensional lattice. Jarosites are the prototype for a spin-frustrated magnetic structure, because these materials are composed exclusively of kagomé layers. Notwithstanding, jarosite-type materials have escaped precise magnetic characterization over the past three decades, because they are notoriously difficult to prepare in pure and single-crystal forms. These hurdles have been overcome with the development of redox-based hydrothermal methods. Armed with pure and crystalline materials, several perplexing issues surrounding the magnetic properties of the jarosites have been resolved, yielding a detailed and comprehensive picture of the ground-state physics of this kagomé lattice.

Hydrothermal oxidation-reduction methods for the preparation of pure and single crystalline alunites: synthesis and characterization of a new series of vanadium jarosites.

Three new redox-based, hydrothermal, synthetic methods have been developed for the preparation of a new series of jarosites, AV(3)(OH)(6)(SO(4))(2) (A = Na(+), K(+), Rb(+), Tl(+), and NH(4)(+)), in high purity and in single crystalline form. The V(3+) jarosites have been characterized by single-crystal X-ray and elemental analysis, and by infrared and electronic absorption spectroscopy. The synthetic methods employed here represent a new approach for the preparation of the jarosite class of compounds, which for the past several decades, have been notoriously difficult to prepare in pure form. To demonstrate the impact of our new synthetic techniques on the magnetic properties of jarosites, the V(3+) jarosites were also prepared according to the nonredox techniques used over the past 30 years. A comparative study of these samples and those prepared by our new synthetic methods reveals widely divergent magnetic properties, thus pointing to the importance of the new redox synthetic methods to future magnetism studies of jarosite compounds.

Magnetic properties of a homologous series of vanadium jarosite compounds.

Redox-based, hydrothermal synthetic methodologies have enabled the preparation of a new series of stoichiometrically pure jarosites of the formula, AV(3)(OH)(6)(SO(4))(2) with A = Na(+), K(+), Rb(+), Tl(+), and NH(4)(+). These jarosites represent the first instance of strong ferromagnetism within a Kagomé layered framework. The exchange interaction, which is invariant to the nature of the A(+) ion (theta(CW) approximately equal to +53(1) K), propagates along the d(2) magnetic sites of the triangular Kagomé lattice through bridging hydroxyl groups. An analysis of the frontier orbitals suggests this superexchange pathway to possess significant pi-orbital character. Measurements on a diamagnetic host jarosite doped with magnetically dilute spin carriers, KGa(2.96)V(0.04)(OH)(6)(SO(4))(2), reveal significant single-ion anisotropy for V(3+) ion residing in the tetragonal crystal field. This anisotropy confines the exchange-coupled moments to lie within the Kagomé layer. Coupling strengths are sufficiently strong to prevent saturation of the magnetization when an external field is applied orthogonal to the Kagomé layer. Antiferromagnetic ordering of neighboring ferromagnetic Kagomé layers becomes dominant at low temperatures, characteristic of metamagnetic behavior for the AV(3)(OH)(6)(SO(4))(2) jarosites. This interlayer exchange coupling decreases monotonically with increasing layer spacing along the series, A = Na(+), K(+), Rb(+), NH(4)(+), and Tl(+), and it may be overcome by the application of external field strengths in excess of approximately 6 kOe.

NaV3 (OH)6 (SO4 )2 : A Kagomé-Type Vanadium(III) Compound with Strong Intralayer Ferromagnetic Interactions.

A dominant ferromagnetic exchange interaction propagates about the magnetic sites of the Kagomé lattice of the title compound through the bridging hydroxy groups (see section of the structure). This is at variance with the antiferromagnetic exchange observed for jarosite and its derivatives. The ferromagnetism probably arises from the d2 electron count of the VIII centers.

Syntheses and Crystal Structures of a Linear-Chain Uranyl Phenylphosphinate UO(2)(O(2)PHC(6)H(5))(2) and Layered Uranyl Methylphosphonate UO(2)(O(3)PCH(3)).

Two extended uranyl organophosphorus compounds have been synthesized and structurally characterized. Linear-chain uranyl bis(phenylphosphinate), UO(2)(O(2)PHC(6)H(5))(2), was synthesized at 60 degrees C, and its structure was solved by single-crystal methods. UO(2)(O(2)PHC(6)H(5))(2) crystallizes in the triclinic space group P&onemacr; with unit cell parameters a = 5.648(1) Å, b = 8.115(2) Å, c = 9.171(2) Å, alpha = 64.97(3) degrees, beta = 80.59(3) degrees, gamma = 83.34(3) degrees, and Z = 1. The geometry of the uranium atom is tetragonal bipyramidal, and the neighboring uranyl ions are bridged by pairs of phenylphosphinate anions. The phenyl groups form two rows pointing in opposite directions of each chain, and neighboring chains arrange in a staircase fashion. Layered uranyl methylphosphonate, UO(2)(O(3)PCH(3)) (UPMe), was synthesized hydrothermally at 200 degrees C, and its structure was solved by powder pattern X-ray methods and refined by the Rietveld method. UPMe crystallizes in space group P&onemacr; with unit cell parameters a = 6.4027(3) Å, b = 6.6912(3) Å, c = 7.0983(3) Å, alpha = 90.473(2) degrees, beta = 99.684(2) degrees, gamma = 97.333(2) degrees, and Z = 2. The uranyl ions, connected by the phosphonate anions, form parallel inorganic layers, and the methyl groups point perpendicularly between the layers.

Intercalative Ion Exchange of Polyamine Transition Metal Complexes into Hydrogen Uranyl Phosphate.

A series of derivatives of hydrogen uranyl phosphate (HUP) was prepared by displacing the butylammonium ions of butylammonium uranyl phosphate with the transition metal complexes Cu(en)(2)(2+), Cu(pn)(2)(2+), Cu(trien)(2+), Cu(14-ane)(2+), Cu(15-ane)(2+), Ni(trien)(2+), Ni(14-ane)(2+), and Ni(diene)(2+). X-ray powder patterns proved that the original tetragonal structure of the UP layers remained intact in all derivatives. The extent of the ion exchange and the interlamellar distances were found to depend mainly on the size and on the shape of a particular coordinating ligand. Electronic absorption spectra indicated that the intercalated complexes had four-coordinate square-planar geometry inside the UP lattice. Extents of hydration of the intercalates varied significantly, and they depended mostly on the shapes of the coordinating ligands and on their abilities to regularly pack between the UP layers. Results of the above-mentioned characteristics allow one to divide these transition metal complexes into two groups which differ from each other in their abilities to efficiently fill up the space between the UP layers. The steady-state luminescence spectra of intercalates showed a very weak uranyl emission which was partly due to quenching by the Cu(2+) and Ni(2+) guest complexes and partly to self-absorption.