Half-metallic Co-based quaternary Heusler alloys for spintronics: Defect- and pressure-induced transitions and properties

Heusler compounds offer potential as spintronic devices due to their spin polarization and half-metallicity properties, where electron spin-majority (minority) manifold exhibits states (band gap) at the electronic chemical potential, yielding full spin polarization in a single manifold. Yet, Heuslers often exhibit intrinsic disorder that degrades its half-metallicity and spin polarization. Using density-functional theory, we analyze the electronic and magnetic properties of equiatomic Heusler (L21) CoMnCrAl and CoFeCrGe alloys for effects of hydrostatic pressure and intrinsic disorder (thermal antisites, binary swaps, and vacancies). Under pressure, CoMnCrAl undergoes a metallic transition, while half-metallicity in CoFeCrGe is retained for a limited range. Antisite disorder between Cr-Al pair in CoMnCrAl alloy is energetically the most favorable, and retains half-metallic character in Cr-excess regime. However, Co-deficient samples in both alloys undergo a transition from half-metallic to metallic, with a discontinuity in the saturation magnetization. For binary swaps, configurations that compete with the ground state are identified and show no loss of half-metallicity; however, the minority-spin band gap and magnetic moments vary depending on the atoms swapped. For single binary swaps, there is a significant energy cost in CoMnCrAl but with no loss of half-metallicity. Although a few configurations in CoFeCrGe energetically compete with the ground state, the minority-spin band gap and magnetic moments vary depending on the atoms swapped. This information should help in controlling these potential spintronic materials

Chirality at metal and helical ligand folding in optical isomers of chiral bis(naphthaldiminato)nickel(II) complexes

Enantiopure bis[(R or S)-N-1-(Ar)ethyl-2-oxo-1-naphthaldiminato-κ2N,O]nickel(ii) complexes Ar = C6H5 (1R or 1S), p-OMeC6H4 (2R or 2S), and p-BrC6H4 (3R or 3S) are synthesized from the reactions between (R or S)-N-1-(Ar)ethyl-2-oxo-1-naphthaldimine and nickel(ii) acetate. Circular-dichroism spectra and their density-functional theoretical simulation reveal the expected mirror image relationship between the enantiomeric pairs 1R/1S and 3R/3S in solution. CD spectra are dominated by the metal-centered Λ- or Δ-chirality of non-planar four-coordinated nickel, this latter being in turn dictated by the ligand chirality. Single crystal structure determination for 1R and 1S shows that there are two symmetry-independent molecules (A and B) in each asymmetric unit that give a Z′ = 2 structure. Two asymmetric and chiral bidentate N^O-chelate Schiff base ligands coordinate to the nickel atom in a distorted square planar N2O2-coordination sphere. The conformational difference between the symmetry-independent molecules arises from the "up-or-down" folding of the naphthaldiminato ligand with respect to the coordination plane, which creates right- (P) or left-handed (M) helical conformations. Overall, the combination of ligand chirality, chirality at the metal and ligand folding gives rise to discrete metal helicates of preferred helicity in a selective way. Cyclic voltammograms (CV) show an oxidation wave at ca. 1.30 V for the [Ni(L)2]/[Ni(L)2]+ couple, and a reduction wave at ca. -0.35 V for the [Ni(L)2]/[Ni(L)2]- couple in acetonitrile

Spin state behavior of iron(II)/dipyrazolylpyridine complexes. New insights from crystallographic and solution measurements

The isomeric complexes [Fe(1-bpp)2]2+ and [Fe(3-bpp)2]2+ (1-bpp=2,6-di[pyrazol-1-yl]pyridine; 3-bpp=2,6-di[1H-pyrazol-3-yl]pyridine) and their derivatives are some of the most widely investigated complexes in spin-crossover research. This article addresses two unique aspects of their spin-state chemistry. First, is an unusual structural distortion in the high-spin form that can inhibit spin-crossover in the solid state. A new analysis of these structures using continuous shape measures has explained this distortion in terms of its effect on the metal coordination geometry, and has shown that the most highly distorted structures are a consequence of crystal packing effects. Second, solution studies have quantified the influence of second-sphere hydrogen bonding on spin-crossover in [Fe(3-bpp)2]2+, which responds to the presence of different anions and solvents (especially water). Previously unpublished data from the unsymmetric isomer [Fe(1,3-bpp)2]2+ (1,3-bpp=2-[pyrazol-1-yl]-6-[1H-pyrazol-3-yl]pyridine) are presented for comparison. Modifications to the structure of [Fe(3-bpp)2]2+, intended to augment these supramolecular effects, are also described