Kinetics and mechanism of the formation of manganese(III) from manganese(II) and (VII) in aqueous perchlorate solution

The reaction 4MnIIfMnVxI = 5MnIII obeyed the rate equation, rate = k[Mnu]2[MnvII]. In lithium perchlorate media the [H+]-dependence was found to be k = k~H+]+ko. Activation energies and entropies were compared with standard values for formation of intermediate species in an examination of possible mechanisms

Kinetics of the aqueous manganese(III)+ iron(II) reaction by platinum-electrode polarography

The rate equation –d[FeII]/dt=kobs[FeII][MnIII] was established by measurements of the iron(II) diffusion current. For 0.3–15°C in 0.54-3 M HClO4 the variation of kobs was consistent with the relation kobs=(ko[H+]+k1Kh)/([H+]+Kh), where Kh is the manganese(III) hydrolysis constant. Values of k0 are just less than k1 as in the comparable oxidation of vanadium(IV). It is concluded from further comparisons of rates that the activated complexes are outer-sphere

A kinetic measurement for fast cation-cation oxidations in solution

The use of rotating platinum electrodes in polarography, hitherto confined to oxidations involving only one ionic reactant can be extended to kinetic investigation of fast calomel-cation oxidations provided there exists a suitable applied voltage at which only one species gives a diffusion current..

Electron transfer reactions of vanadium(IV) with some oxyanion oxidants in aqueous perchloric acid. Part I. Reaction with chromium-(VI) and manganese(VII)

The rate law k[CrVI][VIV]2[H+]n/VV], with n < 1, is confirmed, and differences from the iron(II)–chromium(IV) result are explained. With proper interpretation the comparability of iron(II) with vanadium(IV) reactions is preserved. The reaction of vanadium(IV) with manganese(VIII) appears to be initially just of the second order, but non-stoicheiometry and acceleration by product manganese(II) introduce later complications only qualitatively resolvable

The iron(II)-chromium(VI) reaction: An additional pathway first order in iron(II)

The iron(II)–chromium(VI) reaction has been studied by platinum-electrode polarography at low (micromolar) iron(II) concentrations. Here an additional term first order in iron(II) appears in the rate equation, which is found to be –d[FeII]/dt=k1H[H+][HCrO4–][FeII]+k2H[HCrO4–][FeII]2/[Fe3+], with k1H= 234 l2 mol–4 s–1 and 10–8k2H= 6·92 l4 mol–4 s–1 at 20 °C and ionic strength 1·00 mol l–1 in sodium perchlorate medium with 0·025–0·065M perchloric acid

Jahn-Teller orbital glass state in the expanded fcc Cs3C60 fulleride

The most expanded fcc-structured alkali fulleride, Cs3C60, is a Mott insulator at ambient pressure because of the weak overlap between the frontier t1u molecular orbitals of the C603− anions. It has a severely disordered antiferromagnetic ground state that becomes a superconductor with a high critical temperature, Tc of 35 K upon compression. The effect of the localised t1u3 electronic configuration on the properties of the material is not well-understood. Here we study the relationship between the intrinsic crystallographic C603− orientational disorder and the molecular Jahn–Teller (JT) effect dynamics in the Mott insulating state. The high-resolution 13C magic-angle-spinning (MAS) NMR spectrum at room temperature comprises three peaks in the intensity ratio 1:2:2 consistent with the presence of three crystallographically-inequivalent carbon sites in the fcc unit cell and revealing that the JT-effect dynamics are fast on the NMR time-scale of 10−5 s despite the presence of the frozen-in C603− merohedral disorder disclosed by the 133Cs MAS NMR fine splitting of the tetrahedral and octahedral 133Cs resonances. Cooling to sub-liquid-nitrogen temperatures leads to severe broadening of both the 13C and 133Cs MAS NMR multiplets, which provides the signature of an increased number of inequivalent 13C and 133Cs sites. This is attributed to the freezing out of the C603− JT dynamics and the development of a t1u electronic orbital glass state guided by the merohedral disorder of the fcc structure. The observation of the dynamic and static JT effect in the Mott insulating state of the metrically cubic but merohedrally disordered Cs3C60 fulleride in different temperature ranges reveals the intimate relation between charge localization, magnetic ground state, lifting of electronic degeneracy, and orientational disorder in these strongly-correlated systems

Spin frustration and magnetic ordering in theS=12molecular antiferromagnetfcc−Cs3C60

We have investigated the low-temperature magnetic state of face-centered-cubic (fcc) Cs3C60, a Mott insulator and the first molecular analog of a geometrically frustrated Heisenberg fcc antiferromagnet with S=1/2 spins. Specific heat studies reveal the presence of both long-range antiferromagnetic ordering and a magnetically disordered state below TN=2.2 K, which is in agreement with local probe experiments. These results together with the strongly suppressed TN are unexpected for conventional atom-based fcc antiferromagnets, implying that the fulleride molecular degrees of freedom give rise to the unique magnetic ground state

Manganese(III) and its hydroxo- and chloro-complexes in aqueous perchloric acid: comparison with similar transition-metal(III) complexes

At 25°C the formation constant of MnCl2+ is found by spectrophotometry to be 13.2 ± 0.9 dm3 mol–1 at ionic strength 3.26 mol dm–3; for MnCl+2 the value is 1.1 ± 0.7 dm3 mol–1. Increase in the number of chloride ions in complexes results in longer wavelengths for the corresponding absorption maxima. In the absence of chloride the hydrolysis constant of MnIII at ionic strength 5.6 mol dm–3 is found from voltammetry and potentiometry to be 1.05± 0.26 mol dm–3. Aged managanese(III) is found to be 15–25% polymeric, from both kinetic and e.m.f. measurements. Comparison of formation constants for halogeno- and hydroxo-complexes of M3+(first transition series) shows that a combination of charge-transfer, ligand-field and coulomb interactions underlies the observed sequences; the dipole moment of OH– is also a factor

Sensitivity of the Mott Transition to Non-cubic Splitting of the Orbital Degeneracy: Application to NH3 K3C60

Within dynamical mean-field theory, we study the metal-insulator transition of a twofold orbitally degenerate Hubbard model as a function of a splitting \Delta of the degeneracy. The phase diagram in the U-\Delta plane exhibits two-band and one-band metals, as well as the Mott insulator. The correlated two-band metal is easily driven to the insulator state by a strikingly weak splitting \Delta << W of the order of the Kondo-peak width zW, where z << 1 is the metal quasiparticle weight. The possible relevance of this result to the insulator-metal transition in the orthorhombic expanded fulleride NH3 K3C60 is discussed.Comment: revtex, 15 pages including 6 ps figures. Submitted to Phys. Rev.

Separation of K+ and Bi3+ displacements in a Pb-free, monoclinic piezoelectric at the morphotropic phase boundary

The best piezoelectric properties of any perovskite oxide known are found in the solid solution of the relaxor Pb(Mg1/3Nb2/3)O3 and ferroelectric PbTiO3. Despite its impressive properties, this system has limited analogy. We present the compositional exploration of the Pb-free analogue (1-x)(K1/2Bi1/2)(Mg1/3Nb2/3)O3-x(K1/2Bi1/2)TiO3 (KBMN-KBT). We locate the morphotropic phase boundary between x = 0.86 and 0.88 changing from Cm to Pm symmetry and the optimally performing composition at x = 0.88. We report a piezoelectric figure of merit (d33*) of 192 pm V−1 from strain measurements. Diffraction methods reveal disordered displacements of K+ and Bi3+ which persist from the KBMN endmember through multiple changes in symmetry. Rearrangement of the Bi3+ displacements along the uncommon [011]c direction drives the physical response. Ferroelectric, dielectric, and piezoresponse force microscopy are used to study the progression of physical properties through the MPB and attribute the mechanism to a polarization rotation. Taking account for local, short-range, and average structural features yield a balanced perspective on the structure and properties of this system, isolating the driving force within this system to the Bi3+ bonding configuration. This work yields a strong analogy to the Pb-based analogue, and provides strategies for further optimization