Dilute ferrimagnetic semiconductors in Fe-substituted spinel ZnGa2_2O4_4

Solid solutions of nominal composition [ZnGa$_2$O$_4$]$_{1-x}$[Fe$_3$O$_4$]$_x$, of the semiconducting spinel ZnGa$_2$O$_4$ with the ferrimagnetic spinel Fe$_3$O$_4$ have been prepared with $x$ = 0.05, 0.10, and 0.15. All samples show evidence for long-range magnetic ordering with ferromagnetic hysteresis at low temperatures. Magnetization as a function of field for the $x$ = 0.15 sample is S-shaped at temperatures as high as 200 K. M\"ossbauer spectroscopy on the $x$ = 0.15 sample confirms the presence of Fe$^{3+}$, and spontaneous magnetization at 4.2 K. The magnetic behavior is obtained without greatly affecting the semiconducting properties of the host; diffuse reflectance optical spectroscopy indicates that Fe substitution up to $x$ = 0.15 does not affect the position of the band edge absorption. These promising results motivate the possibility of dilute ferrimagnetic semiconductors which do not require carrier mediation of the magnetic moment.Comment: 9 pages and 6 figure

Defects, Dopants and Lithium Mobility in Li ₉ v ₃ (P ₂ O ₇ ) ₃ (PO ₄ ) ₂

Layered Li9V3(P2O7)3(PO4)2 has attracted considerable interest as a novel cathode material for potential use in rechargeable lithium batteries. The defect chemistry, doping behavior and lithium diffusion paths in Li9V3(P2O7)3(PO4)2 are investigated using atomistic scale simulations. Here we show that the activation energy for Li migration via the vacancy mechanism is 0.72 eV along the c-axis. Additionally, the most favourable intrinsic defect type is Li Frenkel (0.44 eV/defect) ensuring the formation of Li vacancies that are required for Li diffusion via the vacancy mechanism. The only other intrinsic defect mechanism that is close in energy is the formation of anti-site defect, in which Li and V ions exchange their positions (1.02 eV/defect) and this can play a role at higher temperatures. Considering the solution of tetravalent dopants it is calculated that they require considerable solution energies, however, the solution of GeO2 will reduce the activation energy of migration to 0.66 eV

Li2SnO3 as a Cathode Material for Lithium-ion Batteries:Defects, Lithium Ion Diffusion and Dopants

Tin-based oxide Li2SnO3 has attracted considerable interest as a promising cathode material for potential use in rechargeable lithium batteries due to its high- capacity. Static atomistic scale simulations are employed to provide insights into the defect chemistry, doping behaviour and lithium diffusion paths in Li2SnO3. The most favourable intrinsic defect type is Li Frenkel (0.75 eV/defect). The formation of anti-site defect, in which Li and Sn ions exchange their positions is 0.78 eV/defect, very close to the Li Frenkel. The present calculations confirm the cation intermixing found experimentally in Li2SnO3. Long range lithium diffusion paths via vacancy mechanisms were examined and it is confirmed that the lowest activation energy migration path is along the c-axis plane with the overall activation energy of 0.61 eV. Subvalent doping by Al on the Sn site is energetically favourable and is proposed to be an efficient way to increase the Li content in Li2SnO3. The electronic structure calculations show that the introduction of Al will not introduce levels in the band gap

Efficient Palladium-Catalyzed Cyclotrimeriza- tion of Arynes: Synthesis of Triphenylenes**

Over the last 15 years much effort has been devoted to the preparation and characterization of transition metal complexes of arynes. [1] Parallel studies on the reactivity of these complexesÐparticularly those of Ti, Zr, As part of a project aimed at the development of new reactions of arynes promoted by metal complexes, here we report on the metal-mediated cyclotrimerization of arynes. These preliminary results show that the reaction proceeds in the presence of catalytic amounts of metal and that it has great potential for the preparation of triphenylenes, which are found at the core of many discotic liquid crystals [9] An example of the formation of triphenylene as side product of a palladium-catalyzed domino reaction has also been reported. [10] However, to the best of our knowledge, efficient preparation of triphenylenes by metalcatalyzed reaction of arynes is without precedent. Development of a catalytic procedure for the trimerization of arynes requires careful selection of the catalyst and the method for generation of the aryne. The catalyst was chosen from among the various metal systems used for trimerization of alkynes; suitable candidates contained metals such as Ni, Co, Pd, and Pt. We decided to carry out the first trials with palladium complexes because they are easy to handle and in general stable. Among the many procedures available for the generation of arynes [9] S

Ferric Phosphite: Dimers of Face-Sharing Fe^IIIO₆ Octahedra. Crystal Structure Redetermination, Mössbauer Spectra, Magnetic Susceptibility, and Heat Capacity Data

Synthesis, structural characterization, and physical properties of Fe2(HPO3)3 are reported. The structure establishes the phosphite anion as a bridging unit producing dimers of face-sharing Fe(III)O6 octahedra. The Fe2O9 units are interconnected by sharing common apices with HPO3 tetrahedra, resulting in a three-dimensional arrangement. These structural characteristics determine the physical properties of the compound. A weak antiferromagnetic exchange interaction in the dimeric entities and a three-dimensional ordering below 20 K of antiferromagnetic nature can be detected using Mossbauer, susceptibility, and beat capacity data

MÖSSBAUER EFFECT STUDY ON THE HIGH-SPIN (5T2) ⇌ LOW SPIN (1A1) TRANSITION IN [Fe(2-pic)3]Cl2. DILUTION EFFECT IN [FexZn1-x(2-pic)3] Cl2. EtOH AND CRYSTAL SOLVENT EFFECT

L'influence de la température sur la transition 5T2(Oh) ↔ lA1(Oh) dans [Fe(2-picolylamine)3]Cl2 a été étudiée très en détail par spectrométrie Mössbauer, à la suite d'une brève approche faite par Renovitch et Baker. On a particulièrement analysé : 1) L'effet de la substitution du fer par du zinc dans[FexZn1-x(2-pic)3]Cl2.EtOH et 2) l'effet dû au cristal solvant. Les spectres d'absorption des solutions solides avec x = 1,0-0,8-0,6 et 0,2 ont été obtenus sur un intervalle de température compris entre 10 et 300 K. Tous les échantillons montrent une transition de phase coopérative haut-spin (5T2) ↔ bas-spin (lAl). Les spectres montrent la coexistence de deux doublets quadrupolaires provenant de fer (II) dans des états bas-spin (1A1) et à haut-spin (5T2). L'intensité du doublet bas-spin augmente aux dépends de l'intensité du doublet haut-spin lorsque la température diminue. A toute température la contribution bas-spin diminue quand on augmente le degré de dilution (1 - x). La température critique de transition Tc, définie comme la température à laquelle les contributions des deux états de spin sont équivalentes, varie linéairement avec la concentration en fer dans l'intervalle 0,2 ≤ x ≤ 1,0, suivant la relation : Tc(x)/K = 40 x + 82. L'effet quadrupolaire et le déplacement isomérique sont tous deux indépendants de x à toute température. L'influence de la nature du cristal solvant a été étudiée à partir des systèmes [Fe(2-pic)3]Cl2mCnH2n+1OH, avec (m2n) = (1,2) → + C2H5OH, (1,1) → CH3OH, (1,0) → H20, (2,0) → 2 H2O. Ces résultats suggèrent que la transition de spin est gouvernée par un couplage coopératif entre les états électroniques des ions et le spectre de phonon du réseau. Le calcul du champ de ligand conduit à la nature de l'état fondamental haut-spin.The temperature dependent spin transition 5T2(Oh) ↔lAl(Oh) in [Fe(2-pic)3]Cl2 (2-pic = 2-picolylamine), shortly reported on by Renovitch and Baker, has been studied by Mössbauer spectroscopy in great detail, with particular emphasis on two effects : 1) The substitution of zinc for iron in [FexZn1-x(2-pic)3]Cl2.EtOH ; 2) different solvent molecules in the crystal lattice. Absorption spectra of the solid solutions [FexZn1-x(2-pic)3]Cl2. EtOH, x = 1.0, 0.8, 0.6, and 0.2, have been measured in the temperature range 10-300 K. All specimens exhibit a cooperative phase transition due to high-spin (5T2) ↔ low-spin 1A1) transition. The Mössbauer spectra at various temperatures demonstrate the coexistence of two quadrupole doublets arising from the iron (II) low-spin (1Al) and high-spin (5T2) states. The intensity of fhe low-spin doublet increases with decreasing temperature at the expense of the high-spin intensity. At any temperature the low-spin intensity decreases with increasing degree of dilution (1 - x). The critical transition temperature Tc (temperature of equal intensity of the spin states) varies linearly with the iron concentration in the range 0.2 ≤ x → 1.0 : Tc(x)/K = 40 x + 82. The quadrupole splitting and the isomer shift of both spin states are independent of x at all temperatures. The influence of various crystal solvent molecules has been investigated in the system [Fe(2-pic)3]Cl2. mCnH2n+1OH, with (m, n) = (1, 2) = C2H5OH, (1, 1) = CH3OH, (1, 0) = H2O, (2, 0) = 2 H2O. Temperature dependent spin transitions also take place in the (1, 1) and (1, 0) systems. Iron (II) in the system with two H2O molecules exists in the low-spin state over the whole temperature range. The (1, 0) system shows a hesteresis upon cooling and heating. It is suggested that the spin transition is governed by a cooperative coupling between the electronic states of the constituent ions and the phonon system of the lattice. Ligand field calculations have been carried out to establish the nature of the high-spin electronic ground state

Decacyclene as Complexation Manifold: Synthesis, Structure and Properties of Its Fe₂ and Fe₄ Slipped Triple‐Decker Complexes

Reaction of [(η5‐Me4EtC5)FeIICl(tmeda)] (tmeda=N,N,N′N′‐tetramethylethylenediamine) with a polyanion solution of decacyclene (1) results in the formation of the triple‐deckers [{(η5‐Me4EtC5)Fe}2‐μ2‐(η6:η6‐decacyclene)] (3) and [{(η5‐Me4EtC5)Fe}4‐μ4‐(η6:η6:η6:η6‐decacyclene)] (4). Metal complexation in 3 and 4 occurs on opposite faces of the π perimeter in an alternating mode. The decacyclene ring adopts a gently twisted molecular propeller geometry with twofold crystallographic symmetry (C2). Complex 4 crystallizes in the chiral space group C2221; the investigated crystal only contains decacyclene rings with M chirality. The handedness can be assigned unambiguously to the presence of the iron atoms. Cyclovoltammetric studies revealed quasireversible behavior of the redox events and a strong interaction of the Fe atoms in 3 and 4, exemplified by potential differences ΔE of 660 and 770(780) mV between the first and the second individual oxidation processes. This corresponds to a high degree of metal–metal interaction for 3 and 4. The sucesssful syntheses of 3 and 4 together with earlier results from our laboratory proves that all five‐ and six‐membered π subunit sets of 1 are prone to metal complexation. A clear site preference in 1 towards the complexation of [CpR]iron, ‐cobalt, and ‐nickel fragments exists