Quantum Versus Jahn-Teller Orbital Physics in YVO3_3 and LaVO3_3

We argue that the large Jahn-Teller (JT) distortions in YVO$_3$ and LaVO$_3$ should suppress the quantum orbital fluctuation. The unusual magnetic properties can be well explained based on LDA+$U$ calculations using experimental structures, in terms of the JT orbital. The observed splitting of the spin-wave dispersions for YVO$_3$ in C-type antiferromagnetic state is attributed to the inequivalent VO$_2$ layers in the crystal structure, instead of the ``orbital Peierls state''. Alternative stacking of $ab$-plane exchange couplings produces the c-axis spin-wave splitting, thus the spin system is highly three dimensional rather than quasi-one-dimensional. Similar splitting is also predicted for LaVO$_3$, although it is weak.Comment: 4 pages, 2 tables, 2 figures, (accepted by PRL

Crystal chemical and quantum chemical studies of Ba(Sr)-Nb oxide compounds

The information available on the BaO(SrO)-NbO-NbO2 system with the niobium atom in the lower oxidation degree is very limited. Very few compounds have been found previously in this system. They are BaNbO3, SrxNbO3(0,7=x=1), Ba2Nb2O9, SrNb8O14; and some suggestions on the BaNb8O14 existence have been made also. At the same time Nb-based oxide compounds could be quite interesting in the search of new noncopper high T(sub c) superconductors Researchers studied Ba(Sr) NbxO2x-2 and Ba2(Sr2)-NbxO2x-1 compositions in the phase diagram of BaO(SrO)-NbO-NbO2 system. The synthesis of the materials was carried out in vacuum at the temperatures of 1000 to 1500 C. Barium carbonate and niobium pentoxide were used as initial components. X-ray analysis was carried out

Methyl 7,8-diacet­oxy-11-oxo-5-(2-oxo­pyrrolidin-1-yl)-7,9-epoxy­cyclo­penta­[4,5]pyrido[1,2-a]quinoline-10-carboxyl­ate sesquihydrate

The title compound, C26H28N2O9·1.5H2O, the product of an acid-catalysed Wagner–Meerwein skeletal rearrangement, crystallizes as a sesquihydrate with the O atom of one of the two independent water mol­ecules occupying a special position on a twofold axis. The organic mol­ecule comprises a fused penta­cyclic system containing two five-membered rings (cyclo­pentane and tetra­hydro­furan) and three six-membered rings (piperidinone, tetra­hydro­pyridine and benzene). The five-membered rings have the usual envelope conformations, and the central six-membered piperidinone and tetra­hydro­pyridine rings adopt boat and sofa conformations, respectively. In the crystal, there are three independent O—H⋯O hydrogen bonds, which link the organic mol­ecules and water mol­ecules into complex two-tier layers parallel to (001). The layers are further linked into a three-dimensional framework by attractive inter­molecular carbon­yl–carbonyl inter­actions

Lattice Distortion and Magnetism of 3d-t2gt_{2g} Perovskite Oxides

Several puzzling aspects of interplay of the experimental lattice distortion and the the magnetic properties of four narrow $t_{2g}$-band perovskite oxides (YTiO$_3$, LaTiO$_3$, YVO$_3$, and LaVO$_3$) are clarified using results of first-principles electronic structure calculations. First, we derive parameters of the effective Hubbard-type Hamiltonian for the isolated $t_{2g}$ bands using newly developed downfolding method for the kinetic-energy part and a hybrid approach, based on the combination of the random-phase approximation and the constraint local-density approximation, for the screened Coulomb interaction part. Then, we solve the obtained Hamiltonian using a number of techniques, including the mean-field Hartree-Fock (HF) approximation, the second-order perturbation theory for the correlation energy, and a variational superexchange theory. Even though the crystal-field splitting is not particularly large to quench the orbital degrees of freedom, the crystal distortion imposes a severe constraint on the form of the possible orbital states, which favor the formation of the experimentally observed magnetic structures in YTiO$_3$, YVO_, and LaVO$_3$ even at the HF level. Beyond the HF approximation, the correlations effects systematically improve the agreement with the experimental data. Using the same type of approximations we could not reproduce the correct magnetic ground state of LaTiO$_3$. However, we expect that the situation may change by systematically improving the level of approximations for dealing with the correlation effects.Comment: 30 pages, 17 figures, 8 tables, high-quality figures are available via e-mai

Експериментальні та теоретичні дослідження таутомерії серед 3-заміщених 2-метилхінолін-4(1H)-онів

4-Hydroxy-/4-oxo tautomerism in the series of 3-substituted 2-methyl-quinolin-4(1H)-ones has been studied by 13C NMR-spectroscopy and quantum-chemical methods in various approximations (restricted Hartree-Fock method, DFT and MP2) for the isolated molecules and for solutions using empirical correction of effects for solvents (PCM COSMO procedure). Substituents that are different in their nature have no significant influence on the value of the chemical shift of carbon in position C4 of the quinolone cycle. The only exception is the carbon shielding associated with the bromine atom in the molecule of 3-bromo-2-methyl-1,4-dihydroquinoline-4-one. Significant deshielding detected in all cases in 13C NMR-spectra of the carbon nuclei in position 4 of the ring is in favour of the existence of all derivatives studied as 4-oxo forms in DMSO-d6 solution. The experimental and calculated values for the chemical shift of carbon in position C4 of 4-oxo and 4-hydroxy isomers differ considerably and can be used as a criterion for assigning quinolin-4 (1H)-ones to a particular tautomeric form.С помощью ЯМР 13С спектроскопии и квантово-химическими методами в различных приближениях (ограниченный метод Хартри-Фока, DFT и МР2) для изолированных молекул и растворов с использованием эмпирической коррекции эффектов растворителей (процедура РСМ COSMO) исследована 4-гидрокси 4-оксо-таутомерия в ряду производных 3-замещенных 2-метилхинолин-4(1Н)-онов. Различные по своему характеру заместители не оказывают существенного влияния на значение химического сдвига углерода в положении С4 хинолонового цикла. Исключение составляет лишь экранирование углерода, связанного с атомом брома в молекуле 3-бромо-2-метил-1,4-дигидрохинолин-4-oна. Значительное дезэкранирование, обнаруженное во всех случаях в спектрах ЯМР 13С для ядер углерода в 4-ом положении кольца, говорит в пользу существования всех исследованных производных в растворе в DMSO-d6 в виде 4-оксо-форм. Экспериментальные и расчетные значения химического сдвига для углерода в положении С4 для 4-оксо- и 4-гидрокси-изомеров заметно отличаются и могут быть использованы в качестве критерия для отнесения хинолин-4(1Н)-онов к той или иной таутомерной форме.За допомогою ЯМР 13С спектроскопії і квантово-хімічними методами в різних наближеннях (обмежений метод Хартрі-Фока, DFT і МР2) для ізольованих молекул і розчинів з використанням емпіричної корекції ефектів розчинників (процедура РСМ COSMO) досліджена 4-гідрокси оксо-таутомерія в ряду похідних 3-заміщених 2-метилхінолін-4(1Н)-онів. Різні за своїм характером замісники не чинять істотного впливу на значення хімічного зсуву вуглецю в положенні С4 хінолонового циклу. Виняток становить лише екранування вуглецю, пов’язаного з атомом брому в молекулі 3-бромо-2-метил-1,4-дигідрохінолін-4-oну. Значне дезекранування виявлене у всіх випадках у спектрах ЯМР 13С для ядер вуглецю в 4-му положенні кільця вказує на користь існування всіх досліджених похідних у розчині в DMSO-d6 у вигляді 4-оксо-форм. Експериментальні та розрахункові значення хімічного зсуву для вуглецю в положенні С4 для 4-оксо- і 4-гідрокси-ізомерів помітно відрізняються і можуть бути використані в якості критерію для віднесення хінолін-4 (1Н)-онів до тієї чи іншої таутомерної форми

Precision determination of band offsets in strained InGaAs/GaAs quantum wells by C-V-profiling and Schroedinger-Poisson self-consistent simulation

The results of measurements and numerical simulation of charge carrier distribution and energy states in strained quantum wells In_xGa_{1-x}As/GaAs (0.06 < x < 0.29) by C-V-profiling are presented. Precise values of conduction band offsets for these pseudomorphic QWs have been obtained by means of self-consistent solution of Schroedinger and Poisson equations and following fitting to experimental data. For the conduction band offsets in strained In_xGa_{1-x}As/GaAs - QWs the expression DE_C(x) = 0.814x - 0.21x^2 has been obtained.Comment: 9 pages, 12 figures, RevTeX