A Room-Temperature Verwey-type Transition in Iron Oxide,Fe5O6

Functional oxides whose physicochemical properties may be reversibly changed at standard conditions are potential candidates for the use in next-generation nanoelectronic devices. To date, vanadium dioxide (VO₂) is the only known simple transition-metal oxide that demonstrates a near-room-temperature metal–insulator transition that may be used in such appliances. In this work, we synthesized and investigated the crystals of a novel mixed-valent iron oxide with an unconventional Fe₅O₆ stoichiometry. Near 275 K, Fe₅O₆ undergoes a Verwey-type charge-ordering transition that is concurrent with a dimerization in the iron chains and a following formation of new Fe−Fe chemical bonds. This unique feature highlights Fe₅O₆ as a promising candidate for the use in innovative applications. We established that the minimal Fe−Fe distance in the octahedral chains is a key parameter that determines the type and temperature of charge ordering. This model provides new insights into charge-ordering phenomena in transition-metal oxides in general.     

Revisiting van der Waals Radii: From Comprehensive Structural Analysis to Knowledge-Based Classification of Interatomic Contacts

Weak noncovalent interactions are responsible for structure and properties of almost all supramolecular systems, such as nucleic acids, enzymes, and pharmaceutical crystals. However, the analysis of their significance and structural role is not straightforward and commonly requires model studies. Herein, we describe an efficient and universal approach for the analysis of noncovalent interactions and determination of van der Waals radii using the line-of-sight (LoS) concept. The LoS allows to unambiguously identify and classify the “direct” interatomic contacts in complex molecular systems. This approach not only provides an improved theoretical base to molecular “sizes” but also enables the quantitative analysis of specificity, anisotropy, and steric effects of intermolecular interactions.     

An NMR Study of Outer-Sphere Interactions of BF4 − Anions with Paramagnetic Ions

The significance and role of outer-sphere interactions for the chemistry of complex compounds and solutions have been discussed in the literature time and again.^1,2 The existence of the complexes per se in solutions is determined not only by the interactions of the central ion with the near-by ligands, but also with the particles of the outer or secondary coordination sphere. Now, while “intra-sphere” complexes may be both labile and inert with respect to their kinetic behavior in solutions, outer-sphere compounds are predominantly labile. Inasmuch as the majority of outer-sphere compounds cannot be isolated from solutions, NMR-spectroscopy is essentially one of the most convenient methods for their study. Naturally, such investigations are complicated by an exchange of particles constituting associate compositions, and this exchange may be sufficiently rapid to present interest in itself.     

Temperature Dependencies of 19F NMR Spectra Parameters in BF4 − Solutions

Abstract

Despite numerous studies on the BF₄ ^? anion ^1-6 indicating to its complex behavior in different solutions in the presence of different cations, it is still quite difficult to explain this unambiguously. This is ostensibly due to the incompleteness of experimental data both for temperature dependencies and for solution compositions³. Taking this into account, we measured the dependencies of ¹⁹F NMR spectra in a broad range of temperatures and concentrations in aqueous and water-acetone solutions of Co²⁺, Ni²⁺ and Zn²⁺ tetrafluoroborate salts on “RYA-2305” and “SWL 2-31/10” instruments in spherical and cylindrical ampoules. The said dependencies were recorded both for BF₄ ^? and BF₃OH^? signals, the latter forming in insignificant quantities due to the hydrolysis of the tetrafluoroborate anion in positively hydrated cation solutions¹.     

Temperature-induced changes in the single-crystal structure of K9H7(SO4)8 · H2O

Interest in superprotonic crystals of M _m H _n (XO₄)_{(m + n)/2} is associated with the solution to the fundamental problem of modern condensed matter physics: investigations of structural phase transitions and the stabilization of phases with high proton conductivity with the aim of designing new functional materials. The available data suggest that changes in the physical properties in these crystals can occur through different structural mechanisms. To reveal the structural conditionality for anomalies in the physical properties, the crystals of K₉H₇(SO₄)₈ · H₂O were studied by X-ray diffraction in the temperature range of 25–463 K, and the crystal structure of the high-temperature phase was determined at 418 K (sp. gr. Pcan). The results of the study indicate that the occurrence of high conductivity in K₉H₇(SO₄)₈ · H₂O crystals at high temperatures is associated with the diffusion of water of crystallization, the hydrogen-bond network rearrangement, and the motion of K ions. The hydrogen-bond rearrangement and the hindered back diffusion of water to the crystal stabilize the high-temperature phase and ensure its supercooling to low temperatures.     

Torsion of an elastic rod whose cross-section is a parallelogram,trapezoid, or triangle,or has an arbitrary shape by the method of transformation to a rectangular domain

A coordinate transformation is used to take the domain of the rod cross-section to a rectangular domain for which the spectra of eigenfunctions and eigenvalues are known. The torsion function is represented as a generalized Fourier series to reduce the problem to solving a closed linear system of algebraic equations for the expansion coefficients. It is shown that these Fourier series converge absolutely, because the expansion coefficients decrease by a cubic law depending on the term number. We prove that the approximate solution in the form of a finite sum of the Fourier series converges to the exact solution. This theorem is generalized to the case of a rod cross-section of arbitrary shape.     

Improving the differentiability of generalized fourier series solutions of boundary value problems of mechanics by using boundary functions

We prove a theorem on conditions for the differentiation of generalized Fourier series. We show that Fourier series solutions of boundary value problems can in general be differentiated term by term only once. To improve the differentiability properties of such series, we suggest to use pth-order boundary functions. We suggest an algorithm for constructing boundary functions for classical domains. This approach is illustrated by a new solution, with improved differentiability properties, of the problem on the torsion of an elastic rod of rectangular cross-section.     

Order-Parameter Temperature Dependences in Nanocomposites of Porous Glass–Sodium Nitrite

Temperature dependences of the order parameter in nanocomposites based on porous borosilicate glasses with mean pore diameters of 20 and 46 nm and filled with sodium nitrite are studied via the diffraction of synchrotron radiation. The mean diffraction sizes of NaNO₂ nanoparticles in the pores of these matrices and the temperatures of the transition of sodium nitrite to the ferroelectric state are determined. It is shown that the phase transition for these nanoparticles remains a first-order phase transition.