Mathematical dynamics and migration of silver ions in superionic conductor AgI

It is established that the mathematical dynamics and the dynamics of silver ion migration represented by equipotential surfaces of interionic potential for the superionic conductor AgI are rather similar. The parameters of the interionic potential are determined in terms of the effective pair potential of the EXAFS method.     

Geometrical Aspects of the Patterns of Conduction Paths in Fast Ion Conductors

A method for a straightforward calculation of geometrically privileged conduction paths in fast ion conductors is presented. It makes use of a special property of Dirichlet domains, namely that their vertices and edges define all polyhedral voids and all least narrow connecting passages within a rigid framework. The results of the procedure are exhaustive in the geometrical sense and may be used as a basis for physical investigations. The method is illustrated by two simple examples, α-AgI and sodium β-alumina. Restrictions are discussed.     

Entropy theory of metamorphic transitions in coals

A model of a metamorphic phase transition in coals has been suggested, which explains the change of the superionic conduction of coals at early stages of metamorphism by the electronic conductivity of anthracites. The model is based on a concept of quasiplane fragments of aromatic rings, the main component of microstructure of coals, the percolation approximation for determining the carbon concentration responsible for the formation of infinite clusters of aromatic rings, and the Lichtenecker approximation for determining the conductivity of the object “coal substance + rock”.     

Coordination sequences and coordination waves in matter

A possible way of partitioning a space into polycubes (n-dimensional modifications of Golomb polyominoes, which are generally nonconvex) is used as a basic model of ordered matter structure. It is suggested that layer-by-layer growth of a structure, occurring along the geodetics of the digraph of a net defined by the local rules of bonding of polycubes, justifies the phenomenological laws of shaping (self-similarity during the growth, independence of the polyhedron shape on the “seed,” the symmetry of the growth polyhedron, etc.). Specific results of the analysis of number sequences of the increase in coordination circles for planar periodic partitions of model and real crystal structures, as well as the preliminary results of investigation of standing coordination topological waves, revealed for the first time in computer experiments, are reported.     

Structure of the RbTi<Subscript>0.98</Subscript>Zr<Subscript>0.02</Subscript>OPO<Subscript>4</Subscript> single crystal at temperatures of 293 and 105 K

This paper reports on the results of precision X-ray structural investigations of a RbTi_0.98Zr_0.02OPO₄ single crystal at temperatures of 293 and 105 K. It is established that the observed decrease in the temperature of the ferroelectric phase transition in RbTiOPO₄ crystals doped with zirconium is associated with the increase in the Rb-O bond lengths. The structural factors responsible for the decrease in the electrical conductivity in these crystals are revealed. An analysis is made of the structure of the helical channels which in crystals of this family are considered to be a decisive factor for the manifestation of superionic conduction. It is shown that, in structures of the KTiOPO₄ (KTP) type, the migration of ions in channels is most hindered inside the cavities.     

Atomistic and Ab initio modeling of CaAl<Subscript>2</Subscript>O<Subscript>4</Subscript> high-pressure polymorphs under Earth’s mantle conditions

Semi-empirical and ab initio theoretical investigation of crystal structure geometry, interatomic distances, phase densities and elastic properties for some CaAl₂O₄ phases under pressures up to 200 GPa was performed. Two independent simulation methods predicted the appearance of a still unknown super-dense CaAl₂O₄ modification. In this structure, the Al coordination polyhedron might be described as distorted one with seven vertices. Ca atoms were situated inside polyhedra with ten vertices and Ca–O distances from 1.96 to 2.49 Å. It became the densest modification under pressures of 170 GPa (density functional theory prediction) or 150 GPa (semi-empirical prediction). Both approaches indicated that this super-dense CaAl₂O₄ modification with a “stuffed α-PbO₂” type structure could be a probable candidate for mutual accumulation of Ca and Al in the lower mantle. The existence of this phase can be verified experimentally using high pressure techniques.     

Hg X n coordination polyhedra (X = F, Cl, Br, and I) in crystal structures

The crystallochemical analysis of 188 structures containing 252 crystallographic kinds of Hg(II) atoms in Hg X _n coordination polyhedra (X = F, Cl, Br, and I) has been performed by the method of intersecting spheres with the use of Vorono-Dirichlet polyhedra. It was found that halogen atoms surrounded by Hg(II) atoms are characterized by the coordination numbers from 3 to 8. It was demonstrated that, at the unvarying nature of the X atoms, the average Hg-X interatomic distance increases by 0.13–0.57 Å with an increase of the coordination number, whereas the average radius of spheres with the volume equal to that of the Vorono-Dirichlet polyhedron of a Hg atom is either independent of the coordination number (at X = F) or varies with it only by about 0.02–0.07 Å (at X = Cl, Br, and I). This allows the approximation of mercury atoms in the crystal structures by soft (deformable) spheres of a constant volume. Some results obtained in the analysis of the topology and the geometry of [Hg_m X _n ] ^z?-complexes in crystal structures are also presented.     

Neutron diffraction study of phase transitions in the superionic conductor Li<Subscript>0.25</Subscript>Cu<Subscript>1.75</Subscript>Se

Neutron diffraction studies of the superionic conductor Li_0.25Cu_1.75Se are reported. It is shown that gradual disordering in the cation sublattice with an increase in temperature leads to changes in the symmetry of the crystal lattice at 400 and 500 K. At temperatures close to these values, anomalies in the temperature dependence of both ionic and electron conductivity are observed.     

A Mechanism of High-Tc Superconductivity Relating to Perovskite Crystal Structure

A mechanism of high-T_c superconductivity related to the perovskite crystal structure is proposed by a “phonon attenuation model”, which means that in the perovskite-like crystal structure, a number of “ordering” vacancies or holes occur due to the non-stoichiometric structure features. Such a reticular “spongly-like” structure facilitates lattice thermovibration attenuation perovskite (phonon attenuation) during temperature reduction. This leads to an abrupt drop of the resistivity, owing to the conduction electrons transmitted through the media without collision with irregularities in the lattice due to the lattice thermovibration. This probably is the main reason for high-T_c superconductivity of the perovskite cuprates.     

Light Interference Study of Superionic Transition in Synthetic Metal—Solid Electrolyte Hybrid (BEDT-TTF)3AgxI8

Abstract

First order phase transition at 163–172 K is found in the (BEDT-TTF)₃Ag_xI₈ compound in close vicinity to the temperature range of superionic conductivity. This observation indicates formation of the mobile silver ions in the salt as a consequence of sublattice melting.     

Atomic layer deposition of high-k dielectrics on III–V semiconductor surfaces

The goal of this article is to provide an overview of the state of knowledge regarding the Atomic Layer Deposition (ALD) of metal oxides on III–V semiconductor surfaces. An introduction to ALD, the band structure, various defects present on the III–V surface and how they relate to Fermi level pinning are discussed. Surface passivation approaches are examined in detail in conjunction with experimental and computational results. The “interface clean-up” reaction that leads to the formation of a sharp gate oxide/semiconductor interface is related to the surface chemistry and the transport of the surface oxides through the growing dielectric film. Finally, the deposition of metal oxides on semiconductors is discussed in the context of interface quality and some examples of devices using III–V channels and ALD metal oxides are given.     

Ionic conductivity of Bi12(V,Bi)O20 + δ single crystal (δ = 0.27) with a sillenite-type structure

The ionic conductivity of nonstoichiometric Bi₁₂(V_0.89Bi_0.03)O_20.27 single crystal with a sillenite-type structure has been investigated by impedance spectroscopy; its conductivity at 673 K is 2 × 10^?3 S/cm, which is about two orders of magnitude higher than the conductivity of oxide superionic conductor single crystal Zr_0.88Y_0.12O_1.94. As follows from crystallochemical analysis, ion transport in Bi₁₂(V_0.89Bi_0.03)O_20.27 is due to additional O^2? ions, which arise due to oxygen nonstoichiometry.     

Strukturelle Umklapp-Vorgänge in metastabilen β1-Kupfer-Zink-Mischkristallen bei tiefen und höheren Temperaturen (III). Zur Gitterkinetik der isothermen Phasen-Umwandlungen

Whereas part (I) of the present series treated chiefly lattice-dynamical aspects, being important for the initiation of the β₁/α-transformation processes, and the preceding part (II) discussed explicitely the phase transformation of β₁Cu/Zn-crystals, the submitted third part, (III), handles of a lattice kinetic detailing of the results, exclusively received by X-ray measurements (transformation isotherm 360°C). Keeping the subdivision of the transformation processes into the 3 main periods (A) PREDIFFUSION, (B) “UM-.KLAPPING”, (C) POSTDIFFUSION there are the following results: (A) The quickly running heterogenization of the initial β₁-matrix lattice, β₁ ⟹ β₁ + β₁ takes place with an activation energy Q_w for the wandering of vacancies, frozen- in by quenching: “Primary vacancies”, which will disappear in sinks already after about 2 sec. (B)The first structural actions of “Umklapping”,β₁⟹β₁⟹β₁, occurring in Zn-poorβ₁-districts, will be caused dynamically by longwavy thermo-acoustic sher-vibrations – “soft modes” of the type ({110)}, 〈110〉 with wavelengths of about 86 Ä – and succeeding dissociations of dislocations in the momentary structure β₁. The activation energy of this transformation stage, being isolated from the increase of the lattice period aβ₁, is QII= 0.8 eV> QI; therefore the energy difference QII- QI⋍ 5.10⁻² eV can be interpreted as energy barrier for the structural transitions β₁⟹ β₁⟹ β₂ – As the “mean atomic volume” of the diffusionless period is smaller than that of β₁, “secondary vacancies” will be formed during this period, and so the following diffusion processes become accelerated for some time. In this subperiod, in which the first α-crystals appear and begin to grow, the activation energy is 1.0 eV. the α-crystals are formed with 12-fold crystallographical strong defined orientations relative to the β₁-matrix. (C) During the postdiffusion period, which will begin with the disappearance of the transition lattice β₂ and which is ruled by growth of the original (coherent) fcc α-crystallets, there are only diffusive direct transitions of (Zn-rich) β₁+− and of residual (Zn-poor but not more “umklapping”) β₁-districts; β₁+ α and β₁ α respectively. The growth of the α-crystallets takes place with the activation energy QIV = 1.45 eV. This energy is the sum of the activation energies for the formation and wandering of a vacancy, Q_IV=Q_B + QW, because in the meantime the excessive “secondary vacancies” are annihilated and the “mean atomic volume” of thc reacting associations is increasing; consequently QB = 0.7 eV. Caused by the increase of the “mean atomic volume” and exhibited by X-ray diffraction photographs, near the end of the isotherm reaction the (at least partially) coherent β₁/α-association suffers a self-deformation. This deformation, only taking place in the α-members of the association, will be signified already in much former reaction stages by weak satellite reflections of some α-crystal interferences (to this s. part (II)). A texture-like diffuse prolongation of several a-reflexions shows the β₁/β₂/α-lattice coherence to be removed partially in some micro-regions (very weak X-ray intensities? ) For this there are a few reasons: (1) In the moment of “Umklapping” the concerned micro-lattice regions are superplastic so that the other strong crystallographical Pβ₁α-orientation relationship can easy get lost, at least partially. (2) It is possible that also the so called “massive transformation” contributes to this effect, since this transformation operates without lattice coherency-the rather the lower the Zn-content of thc transforming lattice regions, as it is in fact realized for the Zn-poorest β₁-districts in the moment of “Umk1apping”.     

Molecular dynamics simulation of premelting effect in AgBr

The effect of premelting in silver bromide crystals has been simulated for the first time. It is shown that at the temperature about 150°C lower than the melting point of silver bromide, a considerable increase in the mobility in the cationic sublattice is observed, whereas the (self-)diffusion coefficient of silver ions attains values exceeding 10^?6 cm²/s. The assumption about the superionic nature of conductivity in the region of premelting is confirmed by the break of the long-range order in the cationic subsystem, which, in turn, is confirmed by the comparison of the pair cation-cation correlation functions far from and in the vicinity of the melting point. It is established that the premelting effect correlates with the experimentally observed effect of a considerable increase in ionic conductivity in the vicinity of the melting point. It is shown that the premelting effect in AgBr is similar to the diffuse superionic phase transition in anionic conductors of the MF₂ family (M = Ca, Ba, Sr, and Pb).     

Mechanism of thermal expansion of structural modifications of zinc pyrovanadate

Crystal chemical characteristics of the α and β modifications of Zn₂V₂O₇ are calculated based on in situ high-temperature X-ray measurements. The expansion of the structure is found to be strongly anisotropic up to the negative volumetric thermal expansion of the α-Zn₂V₂O₇ unit cell in the temperature range of 300–600°С, α _V =–17.94 × 10^–6 1/K. The transformations of the “hard” and “soft” sublattices with an increase temperature and at the phase transition are considered in detail. It is shown that the negative volumetric thermal expansion of α-Zn₂V₂O₇ is due to the degeneracy of the zigzag-like shape of zinc–oxygen columns at constant distances between their vertices.     

Negative magnetoresistance in Si atomic-layer-doped GaAs

Magnetoresistance of a Si atomic-layer-doped (δ-doped) GaAs is measured to study the transport properties of the δ-doped structure. The magnetoresistance decreases as the temperature is lowered and the depth of the δ-doped layer, i.e., the distance between the surface and the δ-doped layer, becomes less than 300 nm. Negative magnetoresistance is observed below 80 K in a sample having a doping density of 1.67×10¹² cm⁻² and a δ-doped layer depth of 40 nm. The conduction channel in the δ-doped structure consists of two parts, the quantized states in the conduction band and the impurity band. The origin of the negative magnetoresistance is suggested to be the conduction in the inhomogeneous potential due to localized impurity potential.     

Atomic force microscopy modified for studying electric properties of thin films and crystals. Review

Probe force microscopy continues growing in popularity as a method for studying surfaces of solids and control over crystals and thin films that are grown on various scientific and industrial setups. New modifications of the method increase the possibilities for recording various characteristics of the objects studied. An important role here is played by “electrical” force microscopy, the various modifications and practical applications of which are considered below, as well as the results obtained by this method.     

Block Lattices with Asymmetric Interaction Forces

A series of 2D block lattices constructed by identical building elements and having asymmetric interaction forces is proposed as a generalisation of the well‐known Kossel crystal. Different polymorph structures are created depending on the mutual orientation of the neighbour elements. The lattice energy for some of them is calculated at low temperatures using the repeatable step of blocks (monomers, dimers, quadrumers). A variety of “surface” structures (including alternating “surfaces”) is established. The corresponding coexisting equilibrium forms are determined following the method of Stranski and Kaischew. The thermodynamic stability of these polymorph structures is discussed. An orientation order/disorder surface transition is expected to appear as a novel phenomenon at high temperatures.     

An unusual type of unmixing in NaCl-KCl mixed crystals at room temperature

Water-catalysed decomposition of solid solutions of NaCl–KCl has been investigated at room temperature. The “wet” decomposition proceeds at abnormally high rate as compared to the usual “dry” decomposition. The formation and movement of the decomposition boundary is characteristic of the “wet” process. The influence of divalent impurities and partial decomposition of the usual type has been studied on the rate of the decomposition boundary movement. The mechanism of the unmixing is suggested which involves two interrelated process: the phase separation and formation of a system of interlocking microcracks acting as channels for the water transport from the surface into the bulk of the crystals. The open porosity resulting from the unmixing is confirmed experimentally.