Peculiarities of spontaneous crystallization of cubic boron nitride single crystal powders in the Li-B-N (H,Be) system

Abstract

We have investigated the effect of beryllium additions on the kinetics of conversion of hexagonal graphite-like BN to cubic BN (hBN → cBN) as well as on the linear rate of cBN crystal growth in a Li-B-N (H, Be) system. Experiments were performed in the temperature range 1940-2080K at a pressure of 4.3GPa. With the addition of 0.25 and 0.50wt.% beryllium the activation energy of the cBN formation process is found to be 45.8 and 42.0kJ/mol, respectively. The resulting crystals showed p-type conductivity. The activation energy of the impurity carriers for the samples with 0.25 and 0.50 wt.% beryllium additions equalled 0.22 and 0.17eV, respectively.     

Ordered porous TiO2 films obtained by freezing and the application in dye sensitized solar cells

TiO₂ films with ordered porous structure were prepared by freezing. By simply freezing wet TiO₂ film on a cold copper plate, ice crystals could grow from bottom of film. Removing such ice crystals then led to ordered, micro-sized pores in the films. With assistance of scanning electron microscopy and N₂ adsorption–desorption isotherms, micro-structural properties of the films were studied. Coarsening behavior of ice crystals was analyzed based on evolvement of the micro-structure. It was found that, both larger film thickness and longer freezing time were beneficial for the formation of ordered porous structure, which was caused by enhanced ice crystals growth. The films were then used to fabricate photoanode of dye sensitized solar cells, of which the photo-to-electric power conversion efficiency was evaluated. It was observed that, solar cell made of ordered porous TiO₂ film came out with 13% larger photo-current density comparing to that made of conventional doctor blading method, which is due to the enhanced light scattering by the ordered porous structure.     

Structure et proprietes de conduction du compose K0,1Li0,9SbO3

Single-crystals of K_0,1Li_0,9SbO₃ are obtained by ion exchange method from crystals of KSbO₃, $\text{1}\text{6}$ KF and crystallize in the cubic system, space group Pn3, a = 9,681 (3) Å, Z = 4. Using 398 independent reflections, the structure has been refined by full-matrix least squares, to factor R = 0,043.In this space group, the alkaline ions are in an ordered state.The ionic conductivity of cubic KSbO₃ phases is very high and the motion of cations is three-dimensional along <111> tunnels. The authors discusse the influence of ordered (Pn3) and disordered (Im3) states on the conductivity of these compounds.     

High pressure studies on single crystals of zirconium sulphoselenides

Abstract

Electrical resistance measurements have been performed on ZrS₂, ZrSe₂, and ZrSSe single crystals grown by Iodine vapour transport technique in the pressure range 10 kilobar to 80 kilobar (1 GPa-8GPa). It is observed that all these crystals show a decrease in resistance with increase in pressure. The results have been discussed on the basis of energy band.     

Optical properties of alkaline earth oxide crystals

Abstract

Single crystals have been obtained of undoped calcium, strontium and barium oxides. Recent work on the F-band in these crystals will be reviewed, quoting values for the spin-orbit interaction constants for the first excited states of F-centres. A zero-phonon line has been found in undoped calcium oxide, and there has been a controversy whether or not this line is due to the F-centre. Another zero-phonon line occurs in bismuth-doped calcium oxide. The properties of both zero-phonon lines have been studied by applying uniaxial stress or a magnetic field to the crystal at liquid helium temperature. Both lines are found to be due to electric dipole transitions at centres of cubic or terahedral symmetry.     

The formation of biaxial nematic phases in binary mixtures of thermotropic liquid-crystals composed of uniaxial molecules

ABSTRACT

Monte Carlo simulations in the isothermal-isobaric ensemble are used to investigate the formation of an ordered, biaxial nematic phase in a binary mixture of thermotropic liquid crystals. The orientational dependence of the interaction between molecules of each pure component is the same as in the well-known Maier-Saupe model; each pure component of the mixture is therefore capable of forming a uniaxial nematic phase. For the interaction between molecules of different components, we use the same Maier-Saupe model but change the sign of the coupling constant. As a consequence a T-shaped arrangement of these molecules is energetically favoured. The formation of the biaxial phase occurs in two steps. At higher temperatures T, one of the components forms a uniaxial nematic phase whereas the other is in a quasi two-dimensional restricted isotropic liquid state. We develop a simple theoretical model to understand the high degree of (ostensible) nematic order in the latter. At lower T, the second component becomes nematic and then the entire mixture of the two compounds has biaxial symmetry. The biaxial nematic phase does not demix into domains rich in molecules of one or the other species.     

Ab initio study of phonons and structural stabilities of the perovskite-type MgSi{O_3}

Using the local-density approximation, calculating the Hellmann-Feynman forces, applying the direct method and deriving the phonon dispersion relations, the stability of the perovskite-like structures of MgSiO₃ at T =0 have been studied. The cubic Pmm phase shows a dispersion-less soft phonon branch spreading from the R to M points of the cubic Brillouin zone. This soft branch persists up to high pressures of 150 GPa. The low-symmetry phases I4/mcm and Imma, P4/mbm can be considered as a result of the soft mode condensation at the M and R points, respectively. These phases prove to be unstable at T =0. The experimentally observed Pmnb phase is a consequence of the intersection of Imma and P4/mbm space groups. Thus, it can be regarded as a simultaneous condensation of two soft modes: one at the M and a second at the R high-symmetry points of the cubic Brillouin zone. The phonon dispersion relations of Pmnb show that this phase is stable and its optical phonons appear above 4.0 THz only. Received 15 October 1999 and Received in final form 14 January 2000     

立方晶体超声物态方程弹性参数的高压测量

 用超声脉冲回波重合技术（PEO），在0.1~500 MPa流体静压力范围，测量了立方晶体Si和Bi₁₂GeO₂₀中沿[100]、[110]和[111]方向传播的纯纵波和切变波自然声速值随压力的变化。根据有限形变理论，由声速与压力关系的实验结果。确定了立方晶体Si和Bi₁₂GeO₂₀超声物态方程的弹性参数（含三阶和四阶弹性常数）。     

Study of crystals in medium-and long-wavelength IR ranges by surface-electromagnetic-wave spectroscopy

First observation of excitation of surface electromagnetic waves (SEW) is reported in the reststrahlen region in for biaxial crystal KTiOPO₄ (KTP) using a tunable CO₂ laser around 10 μm, and for CaF₂, BaF₂, MgO (cubic crystals), and LiNbO₃ (uniaxial crystal) in the far IR using ith a free-electron laser. The parameters of SEW propagation in these crystals have been obtained by the interference method of SEW phase spectroscopy, and the possibility of determining the complex dielectric permittivity of crystals from the SEW propagation parameters demonstrated in the range of SEW existence. Fiz. Tverd. Tela (St. Petersburg) 40, 237–241 (February 1998)     

The effect of chemical composition of a crystallization medium on the cBN stoichiometry

Abstract

The effect of chemical composition of the crystallization medium in synthesis on and some properties of cubic boron nitride (cBN) has been studied. Cubic BN crystals with the lowest free boron content and a boron-nitrogen ratio close to the stoichiometric one are grown in a multicomponent crystallization medium in a Li₃N-BN system. These crystals are characterized by a higher strength and thermal stability.     

Pressure dependence of electronic transitions of Fe,Co and Ni ions in layered dihalides

Abstract

The near infra-red absorption peaks due to transition metal ions in four halides of Fe, Co and Ni have been studied as a function of pressure. The behaviors of these ions' absorption peaks under pressure are found to be quite different. While the energy of the absorption peak in Col₂ increases with pressure similar to the behavior of transition metal ions in cubic crystal fields, the absorption peaks in the Fe halides are found to be almost independent of pressure. In Nil₂ two absorption peaks exhibit level-crossing at about 2GPa. The results have been interpreted in terms of a theory proposed by da Silva and Falicov [Phys. Rev. B 45,11511 (1992)] in which pressure changes the trigonal component of the crystal field at the transition metal ions.     

Ordered orthorhombic phases of titanium monoxide

Symmetry analysis is carried out for the ordered phases of cubic monoxide TiO_y with relative oxygen contents y<1 and y>1. It is established that a partially ordered orthorhombic phase (space group Immm)—a derivative of the orthorhombic M₃X₂□ superstructure (at y<1.0) or the inverse superstructure M₂■X₃ (at y>1)—may arise in TiO_y. The distribution of Ti and O atoms, oxygen vacancies □, and titanium vacancies ■ in unit cells of the orthorhombic ordered phases is determined. The phases are formed through the order-disorder transition channel along two rays of a non-Lifshitz star {k ₄}, and the ordering proceeds as a first-order phase transition. The distribution functions of Ti atoms over the sites of metallic and O atoms over the sites of nonmetallic sublattices are calculated for the orthorhombic superstructures of cubic titanium monoxide TiO_y.     

Dielectric behaviour of anisotropic ionic crystals

In the case of cubic ionic crystals Havinga has shown that the temperature variation of dielectric constant could be described in terms of volume and temperature effects. By extending his formalism to anisotropic, ionic crystals it has been shown that unlike in cubic ionic crystals where the volume effect consists of a change in the number per unit volume of the polarizable particles and their polarizability with volume, in the case of anisotropic ionic crystals, in addition to these, a variation in the anisotropy of polarizability due to uneven thermal expansion also has to be taken into account. This method of analysis has been examined by taking rutile as an example.     

Electro-optical Kerr effect of azo-dye-doped liquid crystals using nulled intensity method

ABSTRACT

The electro-optical Kerr effect and pre-transition behavior have been investigated for different concentration of dimethyl-aminoazobenzene dye-doped nematic liquid crystal by using the nulled intensity method. Measurement results have showed that the molecular reorientation of liquid crystals induced by photoisomerization of azo dyes come about to increase an amount of Kerr constant at low percentages of dye concentration. Additionally, the results of pre-transition temperature which were obtained by extrapolating the invers Kerr constant as a function of temperature have been found to be in good agreement with predictions of the Landau-De Gennes model.     

Low-temperature phase transition in Ba2TiGe2O8

Previous work on Ba₂TiGe₂O₈ crystals has shown an unusual low-temperature (～ 223 K on cooling, ～ 273 K on heating) phase transition. Precession x-ray photographs on Ba₂TiGe₂O₈ single crystals show an incommensurate modulation along b*, and, for the first time, also along a*. Single crystal intensity data confirm the average structure in space group Cmm2. There is positional disorder in the pyrogermanate groups, and this is the probable cause for the modulated structure. The low-temperature phase transition is proposed to be a lock-in transition, with the modulation along a* locking in at a value of 1/3. Several properties, as well as other unusual features of the low-temperature phase transition, are discussed in light of the proposed lock-in transition. Domain studies show that the ferroelastic domains are unstable in the low-temperature phase.     

Dislocations and the mechanical properties of stabilized ZrO2

Abstract

The dislocation substructures and mechanical properties of cubic ZrO₂ single crystals are analyzed. The temperature dependence of the dislocation velocity is complex, the edge segments moving faster than screw segments at low temperatures, while the screws are faster at high temperatures. Using the loop shrinkage technique, the diffusion coefficient of the slowest diffusing species can be determined. The mechanical properties are analyzed in terms of the Peierls mechanism and of interaction of dislocations and point defects.     

Growth and characterization of BaZnGa

Abstract

We report the growth, structure and characterization of BaZnGa, identifying it as the sole known ternary compound in the Ba–Zn–Ga system. Single crystals of BaZnGa can be grown out of excess Ba–Zn and adopt a tI36 structure type. There are three unique Ba sites and three M = Zn/Ga sites. Using DFT calculations we can argue that whereas one of these three M sites is probably solely occupied by Ga, the other two M sites, most likely, have mixed Zn/Ga occupancy. Temperature-dependent resistivity and magnetization measurements suggest that BaZnGa is a poor metal with no electronic or magnetic phase transitions between 1.8 and 300 K.     

非密堆积TiO2空心微球光子晶体的制备与能带分析

利用胶体自组装、高温烧结和HF酸刻蚀技术,制备了SiO₂微球非密堆积面心立方(FCC)结构的胶体晶体,并以此为模板,利用溶胶凝胶方法和NaOH湿法刻蚀技术制备了TiO₂空心微球非密堆积FCC结构光子晶体.利用电子显微镜分析了晶体的结构,用平面波展开法对该结构进行了能带计算与分析.计算结果表明,制备的TiO₂空心微球非密堆积FCC结构光子晶体在低能区的第二、第三能带之间除布里渊区的W点仍保持简并外,其余各点简并都已经消除. 关键词： 空心微球 非密堆积 光子晶体 光子带隙     

Flexomagnetoelectric interaction in cubic,tetragonal and orthorhombic crystals

The phenomenological theory of the flexomagnetoelectric coupling in crystals of the cubic, tetragonal and orthorhombic crystal systems has been suggested. Secondary role of the crystal structure chirality was shown. Oppositely, significant role of the crystallographic point group type (symmetric, alternating, dihedral or cyclic) in the flexomagnetoelectric coupling has been derived. It was shown, that conceptually new features of the flexomagnetoelectric effects are expected in the crystals of the cyclic groups (crystal classes 4/m, 4? and 4). Proposed verification of the theory is investigation of the domain wall bend details (changes of the effect symmetry). Special case of such verification near the compensation point is suggested. First-principles mechanisms of the flexomagnetoelectric interaction were discussed.     

Quantum-mechanical treatment of the energetics of local defects in crystals: A few answers and many open questions

The problem of the Quantum-Mechanical evaluation of energy changes involved in local defect processes in crystals is discussed. The Perturbed Cluster approach [J. Molec. Catal., 82, 229 (1993)] is taken as a reference. It is shown that several contributions to the defect formation energy must be added, which require the use of auxiliary “associated models”. The charge-balance problem is examined in more detail, where the appropriate associated model is the “perturbative-variational” one. Alkali ion migration in alpha-quartz is taken as an example.