The Process of Formation of Epitaxial Cadmium and Zinc Oxide Films by Interaction of Single-Crystal AIIBVI Layers with Oxygen

The results of electron microscopic and electron diffraction investigation of the surface during annealing of single-crystal A^IIB^VI (CdS, CdSe, ZnSe, ZnS) films in the presence of oxygen are reported. It is shown that the behaviour of islands of cadmium oxide formed by chemical interaction between O₂ and the CdS, CdSe substrate in the process of growth resembles that of the nuclei in film condensation. CdO films become “continuous” more quickly, if the size of blocks of substrate-epitaxial A^IIB^VI film is larger. Epitaxial ZnO films formed on the surface of epitaxial ZnS and ZnSe layers are of a wurtzite structure or a cubic structure with the parameter a = 4.5 Å.     

Review: Different types of ordering

Homogeneous structures A_xB_y with identical A–A or B–B self‐coordination numbers T₁, T₂, T₃, of nearest, next‐nearest and third neighbors are selected. The maximum T_i values are 2 2 2 (1D row), 6 6 6 (2D hexagonal net) and 12 6 24 (3D cubic close‐packing). Reduced T₁ or T₂ values for A_xB_y can be related with attractive or repulsive A–A (or B–B) interactions. A single set of T₁ T₂ T₃; y/x values was obtained for y/x = 1–4 and 6. The y/x = 2 structures of three equivalent positions A, B and C are unfavourable for A = ⊕ and B = ⊖ charge or direction of magnetic moments. The spatial distribution of positive and negative potential near A⁺ and B⁻ positions gives rise to neutral C positions. Frustrated magnetic moments at C positions are disordered. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Calculations of homogeneous region in Ga1−xAlxAs and GaAs1−xPx ternary solid solutions

Ternary solid solutions of A^IIIB^V compounds are considered as pseudobinary A(x)^IIIB(x)^v compounds, where the behaviour of A(x)^III and B(x)^v pseudoatoms is quite similar to A^III and B^v atoms in a binary A^IIIB^v crystal. Weak dependence of point defect contribution into Gibb's energy of A^IIIB^v crystal on its defect nature, random character of ternary solid solutions of A^IIIB^v compounds allow to use already for binary compounds developed formalism in the determination of component thermodynamic potentials of solid solution. Basing on literature data for the equilibrium solidus of AlAs the approximation for the temperature dependence of thermodynamic potential of an AB quasimolecule in A^IIIB^v crystal is revised. This result together with the well-known parameters for the equilibrium liquidus in Ga–P, Ga–As, and Al–As systems were used for calculations of the nonstoichiometric factor at the boundary of a homogeneous region in Ga_1−xAl_xAs and GaAs_1−xP_x ternary solid solutions. The results are compared with the known literature data.     

Atomization Energy of the Cu,Ag, Au Crystals in Terms of the Statistical Model

The statistical atom model is used for ab initio calculations of the atom binding energy in crystals. The original calculations by the above method of atomization energy at T = 0 K of the A₁ modification crystals of I^B elements group of the Periodic System are consistent with the experimental data. The atomization energy estimates of the A₂, A₃ modifications are made.     

Strain of interatomic bonds and crystallochemical aspects of AB 1 2/′ B 1 2/″ O3 oxides with perovskite structure

A method for determining the strain characteristics of interatomic bonds in crystals of ternary oxides AB ₁ ^2/′ B ₁ ^2/″ O₃ with perovskite structure, i.e., AB ₁ ^2/′ B ₁ ^2/″ O₃ (B″ = Nb, Ta, Sb, Re, or Bi) and AB ₁ ^2/′2+ B ₁ ^2/″6+ O₃ (B″ = Mo, W, Re, Os, or U), is developed. A linear relationship is established between the effective lengths of unstrained B-O bonds (l _0BO) and the lengths of unstrained B′-O (l _0B′O) and B″-O (l _0B″O) bonds, which differs from the Vegard rule. The found values of l _0B″O for ternary oxides with perovskite structure turned out to be close to the average interatomic B″-O distances in crystals of polymorphic phases of low-symmetry simple oxides. It is shown that the average length of the unstrained Pb-O bond in PbB ₁ ^2/′ B ₁ ^2/″ O₃ perovskites corresponds to the length of the same bond in binary oxides PbBO₃. For ternary oxides with perovskite structure, a linear correlation between the bond-strain energy and the temperature of their transition to the cubic phase is established. A linear correlation is found between the ratios of the Curie temperatures and the bond-strain energy for lead niobates and tantalates.     

Shaped crystal growth of PbTe by the open tube technique

The possibility of growing shaped A^IVB^VI crystals from the vapour phase by the open tube technique was investigated on PbTe. The thermodynamic conditions to reproducible prepare several habits of PbTe crystals — whiskers, tetrahedral prisms and cubes, platelets (a), dendrites, cubic and octahedral skeletons (b) as well as their properties were examined. The crystalline habits (a) are characterized by perfectly mirror-like (100) surfaces and low dislocation densities. The type and concentration of charge carriers are mainly determined by the vapour composition within the crystallization zone and very from n = 3 × 10¹⁷ to p = 1 × 10¹⁹cm⁻³. These properties are the reason of the interest in shaped PbTe crystals for the aim of IR optoelectronic devices.     

Topology of the polypeptide chain in the complex of agglutinin from castor bean seeds with β-D-galactose in the crystalline state

The three-dimensional structure of the complex of agglutinin from Ricinus communis with β-D-galactose was established and refined at 2.5 Å resolution by X-ray structure analysis. Biocrystals were obtained using dialysis through a semipermeable membrane. X-ray intensity data (R _merge = 4.6%) were collected from one crystal at 100 K using synchrotron radiation at the DESY outstation [European Molecular Biology Laboratory (EMBL), Hamburg, Germany]. The initial phases were calculated by the molecular replacement method. The atoms of both protein and sugar molecules were localized. Unlike ricin, the ricinlike heterodimer RcA contains only one galactose-binding center in the region of the Asn46-Gly25-Trp37-Lys40 site in the first domain of the B subunit, whereas the second galactose-binding site of the B subunit is lost. One functionally important water molecule, which is bound to the residues Tyr123-Glu176-Arg179-Glu207, was revealed in the region of the active center in the A subunit.     

Magnetic Susceptibility and Peculiarity of Chemical Bond in the Cd-P(As) System Compounds

Specific features of interatomic interaction of semiconductor compounds of the Cd—P(As) system derived from the analysis of experimental data and theoretical calculations of magnetic susceptibility are discussed. It is shown that the A²B⁵₂ compounds are of predominantly covalent and A²₃B⁵₂ of dominating ionic bond character.     

Neutron diffraction and vibrational spectroscopic study of single crystals of KH(D)(CHCl2COO)2

The hydrogen atoms in the short (2.498 Å) hydrogen bonds of potassium hydrogen bis-dichloroacetate are placed asymmetrically with respect to the oxygen atoms. The geometric mass effect (R 0.02 Å) is well within the range observed with this type of hydrogen bond. The thermal ellipsoids of the bridging protons are slightly elongated, the longer axis being at 40 ° with respect to the O(2)-O(4) direction. This coincides with the direction of the transition dipole moment of thev _a OH vibration. The two infrared components of this vibration are rather different, one (A_u) being very broad and intense and the other (B_u) only moderately broad. Thev _a OH/v _a OD ratio is small (1.05).     

Elastic debye temperature of compounds with sphalerite structure

Elastic Debye temperatures θ_EL of the semiconducting compounds A^IIIB^V, A^IIB^VI and CuCl are evaluated from second order elastic constants C_ik available from the literature. The dependence of θ_EL on temperature is given. The results at 0 K are compared with those calculated from low temperature measurements of specific heat. Generally, the agreement between these values is better than found in the literature up to now.     

Analytical calculation of binding energies of highly symmetrical particles

Analytical formulae of binding energies per atom are derived for icosahedral, cuboctahedral and face-centered cubic particles A_N by means of summation of pair potentials. Asymptotical relations for larger particles E/N = V (A – BN^−1/3) as well as the topological model of clusters are discussed.     

Hydrothermal Synthesis and Single Crystal Structures of New Thorium Fluorides: A<Subscript>3</Subscript>Ba<Subscript>2</Subscript>Th<Subscript>3</Subscript>F<Subscript>19</Subscript> (A = Na,K, Rb)

Abstract  

A new family of mixed alkali–alkaline earth metal thorium fluorides based on the formulation A₃Ba₂Th₃F₁₉ (A = Na, K, Rb) has been synthesized and structurally characterized. The compounds crystallize in the cubic space group Im-3m with a = 12.1222 (14) and 12.3052 (14) ? for the K and Rb end members, respectively. This is only the second thorium fluoride reported as a single crystal structure to incorporate alkali and alkaline earth metals into the same structure. Crystals were obtained from the equimolar reaction of Ba(OH)₂·H₂O, ThF₄ and ABF₄ in a 2 M AF solution under hydrothermal conditions (545 °C and 1 kbar). Barium atoms substitute at the alkali metal sites and preferential site substitution is supported by the crystallographic analysis. Additional substitution at the alkali metal site has also been shown for K_2.2Na_0.8Ba₂Th₃F₁₉ and K_1.85Rb_1.15Ba₂Th₃F₁₉ using dissimilar alkali tetrafluoroborate and fluoride mineralizers in the reaction.     

Trends in the thermal expansion coefficients of the AIBIIIC2VI and AIIBIVC2V chalcopyrite compounds

Empirical relations are derived for the average linear thermal expansion coefficient α_L and the linear thermal expansion coefficients α_a and α_c of the lattice parameters a and c, respectively, of the A^IB^IIIC₂^IV and A^IIB^IVC₂^V compounds. It is shown that the thermal expansion coefficients of all tetrahedrally coordinated compounds can be described within the same model. The anisotropy of the thermal expansion coefficients depends essentially on the lattice constant ratio c/a. There exists a critical c/a value below of which α_c becomes negative.     

The thermodynamic equilibrium of the formation process of High-Tc superconducting phase YBa2Cu3O7−x

Analysing the equilibrium state of the formation reaction of the high-T_c superconducting phase 1:2:3 (YBa₂Cu₃O_7−x) on the base of the pressure-temperature-oxygen content (P-T−x) correlation research and using the Van't-Hoff equation, the Gibbs free energy (ΔG_m) of the reaction has been calculated for the first time. It is seen that the absolute value of ΔG_m is not large and of the order of 2.5 kcal · mole⁻¹. This leads to an instability of the 1:2:3 phase. Also, from the calculated values of ΔG_m relative stability regions of the 1:2:3 phase are determined for different values of x. Combinatively analysing the relation lg P = A−B/T (where A – the x-dependent constant, and value of B was determined to be equal to 10518 atm. K) and the value of ΔG_m, we come to the conclusion that the synthesis of YBa₂Cu₃O_{7 − x} when x → 0 must be carried out under high oxygen pressure and low temperature for a lasting time, because the reaction rate is limited by slow diffusion in the solid phase.     

Confrontation of the BCF theory and computer simulation experiments with measured (R, σ) curves

A brief summary of crystal growth models – the surface diffusion model of Burton, Cabrera and Frank and computer simulation models – is given and these models are confronted with measured (R, σ) curves. It was found that the most (R, σ) curves measured so far can be fitted by BCF curves with 10⁻² ≲ σ₁ ≲ 10⁻¹ and C having an order of magnitude of 10⁻⁴ cm ≲ s⁻¹. ΔG turns out to be 12 ± 5 kcal ≲ mol⁻¹. – Some of the (R, σ) curves can be described better by a nucleus above nucleus formalism. The order of magnitude of A is 10⁻⁴ cm · s⁻¹ and 10⁻² ≲ B ≲ 10⁻¹. – It is concluded what kind of work should be done in future to check theoretical models.     

Group-theoretical study of cationic ordering in perovskite structure

Atomic ordering in the structure of cubic perovskite has been investigated by the group-theoretical method. The possibility of existence of 13 ordered phases in the 1(a) position (including four binary, two ternary, and five quaternary cationic superstructures) and 13 phases with different types of ordering in the 1(b) position (including four binary, two ternary, and five quaternary cationic superstructures) is established. Calculated structures of some types of ordered low-symmetry perovskite modifications are reported.     

Intra- vs intermolecular hydrogen bonding. The crystal structure of 5-methyl-2-hydroxyacetophenone thiosemicarbazone monohydrate and salicylaldehyde-2-methylthiosemicarbazone monohydrate

The crystal structure of 5-methyl-acetophenonethiosemicarbazone monohydrate,A, and salicylaldehyde-2-methylthiosemicarbazone monohydrate,B, were determined using single crystal X-ray diffraction.A crystallizes in the monoclinic space groupC2/c, with lattice parametersa=14.161(2),b=15.753(1) ?,c=11.084(1) ?, β=112.59(1)° andZ=4, yielding a calculated density ofD _calc=1.352 mg/m³.B crystallizes in the triclinic space groupP1, witha=7.233(2) ?,b=7.371(2) ?,c=11.841(2) ?, α=82.77(2)°, β=78.33(2)°, γ=63.06(2)° andD _calc=1.371 mg/m³ forZ=2,. In bothA andB the immine nitrogen and the sulfur atom areanti with respect to N2-C8. WhileA presents the usual intramolecular six membered hydrogen bond ring,B has instead an intermolecular hydrogen bond between the hydroxy moiety of the salicyladehyde and a water molecule. AM1 calculations agree with the experimental conformations observed in both compounds. Contribution No. 1619 of the Instituto de Química, UNAM.     

Combined atomic ordering in the A and B sublattices of perovskite structure

The simultaneous ordering of cations in the A and B sublattices of cubic perovskite structure has been investigated by the group-theoretical method. It is found that 147 ordered phases may exist. Among them, there may be 121 phases with simultaneous cation ordering in the 1(a)- and 1(c) positions of perovskite structure. It is shown that 53 phases are described by improper order parameters related to cation ordering, the formation of 56 phases is related to improper rotations of octahedra, and 15 phases are improper ferroelectrics. Calculated structures of some types of ordered low-symmetry perovskite modifications are presented.     

Synthesis and molecular structure of propylene-bis(benzoylacetoneiminato)copper(II), [Cu(ba)2-1,2-pd]

The title compound C₂₃H₂₄N₂CuO₂ (4 in Scheme 1) was prepared and characterized by IR, UV, and GC measurements. The structure was solved by direct methods, and full-matrix least-squares refinement of structural parameters led to a conventionalR factor of 0.049 for 1879 reflections. The copper(II) atoms are in a nontetrahedrally distorted planar coordination. A disorder of the five-membered chelate ring has been observed: two alternative positions,A andB for C20, C21 and C22 were found and refined with constrained C-N and C-C bond length. In bothA andB rings, connected by pseudosymmetry plane, the ethylene bridge shows the gauche conformation, with the methyl group in the axial position. The geometry of the molecule is discussed.     

Über die röntgenographische Analyse der mittleren quadratischen Atomverschiebungen in einigen Halbleiterkristallen

X-ray measurements of the Debye-Waller factor were performed with single crystals of Ge and of semiconducting compounds A^IIIB^V, A^IIB^VI, A^IVB^VI in the temperature range 100 K — 1000 K. From the results the mean square atomic displacements 〈u²〉 and the Debye characteristic temperatures θ of the materials were calculated. Both parameters are discussed with respect to the influence of the temperature, the influence of point defects and, in the case of the compounds, deviations from the stoichiometric composition on the thermal lattice vibrations. Finally, considerations are presented, concerning the relationship between the parameters 〈u²〉 and θ_M, respectively, and the activation energies of vacancy formation and diffusion in the materials.