V. Valvoda

Composition of phases and phase mixtures of the Bi-Sr-Ca-Cu-O system

Abstract Ceramic samples of nominal composition of BiSrCaCu 2 O 5.5 were analyzed by an electron microscope equipped with an EDAX system. Compositions of phases C37, C30 and C24 (denoted according to the height of their elemental cells) were found to be Bi 2.4 Sr 1.6 Ca 2 Cu 3 O y , Bi 2.4 Sr 1.6 CaCu 2 O y and Bi 2.4 Sr 1.6− k Ca k CuO y ( k =0.4− 1.2) respectively. In most of analyzed volumes of the size of several cubic micrometers, mixtures of two or of all three phases have been found. A mechanism of formation of the C37 phase and of the phase mixtures is proposed.

Preferred orientation in powder samples of magnesium and magnesium-cadmium alloys

Cylindrico-symmetrical needle-type texture has been observed in powder samples of magnesium and magnesium-cadmium alloys. Orientation distributions of crystallites determined by measurements of 002 reflexions on texture goniometer can be described by simple analytical expressions. Corrections for preferred orientation applied to measurements on powder diffractometer enable determination of Debye characteristic temperature≡ from the dependence of intensities on diffraction angle. The values of≡ determined after correction from samples with preferred orientation are in agreement with the value of randomly oriented sample.

The X-ray Debye temperature of CdTe

The X-ray Debye temperature of CdTe has been measured by diffraction on single crystal and on powder samples. From measurements in the temperature range 8 to 335 ‡C the average values 140±3 K and 145±8 K were determined for single crystal and powder samples respectively. The value 141±5 K was obtained from angular measurements on powder samples at room temperature.

X-ray Debye characteristic temperatures of Mg3Cd alloys

Ordered and disordered phases of two alloys of nearly stoichiometric Mg3Cd constitution were investigated by X-ray diffraction at room temperature. Correction for preferred orientation of crystallites in powder samples was made assuming particular type of orientation distribution with one unknown parameter. Debye characteristic temperatures determined from values of corrected integrated intensities lie in the range 258 K to 267 K for all samples under investigation with no definite difference for ordered and disordered phases.

Magnetic properties of REBa2Cu3O7 (RE = Y, Nd, Sm, Eu, Gd, Dy, Ho)

Abstract Magnetic properties of REBa 2 Cu 3 O 7 compounds were studied at low temperatures. The obtained magnetic RE moments are considerably reduced (except for Gd) with respect to free-3+-ion values. These results are ascribed to crystal field effects. The magnetization curves of the superconducting samples exhibit the expected hysteresis behaviour of type-II superconductors.

X-ray diffraction on solid solutions with atoms of different sizes

The formula for intensity of diffraction of X-rays on random binary solid solution is derived using the linear model of rigid atoms of different sizes in close-packed arrangement. The shape and position of the peaks of diffuse scattering resulting as the main diffraction effect are influenced by different sizes of atoms. On the other hand, the integrated intensity of the diffuse peaks is practically unaffected by the size effect.

Lattice vibrations and local order in substitutional solid solutions. III

The nearest neighbour effect on the mean vibrational state is investigated in solid solutions. It is shown that the possibility of A-B pair bond affection in the closest surrounding of the bond is accounted for, when linear, parabolic or some more complicated dependence of the states of A and B atoms on the relative proportions of both types in their first coordination shell is used. The expression is deduced for the mean vibrational state of atoms in the solid solution under the assumption that the dependence of A and B atom states on the number of unlike atoms in the first coordination shell is given as a polynomial ofi-th order. In the case of the parabolic approximation it follows that the structure of the solid solution (i.e. degree of order) is given by atom interactions, which result in configurations with maximum lattice cohesion. There is no assumption about the nature of atom interaction in the present paper. Therefore the results combined with experimental data can serve as a criterion of availability of different physical models and help in a better understanding of atomic bonds in metal systems.