We have prepared the Y Ca Ba Cu O with xF compounds by the solid state reaction technique. The uperconduct-1yx x 2 3 y ing critical temperature was measured. The structural parameters as a function of Ca substitution were obtained through the detailed analysis of the X-ray diffraction for these Ca-doped samples. The variation of the lattice parameters, some meaningful bond lengths, and the microstrain in lattice change with the Ca content have been discussed.

Nd doped Y0.8 Ca0.2 Ba2-x Ndx Cu3 Oy YBNCO compounds with x≤.50 have been prepared by the solid state reaction technique. The structure and the superconductivity for YBNCO compounds are characterized. The zero resistance transition temperature T for each sample was measured. T increases with the increase of the content of Nd in YBNCO as xF0. 15, c0 c0 and decreases as xG0.20. X-ray diffraction studies show that there is no structural phase transition in the whole doping range. The dependencies of the lattice constants and some other structural parameters on the content of Nd for the samples YBNCO with xF0.20 have different behaviour than that for the samples with xG0.25. It seems that the uperconductivity of YBNCO is related to the structural changes due to Ca and Nd co-doping. q1999 Published by Elsevier Science B. V. All rights reserved.

A dynamic model of anisotropic x-ray diffraction is developed using two-wave approximation. A dispersion surface equation is derived for the screw-axis and glide-plane forbidden reflections. Propagation and polarization phenomena of waves are discussed. The deductions show that all these forbidden reflections may be excited except the 00l (l52n11) reflections for a 63 screw axis and the 00l (l56n13) reflections for 61 and 65 screw axes. The general methods are illustrated by their application to the rutile structure. [S0163-1829(99)3330-5]

For GdFe/NiCoO bilayers after field cooling, hysteresis loops at low temperatures are shifted along both the horizontal and vertical axes. The exchange field HE changes from negative values to positive values with increasing cooling field HCF and the coercivity HC acquires a maximum near the crossover of HE.At 5K, HE and HC at HCF=3 T and the peak height DHC are proportional to the inverse GdFe layer thickness. At HCF=3 T, HE is always positive in the temperature region from 5 to 350K. At low HCF, however, HE is negative at low temperatures and becomes positive at high temperatures. These results can be ascribed to antiferromagnetic coupling between GdFe and NiCoO layers.

Crystal tilts in the epitaxial lateral overgrown (ELO) GaN region over SiO2 mask by hydride vapor phase epitaxial away from the opening region were investigated by scanning electron microscope and the four-circle x-ray diffraction method. The increased mask width and ratio of stripe opening width to stripe period (fill factor) lead to increased wing tilts. The introduction of additional HCl and C3H8 /H2 can decrease wing tilts and improve the ELO GaN surface morphologies significantly. Moreover, it has been observed that a high growth rate will bring more defects and large wing tilts into the GaN materials.