The crystal and magnetic structure of Nd2Fe»N45 at room temperature was refined by Rietveld analysis of neutron-powder-difFraction data. We found that Nd2Fe»N4, has a Th2Zn»-type structure (space group R 3m) and the nitrogen atoms occupy both 9e and 18g sites with occupancies 0.80 and 0.35, respectively. A physical model is proposed based on the large isotropic thermal factor for the 18g sites.

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Two samples of NdBa2Cu307+"with zero-resistance transition temperatures of 85 K (x 0.06) and 90 K (x 0.18), respectively, were studied by using neutron-diffraction powder profile analysis. The compounds crystallize in the same orthorhombic, oxygen-deficient perovskite structure with space group Pmmm, in which both le (0, -, ', 0) and lb (-, ', 0,0) sites are occupied partially by oxygen atoms, with a majority of atoms being located in the le sites. The final refinements converge to weighted profile R factors, R~, of 7.05% and 7.58% for the 90and 85-K samples, respectively. The significant diff'erences between the two samples are the oxygen content and the regular changes of all the Cu -0 bond distances in the structures.

The crystal structure of the high-T, superconductor Ba2YCu307+ has been studied using neutron digraction. This compound has an orthorhombic structure with space group Pmmm and lattice parameters a 3.8132 A, b 3.8774 A, and c 11. 653 A. We found an occupancy of 0.1 for oxygen atoms at the 1b site and a total number of oxygen atom per unit cell of 7.1. The high quality of the sample used may be associated with the partial occupation at the 1b site by oxygen atoms.Early this year Wu et al. ' and Zhao et al. reported that a barium, yttrium, and copper ternary oxide becomes a superconductor above 77 K. Soon after the exciting discovery the structure of the superconducting phase was studied by Cava et al. , Michel, et al. , and Beyers et al. Subsequently, Siegrist et al. carried out a structure determination from single-crystal x-ray data, and proposed a structure based on an orthorhombic perovskitelike model. Recently, Beno et al. and Capponi et al. studied the structure by neutron diffraction. The two groups reported that the O(4)(le) site is fully occupied by oxygen atoms.We report here the results of neutron diffraction on Ba2YCu307+". We found a structure that is different from other models proposed for the oxygen-site occupancies.The sample was prepared using the standard powerceramic method from Y203, BaCO3, and CuO powers of high purity. Neutron and x-ray power diffraction data showed a pure orthorhombic phase. We have measured the resistance and magnetic susceptibility as a function of temperature, and the magnetization as a function of magnetic field for the sample investigated in the present neu-tron diffraction study. Figure 1 shows the temperature dependence of the resistance and susceptibility, and Fig. 2 shows the magnetization curve. The sharp transition of the resistance and susceptibility at 92 K with a narrow transition width 1.2 K as well as the very large 0,2, which can be seen in Fig. 2, implies that the superconducting phase is very stoichiometric and homogeneous.Neutron power diffraction data were collected on the triple-axis spectrometer at the Institute of Atomic Energy, Beijing. The neutron wavelength was 1.227 A.. The data were analyzed with the Rietveld structure refinement program RIETAN.were used for the initial values. We refined the lattice parameters and proNe parameters first, subsequently refined the positions and occupation factors of Ba, Y, and Cu atoms, then refined the positions and occupation factors of all oxygen atoms, then made a final run for all refined pa-Xl IQ CQ CL UJ Cf) CO Ba-Y-cu-0 -5