College Park, Maryland      June 6 - 10 , 2004

M3-A1 (13:30): NIST-DCS Neutron Measurements of Lattice Disorder in a dilute Ge-Si Crystal and in a Null-matrix Ni(62)-Pt Alloy Crystal (Invited)

J.A. Rodriguez, S .C. Moss (Physics Department, University of Houston), J.L. Robertson (Oak Ridge National Laboratory), J.R.D. Copley, D.A. Neumann (NIST Center for Neutron Research), J. Major, V. Bugaev, H. Reichert, H. Dosch (Max Planck Institute fuer Metallforschung, Stuttgart)

Using the disk chopper spectometer (DCS) at NIST in an elastic scattering mode, we have investigated two alloy crystals: 1) a dilute alloy of Ge in Si (~10%) and 2) a null-matrix crystal of O.5262Ni-0.48Pt, so chosen (62Ni has a negative scattering length) that all effects depending on the average lattice scattering (Bragg peaks and Huang diffuse scattering (HDS--due to displacement-displacement correlations) vanishes while the short-range order (SRO) and cross correlation between concentration and displacements, often called the size effect scattering (SE), are now given by the sum of the absolute values of the scattering lengths of the two components.

The Si-Ge crystal was initially studied in the (-110) plane in reciprocal space. After removing the can scattering and normalizing the data, the planar plot from this (previously determined) random alloy showed pronounced fans of diffuse scattering emanating strongly on the low angle side of the Bragg peaks. Selected asymmetric line scans confirmed our earlier SE and HDS scan (1) along <110>, a sure sign that the size of Ge is larger than Si and its displacement field extends to distant neighbors. This data is currently being analysed in Stuttgart by Prof. Bugaev and his students and additional scans in the (001) plane are underway.

The null-matrix data are truly extraordinary. There are only tiny specks of Bragg peaks (useful for alignment) and absolutely no HDS about the Bragg positions. The SRO and SE scattering are quite prominent and are being analysed in real space by Rodriguez and Robertson to extract both SRO and individual species-dependent displacements for each coordination shell, and in k-space by Prof. Bugaev and his colleagues. The results as they come out will be discussed in the context of the electronic theory of alloy phase stability, determined from data above Tc, the first order phase transition temperature.

(1) D. LeBolloc'h et al., Phys. Rev. B 63, 035204 (2001)

Research supported in Houston by the NSF on DMR-0099573

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