NSF-EC Activity: Studies of Defects, Disorder and Phase Transitions via Diffuse Scattering

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We propose here a joint University of Houston - Max Planck Institute (MPI-Stuttgart) program on the diffuse scattering of X-rays and neutrons from disordered alloys. There are three projects involved: a) The determination of detailed local and long-range static atomic displacements in a dilute random Ge(0.08)Si(0.92) alloy crystal using anomalous X-ray scattering at the ESRF in Grenoble, along with high energy-resolution X-ray or neutron measurements of the optic modes and in-band resonant modes; b) The use of a null matrix 62Ni(0.52)Pt(0.48) crystal (62Ni, which has a negative scattering length, will be supplied by the MPI) to eliminate all Bragg, thermal diffuse and Huang scattering in order to evaluate both the local order and local atomic displacements for comparison with theoretical models for this alloy. A "normal" Ni-Pt crystal will also be used to assess the phonon dispersion curves and Huang scattering. Companion theoretical work on the elastic and electronic interactions will be performed in collaboration with our MPI colleagues; c) Continuation of theoretical work to understand defect influences on phase transitions, in this case the X-ray observation of the conversion of a bulk first-order transition in V2H to a tricritical transition in a defective (dislocation-dominated) "skin" layer. Students will participate in this research.In this program of work we continue our studies of the detailed local structure of alloys - both semiconducting and metallic - through the application of diffuse (i.e. non-Bragg) X-ray and neutron scattering. The underlying physics of alloys depends on the precise nature of the interatomic interactions, both electronic and elastic. These interactions may be directly probed through a combination of scattering data and theoretical modeling, either via simulation or from first principles. The essential issues are the "size" of an atom, i.e. charge transfer, and the way in which the basic electronic structure is manifested in local order, alloy stability and, ultimately, in the prediction of phase diagrams. Statistical mechanics lies at the heart of these problems, as does the influence of defects, such as dislocations, on phase stability and phase transitions. Our scattering program, combined with theoretical modeling, will address these issues in a comprehensive way using synchrotron X-rays and reactor-based neutrons. This research will be conducted with students who will receive training in one of the contemporary forefront areas of condensed matter physics. Through this they will be prepared to enter the scientific/technological workforce of the next few decades.