| Popis: |
The Debye-Waller factor of nickel has been measured by powder neutron diffraction over the range of scattering vectors Q from 10 to 24 A-1 (sin θ/λ from 0.8 to 2.0 A-l). Pulses of epithermal neutrons produced by the Harwell LINAC were scattered from a flat sample at nearly back-scattering angles (2θ = 150°), and observed as a function of their wavelength determined from their time of flight over a 4.6 m flight path. The resolution in d spacing was observed to be nearly constant at about Δd/d = 1% over the above Q range. After background subtraction, calibration using a standard vanadium scatterer, and corrections for absorption, the powder pattern was analysed using the techniques of profile analysis, in which a computed powder profile is least-squares fitted directly to the observed profile. The profile refined to an χ2 factor of 17%, giving a simple Debye-Waller factor exp (-2B sin2 θ/λ2) with B = 0.34 ± 0.04 A2. This value is appreciably less than the values of order 0.42 A2 obtained by neutron measurements at low Q. The inclusion of the fourth-order term exp (- 2C sin4 θ/λ4) predicted by anharmonic theory raised the fitted harmonic coefficient to B = 0.37 ± 0.04 A2 in good agreement with X-ray and theoretical values, with C = -0.012 ± 0.006 A4. |
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